- Email: [email protected]
Er3+ co-doped sodium zinc bismuth fluorophosphate glasses
Optical Materials 100 (2020) 109616
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Energy transfer induced enhancement in NIR luminescence characteristics of Yb3þ/Er3þ co-doped sodium zinc bismuth fluorophosphate glasses C. Parthasaradhi Reddy a, *, R. Ramaraghavulu b, T. Kalpana c, J. Gajendiran a a
Department of Physics, Vel Tech Rangarajan Dr. Sagunthala R & D Institute of Science and Technology, Chennai, India Department of Humanities & Sciences, Annamacharya Institute of Technology and Sciences, Kadapa, Andhra Pradesh, India c Department of Physics, Andhra Loyola College, Vijayawada, 520 008, Andhra Pradesh, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Melt quenching method Stimulated emission cross-section Yb3þ/Er3þenergy transfer Yb3þ absorption and Er3þ emission
The results pertaining to the energy transfer-based photoluminescence characteristics of Er3þ-doped and Yb3þ/ Er3þ co-doped NaF–ZnO–BiF3–NH4H2PO4 (NZBFP) glasses prepared through melt-quench technique were demonstrated here. By applying McCumber’s theory, the absorption, stimulated emission cross-sections and optical gain have been estimated for Yb3þ (0.97 μm) and Er3þ (1.54 μm) ions. Yb3þ-doped glass exhibited a sharp emission at 979 nm at 808 nm excitation while Er3þ-doped glass displayed a broad NIR emission band at/around 1545 nm for 980 nm excitation. The presence of Yb3þ absorption band along with the Er3þ absorption bands in the co-doped glass suggests the energy transfer (ET) possibility within these ions. On pumping Er3þ-doped & yYb3þ/Er3þ co-doped NZBFP glasses with 980 nm Laser Diode, a broad Er3þ emission at 1545 nm attributed to 4 I13/2 → 4I15/2 is observed in the NIR region. The increasing of Yb3þconcentration with respect to Er3þ (fixed to 1 mol%) in co-doped glasses have resulted in enhancement of Er3þ NIR emission because of the resonant energy transfer (ET) process from Yb3þ→Er3þ. The energy transfer (ET) mechanism has been illustrated from the spectral overlap of Yb3þ emission and Er3þ absorption, Er3þ decay lifetime curves, and partial energy level di agram. Further, the nature of interaction responsible for the energy transfer process (ET) has been explained using Forster-Dexter theory and I–H fitting. In addition, the suitability of the present synthesized fluo rophosphate glasses for optical amplifier and NIR laser applications were explored in terms of optical amplifi cation and gain parameters, like effective band-width (Δλeff), stimulated emission cross-section (σe), optical gain (G), and gain bandwidth (ΔG).
1. Introduction The rapid development of telecommunication system by optical transmission and fibre optical communication have extended their op tical range of operation to the integrated circuits. The device like dielectric waveguides (single/multi-mode fibre optic systems) have been interfaced with the integrated optical circuits for better operation with widespread applications in multiplexing technologies like WDM (1310–1550 nm), Coarse WDM (1270–1610 nm), Dense WDM (C-band: 1530–1565 nm) and EDFA for high-speed digital communication [1–3]. Glasses are considered to be suitable materials for the fabrication of optical fibres because they offer a wide composition range and glass-forming ability with exceptional optical transparency, UV to NIR transmission besides low attenuation. Though, these features promoted glasses for optical communication, they are passive and lack in
electronic transitions. So, to overcome this, glasses are incorporated with optically active ions like lanthanides and transition metal ions. The unique characteristics which distinguish rare earths from the transition metals are their rich electronic structure, where outer 5s and 5p sub shells shield the optical transitions of 4f shell by enabling them as optically active exhibiting narrow emissions with longer lifetimes and PLQYs [4–6]. These diverse spectroscopic and electronic properties of di- and trivalent rare earth ions make them ideal for probing the local structure. In recent past, solid-state materials e.g., glasses, glass ce ramics, ceramic powders, polymers containing rare-earth ions have been investigated for a number of potential applications, e.g., solar cells, bio-imaging, displays devices, lasers, fibre optics, LEDs, spectral con verters (up and down converters) and optical thermometry [7–10]. Lia Mara Marcondes et al. explained the emission characteristics of Er3þ doped and Er3þ/Yb3þ-codoped GeO2-Nb2O5–K2O glasses in the
* Corresponding author. E-mail address: [email protected] (C.P. Reddy). https://doi.org/10.1016/j.optmat.2019.109616 Received 13 October 2019; Received in revised form 8 December 2019; Accepted 14 December 2019 Available online 18 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
region of 1.5 μm covering C, S and L-telecommunication bands [11]. Er3þ and Er3þ/Yb3þ doped zinc tellurite (ZnO/TeO2) glasses optical characteristics for sensor applications were investigated by Sevcan Tabanli et al. [12]. Sensitizing effect of Yb3þ ions on photoluminescence properties of Er3þ ions in lead phosphate glasses for optical fiber am plifiers are studied by Ch. Basavapoornima et al. [13]. Yin Zhang explore the optical properties of Er3þ/Yb3þ co-doped phosphate glass for its application as electronic amplifiers [14]. Shangjiu Nian et al. studied optical properties of Er3þ/Yb3þ co-doped bismuth calcium borate glass system for NIR lasers and fiber amplifiers [15]. Upconversion and 1.53 μm near-infrared luminescence study of the Er3þ-Yb3þ co-doped novel phosphate glasses are carried out by R. J. Amjad et al. [16]. for their applications in optical fibers. Mustafa Dh Hassib et al. [17]. investigated the Er3þ/Yb3þ co-doped boro-silicate glasses for optical amplifier and gamma radiation shielding applications. A. Maaoui et al. [18]. suggested Er3þ/Yb3þ codoped fluoro-tellurite glass can be useful for development of optical devices. The absorption, emission spectra and lifetime mea surements for the infrared and visible fluorescence for Er–Yb codoped TeO2–ZnO–ZnF2 glasses are explained by A. Miguel et al. [19]. Aroua Langar et al. [20]. studied the optical properties of phosphate glasses codoped with Er3þ–Yb3þ for developing the solid-state 1.55 μm optical amplifiers. Among the rare earth ions operating in NIR region for diverse and potential applications [10,21–24], Er3þ (singly or in combination with Yb3þ) doped glasses have gained considerable attention for optical communication because of 1.54 μm emission (4I13/2 → 4I15/2) corre sponding to the C- band window (1530–1565 nm) for telecommunica tion [25]. The broadness of Er3þ emission band enables it as a suitable candidate for wavelength-division multiplexing (WDM) and erbium-doped fiber amplifier (EDFA) systems, operating in the S-and (1460–1530) and L-band (1565–1625 nm) along with C- band [26,27]. Investigating Yb3þ/Er3þ-combinedly incorporated glasses are very fascinating owing to radiative ET from Yb3þ→Er3þ. The absorption cross-section of Yb3þ corresponding to 2F7/2 → 2F5/2 transition is wider compared to the 4I15/2 → 4I11/2 transition of Er3þ at 980 nm. Therefore, excitation energy would be readily absorbed by Yb3þ ions and subse quently transfer it to Er3þ ions efficiently resulting in a higher optical gain. In addition, high pump absorption favours side-pumping, thus allowing the use of large-area diode lasers as a pumping source [28,29]. Silicate based glasses are well studied optical matrices since many years for its commercial applications as fibre optical amplifiers, but its application has been limited in case of broadband applications due to very narrow bandwidth emission at 1530 nm. Therefore, new glass compositions are explored to achieve efficient fibre amplification and flat gain spectra. Fluorophosphate based glasses, which belong to oxy-fluoride glass family have received much attention, because, their chemical composition is comprised of oxides (phosphate and zinc oxide) and fluorides (sodium fluoride and bismuth fluoride). The presence of oxides promotes material strength, stability and longevity whereas fluorides with lower phonon energy shorten the non-radiative losses resulting in improved photoluminescence decay rates and PLQYs of rare earth luminescence. Fluorides also enhance IR transmission ability by lowering OH absorption by forming HF [30,31]. In the present study, stable NZBFP glasses are doped with Er2O3, Yb2O3 in single and dual combination to investigate their NIR emission characteristics, decay curves, energy transfer mechanism, optical gain and amplification factors.
iv. 20NaF–15ZnO–10BiF3–(55-x) NH4H2PO4–1.0 mol % Er2O3– xYb2O3 (x ¼ 1.0 mol % & x ¼ 1.0, 2.0 and 3.0 mol%) The titled glasses were prepared by NH4H2PO4, BiF3, ZnO, NaF, Er2O3 and Yb2O3 chemicals procured from Sigma-Aldrich. All the pre cursors were weighed according to the above composition indepen dently in 7 g set and mixed thoroughly using agate mortar and pestle until a fine powder was obtained. To obtain homogeneous mixture acetone was added to the chemical mix while grinding. Subsequently, each chemical set was taken into Alumina crucible separately and was heated in a muffle furnace for 1 h at 950 � C. Later those melts were poured into the circular moulds and quenched with a smooth-surfaced brass plate to obtain circular glass discs of 3–4 cm in diameter with 0.3 cm (0.03 mm) in thickness. Finally, these glasses were smoothly polished and further used them to carry on measurements. 2.1. Measurements The refractive indices of the Yb3þ and Er3þ doped NZBFP glasses were measured from Abbes refractometer at wavelength 589.3 nm of sodium vapour lamp using 1-bromonaphthalin as a contact liquid are found to in the range of 1.607–1.668. XRD patterns were measured on a Seifert X-ray Diffractometer (model 3003 TT) with Cu Kα radiation (λ ¼ 1.5406 Å) at 40 kV and 20 mA with a Si detector between 10� and 80� at the rate of 2� /min. TGA-DTA measurements were simultaneously done for NZBFP glass on a NetZsch STA 409 at a heating rate of 10 � C/min under N2 as the purging gas. Raman spectrum of the NZBFP glass is measured on a Jobin Yvon Horiba LabRam series (HR-800) microRaman spectrometer at room temperature. The absorption spectra of NZBFP glasses containing Er3þ and Yb3þ/Er3þ ions were recorded at room temperature in the spectral range of 250 nm–2500 nm on a VarianCary-Win Spectrometer (JASCO V-570). NIR emission spectra were ob tained by exciting the Er3þ and Yb3þ/Er3þ ions doped glass samples at 980 nm and 808 nm using a Crystal laser-DL series at 100 mW. The lifetime decay curves were recorded on a multichannel FluoroHub-B using 150 W pulsed Xe-lamp. 3. Results and discussions 3.1. XRD analysis XRD pattern of NZBFP glass with and without Er3þ, Yb3þ ions singly and combinedly in the range of 10–80� at room temperature as shown in Fig. S1. The absence of a sharp peak in the XRD profile provides infor mation on non-crystalline phase existing in the samples. A Broad hump is seen in the XRD pattern hinting that the undoped and Er3þ, Er3þ/ Yb3þ-doped NZBFP glasses comprised of glassy phase exhibiting amor phous state. 3.2. TGA/DTA analysis Thermal behaviour of the base NaF–ZnF–BiF3–NH4H2PO4 (NZBFP) glass has been displayed in Fig. S2. TGA trace explains weight loss in the host glass while DTA illustrates the heat flow in the sample. TGA profile has shown an initial weight loss of 44% around 645 οCtraceand beyond this temperature, the weight loss is noticed to be constant till 1000 οC. A steep endothermic peak is noticed in the DTA curve at 145 οC due to the evaporation of water which corresponds to the weight loss in the TG curve. The other endotherm peaks at 447 οC and 644 οC corresponds to glass-transition temperature (Tg) and crystallization temperatures (Tc). The thermal stability factor, S ¼ Tg -Tc is determined to be 197 � C, suggesting that fibres can be drawn without crystallization from the host glass composition. The thermal stability factor (S) value greater than 100 indicates the synthesized NZBFP glass exhibits better thermalstability towards a crystallization.
2. Experimental studies The base sodium zinc bismuth fluorophosphate glasses incorporated with Er3þ, Yb3þ & Er3þ/Yb3þ ions separately were prepared by a melt quenching technique in the given compositions: i. 20NaF–15ZnO–10BiF3–55 NH4H2PO4 (referred as NZBFP glass) ii. 20NaF–15ZnO–10BiF3–(55-x) NH4H2PO4–1.0 mol %Er2O3 iii. 20NaF–15ZnO–10BiF3–(55-x) NH4H2PO4–1.0 mol % Yb2O3 2
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
3.3. Raman studies
2
3 hc λ7
6ε
Fig. S3 demonstrates the Raman profile of the NaF–ZnF2–BiF3–NH4H2PO4 (NZBFP) glass measured in 400 cm 1 to 1250 cm 1 range. The Raman spectrum of the present glass system can be divided into two regions; the low frequency region (400-800 cm 1) and high frequency region (800-1250 cm 1). In the low frequency re gion, three bands are noticed at 494, 595 and 663 cm 1, respectively. The band at 494 cm 1 is attributed to bending vibrations of (PO4)3– tetrahedral units with modifier cation. The peak at 595 cm 1 is related to the O–P–O bending vibrations. The peak arising at 663 cm 1 is ascribed to the symmetric vibrations of bridging-oxygens joined to two PO4 tetrahedra (P–O–P) in phosphate chain. The bands in the highfrequency region, 916 cm 1 is due to symmetric stretching of (PO4)3– monomer units. The 1028 cm 1 band is attributed to the symmetric stretching of P–O–P non-bridging oxygens. The peaks at 1152 and 1160 cm 1 are attributed to the symmetric stretching mode of O–P–O nonbridging oxygens [32,33].
σ e ðλÞ ¼ σ a ðλÞexp4
kT
5
(1)
, where σe(λ) represents PL cross-section, σa(λ) represents absorption cross-section, c represents light velocity, h denotes Plank’s constant, k denotes the Boltzmann constant, ε denotes the excitation energy at temperature T. Absorption cross-section σa(λ) has been estimated based on absorption spectrum by employing the relation: � � 2:303 log I0=I (2) σ a ðλÞ ¼ NL , where N represents the rare earth ion concentration, L represents glass sample thickness, log (Io/I) optical density. As seen from Fig. 5(a), the Yb3þ ion in NZBFP glass has strong luminescence and absorption crosssections in comparison to Er3þ-doped glass at/around 0.976 μm, sug gesting the possibility of an efficient pumping near 975 nm to achieve multilevel characteristics. Generally, emission of photons followed by the re-absorption leads to emission spectrum broadening and energy loss. Based on the above observations on both Yb3þ and Er3þ ions (Fig. 2 (a and b)) the tendency of radiation trapping (reabsorption of the emitted radiation) has been significant, resulting in broadening of NIR emission spectra which are normally observed in case of a three-level system [35,37,38]. The absorption and stimulated PL cross-sections are calculated to be 7.74 � 10 21 cm2, 7.86 � 10 21 cm2 for Yb3þ doped glass at 0⋅97 μm and 7.69 � 10 21 cm2, 8.15 � 10 21 cm2 Yb3þ/Er3þ-doped glass for 1⋅54 μm. The broadening of 4I13/2 → 4I15/2 transition covered the C- and L-communication windows beneficial for telecommunication. The optical gain coefficient versus wavelength in the light of popu lation inversion rate G(β) is displayed in Fig. 3(a and b). Optical gain plays a key role in assessing laser media performance. The wavelengthdependent optical gain as a function of G(β) with regard to the laser transitions between upper and lower states of the Er3þ is established based on the PL and absorption cross-sections utilizing the relation [11, 37,38]:
3.4. Optical absorption characteristics The absorption (vis-near IR) spectra of NZBFP: Er3þ & NZBFP: Yb3þ/ Er3þ glasses have been exhibited in Fig. 1. The NZBFP:Er3þ glass exhibited peaks corresponding to 4f-4f electronic transitions raised from 4 I15/2 → 4G9/2 (369 nm), 4G11/2 (378 nm), (4G, 4F, 2H) 9/2 (406 nm), 4F5/ 4 2 4 4 2 (450 nm), F7/2 (488 nm), H11/2 (519 nm), S3/2 (542 nm), F9/2 (651 4 4 4 nm), I9/2 (792 nm), I11/2 (976 nm), I11/2 (1527 nm). NZBFP: Yb3þ/ Er3þ glass demonstrated a strong absorption band at 976 attributed to 2 F5/2 transition of Yb3þ alongside the Er3þ bands suggesting that the Er3þ-doped & Yb3þ/Er3þ co-doped glass matrices can absorb the radi ations effectively upon pumping with 980 nm LD. These absorption bands are influenced by the nature of the host lattice into which they were placed owing to the excitation spectra of 4f electrons. The ab sorption bands of rare earth ions are homogenously broad as well as are sensitive to variations in coordination (site-site disorder) and symmetry [28,34,35]. The absorption and derived stimulated PL cross-sections for NZBFP: 1 Er3þ/1 Yb3þ glass for Yb3þ: 4F7/2 → 4F5/2 & Er3þ: 4I15/2 → 4I13/2 transitions are shown in Fig. 2(a and b). In accordance with McCumber’s theory, stimulated PL cross-section (σemi) for both 4F5/2 → 4F7/2 and 4I13/ 4 2 → I15/2 transition are derived from the absorption cross-section (σ abs) 4 of F7/2 → 4F5/2 and 4I15/2 → 4I13/2 transition of Yb3þ and Er3þ, which is provided by the relation [36]:
GðβÞ ¼ N½pσ e ðβÞ
ð1
pÞσa ðβÞ�
(3)
, where N presents rare earth concentration in glass, β represents the population inversion rate between two laser operating levels which depends on the pump energy-density (P), which takes the values be tween 0 and 1 (P ¼ 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0). From Fig. 3(b), it has been noticed that the gain maxima attributed to 1.54 μm shifts to wards higher wavelength side resembling like NIR emission spectra of Er3þ. This represents the typical characteristic of a quasi-three-level laser system. In optical gain versus wavelength plot, the positive gain is achieved when the population inversion rate is around 40% for NZBFP:Yb3þ/Er3þ glass. This suggests that Yb3þ/Er3þ doped glass sample exhibits flat gain band-width in the region from 1400 to 1650 nm covering C- band (1530–1565 nm) & L-band (1565–1625 nm) in low optical telecommunication window. 3.5. NIR luminescence of NZBFP: 1 Yb3þ and NZBFP: 1 Er3þ glasses The emission spectra of NZBFP: 1 mol% Yb3þ and NZBFP: 1 mol% Er glasses is shown in Fig. 4(a and b). On exciting with 808 nm LD, Yb3þ ions in the ground level are pumped to the 4F5/2 level, from which they decay to ground state by radiating photons in the near NIR region around 975 nm (�1 μm) ascribed to 4F5/2 → 4F7/2 transition. The Er3þdoped glass demonstrated a broad luminescence at 1545 nm attributed to 4I13/2 → 4I15/2 transition. The emission spectrum is partially electricdipole (ED) & partially magnetic-dipole (MD) natured. The ED transition corresponds to a broader component of the emission band and MD 3þ
Fig. 1. The Vis-NIR absorption spectra of NZBFP: Er3þ and NZBFP: Yb3þ/ Er3þ glasses. 3
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
Fig. 2. The absorption and derived stimulated PL cross-sections at 975 nm for (a) NZBFP: 1Er3þ and NZBFP: 1 Yb3þ glasses, (b) NZBFP: 1 Yb3þ/1Er3þglass.
Fig. 3. The optical gain coefficient versus wavelength depending on population inversion rate (γ) for (a) NZBFP: 1 Yb3þ glass at 975 nm and (b) NZBFP: 1 Yb3þ/ 1Er3þglass at 1545 nm.
Fig. 4. PL spectra of (a) NZBFP: 1 mol %Yb3þ and (b) NZBFP: 1 mol% Er3þ glasses.
transition corresponds to a sharp central peak in the emission band [37, 38]. On pumping at 980 nm LD, Er3þ ions are promoted to upper 4I11/2 state and then these unstable ions non-radiatively disintegrate to 4I13/2 level from which they proceed to 4I13/2 ground level radiatively through emitting fluorescence in NIR region at 1545 nm labelled to 4I13/2 → 4 I15/2 transition. In Fig. 4(a and b), inset profiles demonstrate their corresponding PL decay curves. NZBFP:1mol% Yb3þ glass exhibited single exponential decay nature with a lifetime of 506.91 μs while NZBFP:1mol% Er3þ glass exhibited bi-exponential nature with a lifetime of 212.6 μs In order to understand the Stark splitting of 4I13/2 and 4I15/2 levels,
which are emitting and ground levels in the phosphate-based glass, a Gaussian de-convolution fit was done for the 1545 nm emission peak of Er3þ displayed in Fig. 5. The inset figure shows an equivalent schematic energy-level model for a four-level system illustrating 1545 nm emission of Er3þ. The de-convoluted Gaussian peaks obtained as a result of fitting the PL spectra of Er3þ-doped NZBFP glass has been expressed as a, b, c and d. The 4I15/2 ground level gives rise to two sub-levels at 0 and 221.3 cm 1, and excited 4I13/2 level also gives rise to two sub-levels one at 6465.95 cm 1 and the other at 6598.39 cm 1 as observed in the inset of Fig. 5. Thus the energy differences ΔE1 ¼ 221 0 ¼ 221.3 cm 1 and ΔE2 ¼ 6598.39–6465.95 ¼ 132.74 cm 1 represents the Stark splitting energy 4
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
where, d represents sample thickness, To represents the transmission at the baseline, T represents the transmittance at the maximum absorption band of OH group, N(OH) represents the OH ion concentration. The N (OH) concentration in the glass is estimated to be 24 � 2 ppm. This provides information that, OH group has some influence on the Er3þ doped NZBFP glasses. 3.7. Energy transfer and luminescence characteristics of NZBFP: Yb3þ/ Er3þ glasses The probability of ET between Yb3þ and Er3þ ions when co-doped in NZBFP glass by fixing the Er3þ concentration to 1 mol% and varying Yb3þ concentration from 1 to 3 mol % is further investigated. In accordance with Forster-Dexter theory, the main criteria for the transfer of excitation energy within two rare-earth ions (acceptor and donor) relay on the gap within them (smaller the distance within them more the energy transfer takes place) and on the range of overlap between donor PL spectrum and acceptor absorption spectrum. From Dieke’s energy level diagram [43,44], transfer of energy is assumed to occur from Yb3þ to Er3þ, because the emitting level (2I5/2) of Yb3þ lies slightly above the fluorescence level (4I13/2) of Er3þ ion. Further, ET probability from Yb3þ to Er3þ is evaluated from the spectral overlap of the Yb3þ PL spectrum and Er3þ absorption spectrum, which is shown in Fig S5. Therefore, this primary observation supports the ET possibility to takes place from Yb3þ to Er3þ, while Yb3þ behaves as a donor (sensitizer) and Er3þ behaves as acceptor (activator). Furthermore, the ET path between Yb3þ and Er3þ is explained in detail from the PL spectra, energy level scheme and decay lifetime rates of the acceptor (Er3þ). On the basis of Foster-Dexter the ory, the ET parameter CYb→Er is determined from the spectral over DA lapping of sensitizer (Yb3þ) emission and activator (Er3þ) absorption using the empirical relation [45,46]: Z 3c (5) CDA ¼ 4 2 σ Dem ðλÞσ Aabs ðλÞdλ 8π n
Fig. 5. Gaussian de-convolution fit for the 1.5 μm peak of Er3þ emis sion spectrum.
range for 4I15/2 and the 4I13/2 multiplets. The 4I15/2 level shows wider Stark splitting in comparison to 4I13/2 level for the present phosphatebased glass system indicating that the bandwidth firmly depends on the Stark splitting of the multiplets. The broad peaks at 6598.39 and 6198.30 cm 1 are assigned to electric-dipole and the sharp bands at 6465.95 and 6377.85 cm 1 are assigned to the magnetic-dipole component [39,40]. 3.6. Influence of OH group The FTIR absorption spectrum of host, 1.0 mol % Er3þ doped NZBFP glass in the 4000 cm 1 to 400 cm 1 range and also energy transfer be tween energy levels of Er3þ and OH vibrational modes is shown in Fig. S4(a and b). The absorption band noticed in the range of 4000–2200 cm 1 (Fig. S4a) is attributed to the OH group i.e., to the presence of water, because absorption band is characteristic of antisymmetric stretching vibrations of free OH groups or free H2O molecules. The ab sorption band centred at 2350 cm 1 is assigned to the P–O–H linkages, suggesting that OH is also bonded to P. The presences of OH group in the rare earth’s doped glasses have a deleterious effect on the optical properties, for instance, quenching of luminescence intensity through non-radiative energy transfer from excited Er3þ ions to OH centres. In Fig. S4(b) the mechanism of luminescence quenching for 1.54 μm emission as a result of non-radiative energy transfer followed by hy droxyl (OH ) quenching is shown. Two OH group vibrational quanta are neded to bridge the energy gap between the 4I13/2 and 4I15/2 states. The quenching effect due to OH is illustrated as; initially, Er3þ ions in the excited states exchange energy by the means of non-radiative transfer with the ions in the ground state. During this process, if any Er3þ ions were coupled to OH group then the excited energy would be lost resulting in luminescence quenching. From the FTIR spectrum it is observed that OH vibrations occur in 2500–4000 cm 1 range centred at 3202 cm 1 of Er3þ doped glasses. The first overtone (ν ¼ 1) of the stretching vibrations of OH group matches with the energy gap of the Er3þ 4I13/2 → 4I15/2 transition (NIR emission in Er3þ ion occurs at 1.54 μm and the first overtone of H–O–H band falls around 1.42 μm and therefore OH group plays an important role in the luminescence quenching of Er3þ. The OH ion concentration NOH (ions/cm3) in the Er3þ doped glass at 3202 cm 1 is estimated from the expression given below [41,42]: � � 1000 To NðOHÞ ¼ log (4) d T
R where σDem ðλÞσAabs ðλÞdλ, relates the overlap of integral spectral crosssections of sensitizer emission and activator absorption, c denotes the light velocity, and n refer to refractive index. On considering the spectral overlap of Yb3þ luminescence and Er3þ absorption cross-sections (as shown in Fig S5), the ET parameter CYb→Er is determined to be 1.26 � DA 10 39 cm6 S 1 for the glass system under investigation. In addition, the ET process within Yb3þ and Er3þ ions are discussed based on NIR pho toluminescence, partial energy-level scheme, PL decay rates and ET parameters of (yYb3þ/Er3þ) combinedly incorporated NZBFP glass matrices. Fig. 6(a) shows the NIR luminescence spectra for 1Er3þ-doped and yYb3þ/1Er3þ (y ¼ 1.0, 2.0, 3.0 mol%) co-doped NZBFP glasses on pumping with 980 nm LD. As seen from the PL spectrum, the emission intensity of yYb3þ/1Er3þ co-doped glass is far greater than the 1Er3þdoped glass. On raising the Yb3þ-doping level, the PL intensity of Er3þ has increased along with the broadening in the spectrum. In the studied range of Yb3þ concentration, no luminescence quenching is noticed, since Yb3þ possess broad absorption band and also larger absorption cross-section compared with Er3þ at 975 nm which resulted in trans ferring of its energy to Er3þ resonantly. Moreover, Yb3þ possess a twolevel system which is less affected by ion-ion interactions or nonradiative losses like OH group, multi-phonon, and cross-relaxations [47]; which could be the reason for the absence of luminescence quenching within the studied range of Yb3þ concentration. At low Yb2O3 doping level, the average gap within the Yb3þ (donor) and Er3þ (acceptor) is greater so, the interaction between them is weak. As the dopant level of Yb2O3 is increased, the average gap within Yb3þ and Er3þ ions decreased, as a result, the interaction between them increased leading to efficient ET within Yb3þ and Er3þ, due to firm overlap of PL and absorption cross-sections. The probability of radiation trapping 5
Optical Materials 100 (2020) 109616
C.P. Reddy et al.
Fig. 6. (a) NIR emission spectra for Er3þ & Yb3þ/Er3þ combinedly doped NZBFP glasses, (b) partial energy-level scheme of Yb3þ & Er3þ ions.
exhibited mono-exponential nature because of resonant ET within Yb3þ and Er3þ ions. Emission decay rates of the Yb3þ/Er3þ combinedly incorporated glass matrices enhanced on rising Yb3þ content. The esti mated lifetimes are 212.6, 267.3, 348.5 and 457.1 μs, respectively. This provides evidence that Yb3þ/Er3þ co-doped NZBFP glasses exhibited longer lifetimes in comparison with the Er3þ-doped glass suggesting a resonant ET from Yb3þ → Er3þ. The optical amplification and gain parameters such as peak wave length (λp), lifetimes (τErþYb), effective band-width (Δλeff), stimulated PL cross-section (σe (λ)), optical gain (G), and gain bandwidth (ΔG) are provided in Table 1. Stimulated PL cross-section (σ e (λ)) and FWHM or effective bandwidth (Δλeff) parameters play a significant part in optical amplifiers in view of achieving high and wideband gain amplification. Moreover, glass samples with larger effective bandwidth (Δλeff) could be potential candidates for WDM (wavelength-division multiplexing) ap plications. The stimulated PL cross-section σ e (λ) for the 4I13/2 → 4I15/2 transition is evaluated using the equation given below [50]:
tends to become dominant for higher donor concentration resulting in the constraining of donor (Yb3þ) excitation energy for a long time in the system enhancing the probability of ET to (Er3þ) acceptors. The ET mechanism is illustrated as on exciting with 980 nm Laser Diode, Yb3þ ions in the ground level are pumped to Yb3þ:2F5/2 and Er3þ ions to Er3þ:4I11/2, when the ions in both the levels come to resonance, some of the excited Yb3þ:2F5/2 ions transfer excitation energy to Er3þ:4I11/2 level. Then, Er3þ ions in excited level relax non-radiatively through populating 4 I13/2 level and finally cascade down to 4I15/2 level radiatively with the emission of photons at 1.54 μm. The ET mechanism from Yb3þ to Er3þ: 2 F5/2(Yb3þ)þ4I15/2(Er3þ)→2F7/2(Yb3þ)þ4I11/2(Er3þ) [48,49]. Fig. 6(b) the energy level scheme demonstrating the ET process within Yb3þ and Er3þ ions. The PL peak at 1545 nm attributed to the 4I15/2 transition is partially electric-dipole, which contributes to emission broadening and partially magnetic dipole which contributes to sharp central peak in emission satisfying the selection rules ΔS ¼ ΔL ¼ 0 and ΔJ ¼ þ1 [46]. In Fig. 6(a), spectral broadening of Er3þ luminescence bands is evidenced on increasing Yb3þ dopant level, because of radiation trapping, which is proportional to dopant content. The PL peak centred around 1.54 μm is extended along the entire low telecommunication window (S-, C-, and L-bands) in the measured region of 1425–1700 nm. The ET process within emission centres of Yb3þ and Er3þ in NZBFP glass matrix is further evaluated from their emission decay profiles. The decay lifetime curves have been recorded for yYb3þ/1Er3þ (y ¼ 1.0, 2.0, and 3.0 mol%) upon pumping with 980 nm LD for Er3þ: 4I13/2 → 4I15/2 transition (1545 nm) at room temperature as shown in Fig. 7. As seen from decay profiles, the lifetime decay curve recorded for the Er3þ singly doped glass exhibited dual exponential nature while Yb3þ-doped glasses
σ e ðλÞ ¼
8π
λ4p 2 cn Δλ
(6)
τ
eff m
, where n represents material’s refractive index, C represents the light velocity, λp refers to PL peak wavelength, τm represents the emission lifetime and Δλeff represents the effective band-width of the PL band. The effective bandwidth is estimated from the ratio of emission peak area to its average height using relation: � Z � Δλeff ¼ IðλÞdλ I λp (7)
Table 1 Peak wavelength (λP), Effective bandwidth (Δ λeff), PL lifetime (τm); Stimulated PL cross-section (σe), Gain bandwidth (σe x Δ λeff), and optical gain (σe x τm) of various concentrations of Er3þ-doped and Er3þ/Yb3þ combinedly doped NZBFP glass matrices assigned to 4I13/2 → 4I15/2 emission transition.
Fig. 7. Emission decay curves for Er3þ: 4I13/2 → 4I15/2 transition (1545 nm) at room temperature. 6
Glass samples
λP (nm) (�1.0)
Δ λeff (nm) (�1.0)
τm
σEP
σEP x Δ λeff
σEP x τm
NZBFP: 1 Er3þ NZBFP: 1 Er3þ/1 Yb3þ NZBFP: 1 Er3þ/2 Yb3þ NZBFP: 1 Er3þ/3 Yb3þ
1545
53.8
2.12
5.71
307.19
12.10
1545
62.16
2.67
7.44
462.47
19.86
1545
59.74
3.48
6.38
381.14
22.20
1545
60.59
4.57
5.12
316.28
23.39
(ms) (�0.2)
(10 21 cm2) (�0.1)
(10 21 cm2 x nm) (�0.2)
(10 21 cm2 x ms) (�0.2)
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
, where I(λp) represents the intensity of the peak wavelength, I(λ) rep resents the PL intensity at the wavelength (λ). The effective band-width (Δλeff) has been chosen over FWHM, owing to the asymmetric PL band at 1.54 μm of Er3þ. In the current NZBFP glass system, on co-doping Yb3þ at different concentrations to 1 mol% Er3þ, effective line width (Δλeff) found to increase which could be because of the spectral overlapping of absorption (Er3þ) and PL bands (Yb3þ) ions and could also be in the wake of energy migration from Yb3þ→Er3þ. Furthermore, gain band width which is the product of stimulated PL cross-section (σe) and effective band-width (Δλeff) are important parameters to characterize laser materials operating in NIR region at/around 1.54 μm emission. Inokuti-Hirayama (I–H) model has been used to determine the coupling factor (CYbþEr) and critical distance (Ro) from the decay life time curve 1 Yb3þ/1Er3þ: NZBFP shown in Fig. 8. The 1 Yb3þ/1Er3þ decay lifetime curve was fitted to the Inokuti-Hirayama relation [51,52]: 2 3 � � � �3S 7 6 IðtÞ ¼ Io exp4 t=t Q t=t (8) 5 =
o
� � 4π Γ 1 3
Fig. 8. Inokuti-Hirayama fit model for NZBFP: 1 Yb3þ/1Er3þ glass.
�� 3S
=
Q¼
o
Na R3o
� CDA ¼ R6o
τYb
reviewing and editing.
(9)
Declaration of competing interest
(10)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
, where I(t) represents PL intensity at time t, to denotes donor lifetimes in the absence of Er3þ ions. Γ(1–3/S) is the Euler’s gamma function, which relates to the values depending on the interaction type. Γ(1–3/S) is equal to 1.77 (dipole-dipole interaction (S ¼ 6)), 1.43 (dipole-quadru pole interaction (S ¼ 8)) and 1.3 (quadrupole-quadrupole interaction (S ¼ 10)), Na presents concentration of acceptor, Q denotes energy transfer parameter, CDA refers to donor-acceptor coupling constant, Ro refers to distance within the Er3þ and Yb3þ ions. From decay lifetime curve of 1 Yb3þ/1Er3þ doped glass, I–H fitting suits better for S ¼ 6 with the experimental values Q ¼ 2.46, Ro ¼ 9.54 Å and CDA is 1.48 � 10 39 cm6 S 1 realizing the nature of interaction between Er3þ and Yb3þ is electric dipole-dipole type. Therefore, the non-radiative ET process between Yb3þ and Er3þ ions is governed by electric dipole-dipole interaction.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109616. References [1] P.J. Winzer, D.T. Neilson, A.R. Chraplyvy, Fiber-optic transmission and networking: the previous 20 and the next 20 years, Opt. Express 26 (2018) 24190–24239. [2] S. Tanabe, Rare-earth-doped glasses for fiber amplifiers in broadband telecommunication, Compt. Rendus Chem. 5 (2002) 815–824. [3] S. Addanki, I.S. Amiri, P. Yupapin, Review of optical fibers-introduction and applications in fiber lasers, Results Phys. 10 (2018) 743–750. [4] P.D. Dragic, M. Cavillon, J. Ballato, Materials for optical fiber lasers: a Review, Appl. Phys. Rev. 5 (2018), 041301. [5] M. Yamane, Y. Asahara, Glasses for Photonics, Cambridge Univ. Press, Cambridge, 2000. [6] G.C. Righini, Passive and active glasses for integrated optics, in: P. Mazzoldi (Ed.), From Galileo’s Occhialino to Optoelectronics, World Scientific, Singapore, 1993, pp. 272–294. [7] J.C.G. Bunzli, Lanthanide luminescence for biomedical analyses and imaging, Chem. Rev. 110 (2010) 2729. [8] J. Cao, X. Li, Z. Wang, Y. Wei, L. Chen, H. Guo, Optical thermometry based on upconversion luminescence behaviour of self-crystallized K3YF6: Er3þ glass ceramics, Sens. Actuators B Chem. 224 (2016) 507–513. [9] D.Y. Wang, P.C. Ma, J.C. Zhang, Y.H. Wang, Efficient down- and up-conversion luminescence in Er3þ Yb3þ co-doped Y7O6F9 for photovoltaics, ACS Appl. Energy Mater. 1 (2018) 447–454. [10] V.A.G. Rivera, F.A. Ferri, L.A.O. Nunes, E. Marega Jr., White light generation via up-conversion and blue tone in Er3þ/Tm3þ/Yb3þ-doped zinc-tellurite glasses, Opt. Mater. 67 (2017) 25–31. [11] L.M. Marcondes, C.R. da Cunha, B.P. de Sousa, S. Maestri, G.Y. Poirier, Thermal and spectroscopic properties studies of Er3þ-doped and Er3þ/Yb3þ-codoped niobium germanate glasses for optical applications, J. Lumin. 205 (2019) 487–494. [12] S. Tabanli, G. Eryurek, Optical investigation of Er3þ and Er3þ/Yb3þ doped zinctellurite glass for solid-state lighting and optical thermometry, Sens. Actuators A Phys. 285 (2019) 448–455. [13] Ch Basavapoornima, T. Maheswari, Shobha Rani Depuru, C.K. Jayasankar, Sensitizing effect of Yb3þ ions on photoluminescence properties of Er3þ ions in lead phosphate glasses: optical fiber amplifiers, Opt. Mater. 86 (2018) 256–269. [14] Y. Zhang, M. Li, J. Li, J. Tang, Z. Wu, Optical properties of Er3þ/Yb3þ codoped phosphate glass system for NIR lasers and fiber amplifiers, Ceram. Int. 44 (2018) 22467–22472.
4. Conclusion It is summarised that the NZBFP glasses with Er2O3, Yb2O3 in single and dual combination were successfully synthesized via melt quench technique and their optical and luminescence characteristics were measured and systematically analysed. The TGA-DTA traces suggested the host glass is thermal stable without devitrification. The Raman profile revealed the functional groups and phonon energy of the host glass. The absorption and stimulated PL cross-sections are calculated to be 7.74 � 10 21 cm2, 7.86 � 10 21 cm2 for 0⋅97 μm and 7.69 � 10 21 cm2, 8.15 � 10 21 cm2 for 1⋅54 μm in 1 Yb3þ/1Er3þ co-doped glass. Pumping at 980 nm Er3þ and yYb3þ/Er3þ doped glasses have exhibited NIR emission bands around 1545 nm. The increment in the NIR PL in tensity of Er3þ with the addition of Yb3þ because of resonant ET from Yb3þ →Er3þ has been discussed depending on the energy level diagram and increment in Er3þ lifetimes. I–H fitting confirms that the ET mech anism between Yb3þ and Er3þ governed by electric dipole-dipole interaction. The computed absorption, PL cross-sections and the gain at 1.54 μm of Yb3þ/Er3þ could suggest these materials useful in the progress of laser systems and fibre optical communications. Author contributions C. Parthasaradhi Reddy: experiment, data collection, manuscript drafting, data plotting; R. Ramaraghavulu: data collection and manu script drafting; T. Kalpana: software, reviewing; J. Gajendiran: 7
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
[15] S. Nian, Y. Zhang, W. Cao, Z. Wu, Y. Shu, Optical properties of Er3þ/Yb3þ co-doped bismuth calcium borate glass system for NIR lasers and fiber amplifiers, J. Lumin. 194 (2018) 440–445. [16] R.J. Amjad, A. Sattar, M.R. Dousti, Upconversion and 1.53 μm near-infrared luminescence study of the Er3þ Yb3þ co-doped novel phosphate glasses, Optik 200 (2020), 163426. [17] M.Dh Hassib, K.M. Kaky, A. Kumar, E. S¸akar, M.A. Mahdi, Boro-silicate glasses codoped Er3þ/Yb3þ for optical amplifier and gamma radiation shielding applications, Phys. B Condens. Matter 567 (2019) 37–44. [18] A. Maaoui, F.B. Slimen, M. Haouari, A. Bulou, H.B. Ouad, Upconversion and near infrared emission properties of a novel Er3þ/Yb3þ codoped fluoro-tellurite glass, J. Alloy. Comp. 682 (2016) 115–123. [19] A. Miguel, M.A. Arriandiaga, R. Morea, J. Fernandez, R. Balda, Down- and upconversion emissions in Er3þ–Yb3þ codoped TeO2–ZnO–ZnF2 glasses, J. Lumin. 158 (2015) 142–148. [20] A. Langar, C. Bouzidi, H. Elhouichet, M. Ferid, Er–Yb co-doped phosphate glasses with improved gain characteristics for an efficient 1.55 μm broadband optical amplifiers, J. Lumin. 148 (2014) 249–255. [21] J. Zhou, Q. Liu, W. Feng, Y. Sun, F. Li, Up-conversion luminescent materials: advances and applications, Chem. Rev. 115 (2015) 395–465. [22] V. Naresh, B.S. Ham, Influence of multiphonon and cross relaxations on 3P0 and 1 D2 emission levels of Pr3þ doped borosilicate glasses for broad band signal amplification, J. Alloy. Comp. 664 (2016) 321–330. [23] M. Deepa, R. Doddoji, C.S.D. Viswanath, A.V. Chandrasekhar, Optical and NIR luminescence spectral studies: Nd3þ-doped borosilicate glasses, J. Lumin. 213 (2019) 191–196. [24] S. Damodaraiah, R.P. Vijaya Lakshmi, Y.C. Ratnakaram, Role of bismuth content on structural and luminescence properties of Ho3þ doped phosphate glasses, J. Mol. Struct. 1200 (2020), 127157. [25] A. Amarnath Reddy, S. Surendra Babu, G. Vijaya Prakash, Er3þ-doped phosphate glasses with improved gain characteristics for broadband optical amplifiers, Opt. Commun. 285 (2012) 5364–5367. [26] T. Xu, X. Zhang, G. Li, S. Dai, Q. Nie, X. Shen, X. Zhang, Spectroscopic properties and thermal stability of Er3þ-doped TeO2–B2O3–Nb2O5–ZnO glass for potential WDM amplifier, Spectrochim. Acta 67 (2007) 559–563. [27] S.K. Taherunnisa, D.V. Krishna Reddy, T. Sambasiva Rao, K.S. Rudramamba, Y. A. Zhydachevskyy, A. Suchocki, M. Piasecki, M. Rami Reddy, Effect of upconversion luminescence in Er3þ doped phosphate glasses for developing ErbiumDoped Fibre Amplifiers (EDFA) and G-LED’s,, Opt. Mater. 3 (2019) 100034. [28] Ch Basavapoornima, T. Maheswari, S.R. Depuru, C.K. Jayasankar, Sensitizing effect of Yb3þ ions on photoluminescence properties of Er3þ ions in lead phosphate glasses: optical fiber amplifiers, Opt. Mater. 86 (2018) 256–269. [29] S. Meruva, B.L. Carlos, F. Julio Jr., A. Peres, Optical characterization, luminescence properties of Er3þ and Er3þ/Yb3þ co-doped tellurite glasses for broadband amplification, Proc. SPIE 8961 Fiber Lasers XI Technol. Syst. Appl. (2014) 896132, https://doi.org/10.1117/12.2037000. [30] H. Sun, L. Hu, C. Yu, G. Zhou, Z. Duan, J. Zhang, Z. Jiang, Investigation of the effect of fluoride ions introduction on structural, OH- content and up-conversion luminescence properties in Er3þ-doped heavy metal oxide glasses, Chem. Phys. Lett. 408 (2005) 179–185. [31] G.L. Agawane, K. Linganna, J.H. In, J.H. Choi, High emission cross-section Er3þdoped fluorophosphate glasses for active device application, Optik 198 (2019), 163228.
[32] C. Ivascu, A.T. Gabor, O. Cozar, L. Daraban, I. Ardelean, FTIR-Raman and thermoluminescence investigation of P2O5-BaO-Li2O glass system, J. Mol. Struct. 993 (2011) 249–253. [33] I. Jlassi, N. Sdiri, H. Elhouichet, M. Ferid, Raman and impedance spectroscopy methods of P2O5-Li2O-Al2O3 glass system doped with MgO, J. Alloy. Comp. 645 (2015) 125–130. [34] F. Wang, F. Song, S. An, W. Wan, H. Guo, S. Liu, J. Tian, Er3þ/Yb3þ-codoped phosphate glass for short-length high-gain fibre lasers and amplifiers, Appl. Opt. 54 (2015) 1198–1205. [35] A.D. Sontakke, K. Annapurna, Energy transfer based efficient infrared emission at 1.57 lm from Yb3þ–Er3þ codoped Zinc-borobismuthate glasses, Opt. Mater. 35 (2013) 472–478. [36] D.E. McCumber, Theory of phonon-terminated optical masers, Phys. Rev. A 134 (1964) A299. [37] V. Naresh, S. Buddhudu, Analysis of visible-NIR emission and photoluminescence quenching in Er3þ:Bi2O3–AlF3–TeO2–B2O3 glasses, Phys. Chem. Glasses Eur. J. Glass Sci. Technol. B 56 (2015) 255–262. [38] J.H. Choi, F.G. Shi, A. Margaryan, A. Margaryan, W. van der Veer, Novel alkalinefree Er3þ-doped fluorophosphate glasses for broadband optical fiber lasers and amplifiers, J. Alloy. Comp. 450 (2008) 540–545. [39] F.R. Lopez, P. Babu, L. Jyothi, U.R. Rodriguez-Mendoza, I.R. Martin, C. K. Jayasankar, Er3þ-Yb3þ codoped phosphate glasses used for an efficient 1.5 μm broadband gain medium, Opt. Mater. 34 (2012) 1235–1240. [40] K. Damak, E. Yousef, S. AlFaify, C. Russel, R. Maalej, Raman, green and infrared emission cross sections of Er3þ doped TZPPN tellurite glass, Opt. Mater. Express 4 (2014) 597–612. [41] R. Zheng, Z. Wang, P. Lv, Y. Yuan, Y. Zhang, J. Zheng, W. Wei, Novel synthesis of low hydroxyl content Yb3þ-doped fluorophosphate glasses with long fluorescence lifetimes, J. Am. Ceram. Soc. (2014) 1–6. [42] F.F. Sene, J.R. Martinelli, L. Gomes, Optical and structural characterization of rare earth doped niobium phosphate glasses, J. Non-Cryst. Solids 348 (2004) 63–71. [43] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals, WileyInterscience Publishers, New York, 1968. [44] B.G. Wybourne, Spectroscopic Properties of Rare Earths, Wiley-Interscience, New York, 1965. [45] T. Forster, Intermolecular energy migration and fluorescence, Ann. Phys. 2 (1948) 55. [46] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 836. [47] P.D. Dragic, M. Cavillon, J. Ballato, Materials for optical fiber lasers: a review, Appl. Phys. Rev. 5 (2018), 041301. [48] L. Zhang, H. Hu, C. Qi, F. Lin, Spectroscopic properties and energy transfer in Yb3 þ /Er3þ-doped phosphate glasses, Opt. Mater. 17 (2001) 371–377. [49] B.P. Kore, A. Kumar, L. Erasmus, R.E. Kroon, J.J. Terblans, S.J. Dhoble, H.C. Swart, Energy transfer mechanisms and optical thermometry of BaMgF4: Yb3þ, Er3þ phosphor, Inorg. Chem. 57 (2018) 288–299. [50] K. Linganna, K. Suresh, S. Ju, W.T. Han, C.K. Jayasankar, V. Venkatramu, Optical properties of Er3þ-doped K-Ca-Al fluorophosphate glasses for optical amplification at 1.53 μm, Opt. Mater. Express 5 (2015) 1689. [51] B. Xu, B. Yang, Y. Zhang, H. Xia, J. Wang, Cooperative energy transfer in Tm3þ and Yb3þ co-doped phosphate glasses, J. Rare Earth 31 (2013) 164–168. [52] T. Wei, Y. Tian, C. Tian, X.F. Jing, J. Zhang, L. Zhang, S. Xu, Optical spectroscopy and population behavior between 4 I11/2 and 4 I13/2 levels of erbium doped germanate glass, Opt. Mater. Express 4 (2014) 2150.
8
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Energy transfer induced enhancement in NIR luminescence characteristics of Yb3þ/Er3þ co-doped sodium zinc bismuth fluorophosphate glasses C. Parthasaradhi Reddy a, *, R. Ramaraghavulu b, T. Kalpana c, J. Gajendiran a a
Department of Physics, Vel Tech Rangarajan Dr. Sagunthala R & D Institute of Science and Technology, Chennai, India Department of Humanities & Sciences, Annamacharya Institute of Technology and Sciences, Kadapa, Andhra Pradesh, India c Department of Physics, Andhra Loyola College, Vijayawada, 520 008, Andhra Pradesh, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Melt quenching method Stimulated emission cross-section Yb3þ/Er3þenergy transfer Yb3þ absorption and Er3þ emission
The results pertaining to the energy transfer-based photoluminescence characteristics of Er3þ-doped and Yb3þ/ Er3þ co-doped NaF–ZnO–BiF3–NH4H2PO4 (NZBFP) glasses prepared through melt-quench technique were demonstrated here. By applying McCumber’s theory, the absorption, stimulated emission cross-sections and optical gain have been estimated for Yb3þ (0.97 μm) and Er3þ (1.54 μm) ions. Yb3þ-doped glass exhibited a sharp emission at 979 nm at 808 nm excitation while Er3þ-doped glass displayed a broad NIR emission band at/around 1545 nm for 980 nm excitation. The presence of Yb3þ absorption band along with the Er3þ absorption bands in the co-doped glass suggests the energy transfer (ET) possibility within these ions. On pumping Er3þ-doped & yYb3þ/Er3þ co-doped NZBFP glasses with 980 nm Laser Diode, a broad Er3þ emission at 1545 nm attributed to 4 I13/2 → 4I15/2 is observed in the NIR region. The increasing of Yb3þconcentration with respect to Er3þ (fixed to 1 mol%) in co-doped glasses have resulted in enhancement of Er3þ NIR emission because of the resonant energy transfer (ET) process from Yb3þ→Er3þ. The energy transfer (ET) mechanism has been illustrated from the spectral overlap of Yb3þ emission and Er3þ absorption, Er3þ decay lifetime curves, and partial energy level di agram. Further, the nature of interaction responsible for the energy transfer process (ET) has been explained using Forster-Dexter theory and I–H fitting. In addition, the suitability of the present synthesized fluo rophosphate glasses for optical amplifier and NIR laser applications were explored in terms of optical amplifi cation and gain parameters, like effective band-width (Δλeff), stimulated emission cross-section (σe), optical gain (G), and gain bandwidth (ΔG).
1. Introduction The rapid development of telecommunication system by optical transmission and fibre optical communication have extended their op tical range of operation to the integrated circuits. The device like dielectric waveguides (single/multi-mode fibre optic systems) have been interfaced with the integrated optical circuits for better operation with widespread applications in multiplexing technologies like WDM (1310–1550 nm), Coarse WDM (1270–1610 nm), Dense WDM (C-band: 1530–1565 nm) and EDFA for high-speed digital communication [1–3]. Glasses are considered to be suitable materials for the fabrication of optical fibres because they offer a wide composition range and glass-forming ability with exceptional optical transparency, UV to NIR transmission besides low attenuation. Though, these features promoted glasses for optical communication, they are passive and lack in
electronic transitions. So, to overcome this, glasses are incorporated with optically active ions like lanthanides and transition metal ions. The unique characteristics which distinguish rare earths from the transition metals are their rich electronic structure, where outer 5s and 5p sub shells shield the optical transitions of 4f shell by enabling them as optically active exhibiting narrow emissions with longer lifetimes and PLQYs [4–6]. These diverse spectroscopic and electronic properties of di- and trivalent rare earth ions make them ideal for probing the local structure. In recent past, solid-state materials e.g., glasses, glass ce ramics, ceramic powders, polymers containing rare-earth ions have been investigated for a number of potential applications, e.g., solar cells, bio-imaging, displays devices, lasers, fibre optics, LEDs, spectral con verters (up and down converters) and optical thermometry [7–10]. Lia Mara Marcondes et al. explained the emission characteristics of Er3þ doped and Er3þ/Yb3þ-codoped GeO2-Nb2O5–K2O glasses in the
* Corresponding author. E-mail address: [email protected] (C.P. Reddy). https://doi.org/10.1016/j.optmat.2019.109616 Received 13 October 2019; Received in revised form 8 December 2019; Accepted 14 December 2019 Available online 18 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
region of 1.5 μm covering C, S and L-telecommunication bands [11]. Er3þ and Er3þ/Yb3þ doped zinc tellurite (ZnO/TeO2) glasses optical characteristics for sensor applications were investigated by Sevcan Tabanli et al. [12]. Sensitizing effect of Yb3þ ions on photoluminescence properties of Er3þ ions in lead phosphate glasses for optical fiber am plifiers are studied by Ch. Basavapoornima et al. [13]. Yin Zhang explore the optical properties of Er3þ/Yb3þ co-doped phosphate glass for its application as electronic amplifiers [14]. Shangjiu Nian et al. studied optical properties of Er3þ/Yb3þ co-doped bismuth calcium borate glass system for NIR lasers and fiber amplifiers [15]. Upconversion and 1.53 μm near-infrared luminescence study of the Er3þ-Yb3þ co-doped novel phosphate glasses are carried out by R. J. Amjad et al. [16]. for their applications in optical fibers. Mustafa Dh Hassib et al. [17]. investigated the Er3þ/Yb3þ co-doped boro-silicate glasses for optical amplifier and gamma radiation shielding applications. A. Maaoui et al. [18]. suggested Er3þ/Yb3þ codoped fluoro-tellurite glass can be useful for development of optical devices. The absorption, emission spectra and lifetime mea surements for the infrared and visible fluorescence for Er–Yb codoped TeO2–ZnO–ZnF2 glasses are explained by A. Miguel et al. [19]. Aroua Langar et al. [20]. studied the optical properties of phosphate glasses codoped with Er3þ–Yb3þ for developing the solid-state 1.55 μm optical amplifiers. Among the rare earth ions operating in NIR region for diverse and potential applications [10,21–24], Er3þ (singly or in combination with Yb3þ) doped glasses have gained considerable attention for optical communication because of 1.54 μm emission (4I13/2 → 4I15/2) corre sponding to the C- band window (1530–1565 nm) for telecommunica tion [25]. The broadness of Er3þ emission band enables it as a suitable candidate for wavelength-division multiplexing (WDM) and erbium-doped fiber amplifier (EDFA) systems, operating in the S-and (1460–1530) and L-band (1565–1625 nm) along with C- band [26,27]. Investigating Yb3þ/Er3þ-combinedly incorporated glasses are very fascinating owing to radiative ET from Yb3þ→Er3þ. The absorption cross-section of Yb3þ corresponding to 2F7/2 → 2F5/2 transition is wider compared to the 4I15/2 → 4I11/2 transition of Er3þ at 980 nm. Therefore, excitation energy would be readily absorbed by Yb3þ ions and subse quently transfer it to Er3þ ions efficiently resulting in a higher optical gain. In addition, high pump absorption favours side-pumping, thus allowing the use of large-area diode lasers as a pumping source [28,29]. Silicate based glasses are well studied optical matrices since many years for its commercial applications as fibre optical amplifiers, but its application has been limited in case of broadband applications due to very narrow bandwidth emission at 1530 nm. Therefore, new glass compositions are explored to achieve efficient fibre amplification and flat gain spectra. Fluorophosphate based glasses, which belong to oxy-fluoride glass family have received much attention, because, their chemical composition is comprised of oxides (phosphate and zinc oxide) and fluorides (sodium fluoride and bismuth fluoride). The presence of oxides promotes material strength, stability and longevity whereas fluorides with lower phonon energy shorten the non-radiative losses resulting in improved photoluminescence decay rates and PLQYs of rare earth luminescence. Fluorides also enhance IR transmission ability by lowering OH absorption by forming HF [30,31]. In the present study, stable NZBFP glasses are doped with Er2O3, Yb2O3 in single and dual combination to investigate their NIR emission characteristics, decay curves, energy transfer mechanism, optical gain and amplification factors.
iv. 20NaF–15ZnO–10BiF3–(55-x) NH4H2PO4–1.0 mol % Er2O3– xYb2O3 (x ¼ 1.0 mol % & x ¼ 1.0, 2.0 and 3.0 mol%) The titled glasses were prepared by NH4H2PO4, BiF3, ZnO, NaF, Er2O3 and Yb2O3 chemicals procured from Sigma-Aldrich. All the pre cursors were weighed according to the above composition indepen dently in 7 g set and mixed thoroughly using agate mortar and pestle until a fine powder was obtained. To obtain homogeneous mixture acetone was added to the chemical mix while grinding. Subsequently, each chemical set was taken into Alumina crucible separately and was heated in a muffle furnace for 1 h at 950 � C. Later those melts were poured into the circular moulds and quenched with a smooth-surfaced brass plate to obtain circular glass discs of 3–4 cm in diameter with 0.3 cm (0.03 mm) in thickness. Finally, these glasses were smoothly polished and further used them to carry on measurements. 2.1. Measurements The refractive indices of the Yb3þ and Er3þ doped NZBFP glasses were measured from Abbes refractometer at wavelength 589.3 nm of sodium vapour lamp using 1-bromonaphthalin as a contact liquid are found to in the range of 1.607–1.668. XRD patterns were measured on a Seifert X-ray Diffractometer (model 3003 TT) with Cu Kα radiation (λ ¼ 1.5406 Å) at 40 kV and 20 mA with a Si detector between 10� and 80� at the rate of 2� /min. TGA-DTA measurements were simultaneously done for NZBFP glass on a NetZsch STA 409 at a heating rate of 10 � C/min under N2 as the purging gas. Raman spectrum of the NZBFP glass is measured on a Jobin Yvon Horiba LabRam series (HR-800) microRaman spectrometer at room temperature. The absorption spectra of NZBFP glasses containing Er3þ and Yb3þ/Er3þ ions were recorded at room temperature in the spectral range of 250 nm–2500 nm on a VarianCary-Win Spectrometer (JASCO V-570). NIR emission spectra were ob tained by exciting the Er3þ and Yb3þ/Er3þ ions doped glass samples at 980 nm and 808 nm using a Crystal laser-DL series at 100 mW. The lifetime decay curves were recorded on a multichannel FluoroHub-B using 150 W pulsed Xe-lamp. 3. Results and discussions 3.1. XRD analysis XRD pattern of NZBFP glass with and without Er3þ, Yb3þ ions singly and combinedly in the range of 10–80� at room temperature as shown in Fig. S1. The absence of a sharp peak in the XRD profile provides infor mation on non-crystalline phase existing in the samples. A Broad hump is seen in the XRD pattern hinting that the undoped and Er3þ, Er3þ/ Yb3þ-doped NZBFP glasses comprised of glassy phase exhibiting amor phous state. 3.2. TGA/DTA analysis Thermal behaviour of the base NaF–ZnF–BiF3–NH4H2PO4 (NZBFP) glass has been displayed in Fig. S2. TGA trace explains weight loss in the host glass while DTA illustrates the heat flow in the sample. TGA profile has shown an initial weight loss of 44% around 645 οCtraceand beyond this temperature, the weight loss is noticed to be constant till 1000 οC. A steep endothermic peak is noticed in the DTA curve at 145 οC due to the evaporation of water which corresponds to the weight loss in the TG curve. The other endotherm peaks at 447 οC and 644 οC corresponds to glass-transition temperature (Tg) and crystallization temperatures (Tc). The thermal stability factor, S ¼ Tg -Tc is determined to be 197 � C, suggesting that fibres can be drawn without crystallization from the host glass composition. The thermal stability factor (S) value greater than 100 indicates the synthesized NZBFP glass exhibits better thermalstability towards a crystallization.
2. Experimental studies The base sodium zinc bismuth fluorophosphate glasses incorporated with Er3þ, Yb3þ & Er3þ/Yb3þ ions separately were prepared by a melt quenching technique in the given compositions: i. 20NaF–15ZnO–10BiF3–55 NH4H2PO4 (referred as NZBFP glass) ii. 20NaF–15ZnO–10BiF3–(55-x) NH4H2PO4–1.0 mol %Er2O3 iii. 20NaF–15ZnO–10BiF3–(55-x) NH4H2PO4–1.0 mol % Yb2O3 2
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
3.3. Raman studies
2
3 hc λ7
6ε
Fig. S3 demonstrates the Raman profile of the NaF–ZnF2–BiF3–NH4H2PO4 (NZBFP) glass measured in 400 cm 1 to 1250 cm 1 range. The Raman spectrum of the present glass system can be divided into two regions; the low frequency region (400-800 cm 1) and high frequency region (800-1250 cm 1). In the low frequency re gion, three bands are noticed at 494, 595 and 663 cm 1, respectively. The band at 494 cm 1 is attributed to bending vibrations of (PO4)3– tetrahedral units with modifier cation. The peak at 595 cm 1 is related to the O–P–O bending vibrations. The peak arising at 663 cm 1 is ascribed to the symmetric vibrations of bridging-oxygens joined to two PO4 tetrahedra (P–O–P) in phosphate chain. The bands in the highfrequency region, 916 cm 1 is due to symmetric stretching of (PO4)3– monomer units. The 1028 cm 1 band is attributed to the symmetric stretching of P–O–P non-bridging oxygens. The peaks at 1152 and 1160 cm 1 are attributed to the symmetric stretching mode of O–P–O nonbridging oxygens [32,33].
σ e ðλÞ ¼ σ a ðλÞexp4
kT
5
(1)
, where σe(λ) represents PL cross-section, σa(λ) represents absorption cross-section, c represents light velocity, h denotes Plank’s constant, k denotes the Boltzmann constant, ε denotes the excitation energy at temperature T. Absorption cross-section σa(λ) has been estimated based on absorption spectrum by employing the relation: � � 2:303 log I0=I (2) σ a ðλÞ ¼ NL , where N represents the rare earth ion concentration, L represents glass sample thickness, log (Io/I) optical density. As seen from Fig. 5(a), the Yb3þ ion in NZBFP glass has strong luminescence and absorption crosssections in comparison to Er3þ-doped glass at/around 0.976 μm, sug gesting the possibility of an efficient pumping near 975 nm to achieve multilevel characteristics. Generally, emission of photons followed by the re-absorption leads to emission spectrum broadening and energy loss. Based on the above observations on both Yb3þ and Er3þ ions (Fig. 2 (a and b)) the tendency of radiation trapping (reabsorption of the emitted radiation) has been significant, resulting in broadening of NIR emission spectra which are normally observed in case of a three-level system [35,37,38]. The absorption and stimulated PL cross-sections are calculated to be 7.74 � 10 21 cm2, 7.86 � 10 21 cm2 for Yb3þ doped glass at 0⋅97 μm and 7.69 � 10 21 cm2, 8.15 � 10 21 cm2 Yb3þ/Er3þ-doped glass for 1⋅54 μm. The broadening of 4I13/2 → 4I15/2 transition covered the C- and L-communication windows beneficial for telecommunication. The optical gain coefficient versus wavelength in the light of popu lation inversion rate G(β) is displayed in Fig. 3(a and b). Optical gain plays a key role in assessing laser media performance. The wavelengthdependent optical gain as a function of G(β) with regard to the laser transitions between upper and lower states of the Er3þ is established based on the PL and absorption cross-sections utilizing the relation [11, 37,38]:
3.4. Optical absorption characteristics The absorption (vis-near IR) spectra of NZBFP: Er3þ & NZBFP: Yb3þ/ Er3þ glasses have been exhibited in Fig. 1. The NZBFP:Er3þ glass exhibited peaks corresponding to 4f-4f electronic transitions raised from 4 I15/2 → 4G9/2 (369 nm), 4G11/2 (378 nm), (4G, 4F, 2H) 9/2 (406 nm), 4F5/ 4 2 4 4 2 (450 nm), F7/2 (488 nm), H11/2 (519 nm), S3/2 (542 nm), F9/2 (651 4 4 4 nm), I9/2 (792 nm), I11/2 (976 nm), I11/2 (1527 nm). NZBFP: Yb3þ/ Er3þ glass demonstrated a strong absorption band at 976 attributed to 2 F5/2 transition of Yb3þ alongside the Er3þ bands suggesting that the Er3þ-doped & Yb3þ/Er3þ co-doped glass matrices can absorb the radi ations effectively upon pumping with 980 nm LD. These absorption bands are influenced by the nature of the host lattice into which they were placed owing to the excitation spectra of 4f electrons. The ab sorption bands of rare earth ions are homogenously broad as well as are sensitive to variations in coordination (site-site disorder) and symmetry [28,34,35]. The absorption and derived stimulated PL cross-sections for NZBFP: 1 Er3þ/1 Yb3þ glass for Yb3þ: 4F7/2 → 4F5/2 & Er3þ: 4I15/2 → 4I13/2 transitions are shown in Fig. 2(a and b). In accordance with McCumber’s theory, stimulated PL cross-section (σemi) for both 4F5/2 → 4F7/2 and 4I13/ 4 2 → I15/2 transition are derived from the absorption cross-section (σ abs) 4 of F7/2 → 4F5/2 and 4I15/2 → 4I13/2 transition of Yb3þ and Er3þ, which is provided by the relation [36]:
GðβÞ ¼ N½pσ e ðβÞ
ð1
pÞσa ðβÞ�
(3)
, where N presents rare earth concentration in glass, β represents the population inversion rate between two laser operating levels which depends on the pump energy-density (P), which takes the values be tween 0 and 1 (P ¼ 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0). From Fig. 3(b), it has been noticed that the gain maxima attributed to 1.54 μm shifts to wards higher wavelength side resembling like NIR emission spectra of Er3þ. This represents the typical characteristic of a quasi-three-level laser system. In optical gain versus wavelength plot, the positive gain is achieved when the population inversion rate is around 40% for NZBFP:Yb3þ/Er3þ glass. This suggests that Yb3þ/Er3þ doped glass sample exhibits flat gain band-width in the region from 1400 to 1650 nm covering C- band (1530–1565 nm) & L-band (1565–1625 nm) in low optical telecommunication window. 3.5. NIR luminescence of NZBFP: 1 Yb3þ and NZBFP: 1 Er3þ glasses The emission spectra of NZBFP: 1 mol% Yb3þ and NZBFP: 1 mol% Er glasses is shown in Fig. 4(a and b). On exciting with 808 nm LD, Yb3þ ions in the ground level are pumped to the 4F5/2 level, from which they decay to ground state by radiating photons in the near NIR region around 975 nm (�1 μm) ascribed to 4F5/2 → 4F7/2 transition. The Er3þdoped glass demonstrated a broad luminescence at 1545 nm attributed to 4I13/2 → 4I15/2 transition. The emission spectrum is partially electricdipole (ED) & partially magnetic-dipole (MD) natured. The ED transition corresponds to a broader component of the emission band and MD 3þ
Fig. 1. The Vis-NIR absorption spectra of NZBFP: Er3þ and NZBFP: Yb3þ/ Er3þ glasses. 3
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
Fig. 2. The absorption and derived stimulated PL cross-sections at 975 nm for (a) NZBFP: 1Er3þ and NZBFP: 1 Yb3þ glasses, (b) NZBFP: 1 Yb3þ/1Er3þglass.
Fig. 3. The optical gain coefficient versus wavelength depending on population inversion rate (γ) for (a) NZBFP: 1 Yb3þ glass at 975 nm and (b) NZBFP: 1 Yb3þ/ 1Er3þglass at 1545 nm.
Fig. 4. PL spectra of (a) NZBFP: 1 mol %Yb3þ and (b) NZBFP: 1 mol% Er3þ glasses.
transition corresponds to a sharp central peak in the emission band [37, 38]. On pumping at 980 nm LD, Er3þ ions are promoted to upper 4I11/2 state and then these unstable ions non-radiatively disintegrate to 4I13/2 level from which they proceed to 4I13/2 ground level radiatively through emitting fluorescence in NIR region at 1545 nm labelled to 4I13/2 → 4 I15/2 transition. In Fig. 4(a and b), inset profiles demonstrate their corresponding PL decay curves. NZBFP:1mol% Yb3þ glass exhibited single exponential decay nature with a lifetime of 506.91 μs while NZBFP:1mol% Er3þ glass exhibited bi-exponential nature with a lifetime of 212.6 μs In order to understand the Stark splitting of 4I13/2 and 4I15/2 levels,
which are emitting and ground levels in the phosphate-based glass, a Gaussian de-convolution fit was done for the 1545 nm emission peak of Er3þ displayed in Fig. 5. The inset figure shows an equivalent schematic energy-level model for a four-level system illustrating 1545 nm emission of Er3þ. The de-convoluted Gaussian peaks obtained as a result of fitting the PL spectra of Er3þ-doped NZBFP glass has been expressed as a, b, c and d. The 4I15/2 ground level gives rise to two sub-levels at 0 and 221.3 cm 1, and excited 4I13/2 level also gives rise to two sub-levels one at 6465.95 cm 1 and the other at 6598.39 cm 1 as observed in the inset of Fig. 5. Thus the energy differences ΔE1 ¼ 221 0 ¼ 221.3 cm 1 and ΔE2 ¼ 6598.39–6465.95 ¼ 132.74 cm 1 represents the Stark splitting energy 4
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
where, d represents sample thickness, To represents the transmission at the baseline, T represents the transmittance at the maximum absorption band of OH group, N(OH) represents the OH ion concentration. The N (OH) concentration in the glass is estimated to be 24 � 2 ppm. This provides information that, OH group has some influence on the Er3þ doped NZBFP glasses. 3.7. Energy transfer and luminescence characteristics of NZBFP: Yb3þ/ Er3þ glasses The probability of ET between Yb3þ and Er3þ ions when co-doped in NZBFP glass by fixing the Er3þ concentration to 1 mol% and varying Yb3þ concentration from 1 to 3 mol % is further investigated. In accordance with Forster-Dexter theory, the main criteria for the transfer of excitation energy within two rare-earth ions (acceptor and donor) relay on the gap within them (smaller the distance within them more the energy transfer takes place) and on the range of overlap between donor PL spectrum and acceptor absorption spectrum. From Dieke’s energy level diagram [43,44], transfer of energy is assumed to occur from Yb3þ to Er3þ, because the emitting level (2I5/2) of Yb3þ lies slightly above the fluorescence level (4I13/2) of Er3þ ion. Further, ET probability from Yb3þ to Er3þ is evaluated from the spectral overlap of the Yb3þ PL spectrum and Er3þ absorption spectrum, which is shown in Fig S5. Therefore, this primary observation supports the ET possibility to takes place from Yb3þ to Er3þ, while Yb3þ behaves as a donor (sensitizer) and Er3þ behaves as acceptor (activator). Furthermore, the ET path between Yb3þ and Er3þ is explained in detail from the PL spectra, energy level scheme and decay lifetime rates of the acceptor (Er3þ). On the basis of Foster-Dexter the ory, the ET parameter CYb→Er is determined from the spectral over DA lapping of sensitizer (Yb3þ) emission and activator (Er3þ) absorption using the empirical relation [45,46]: Z 3c (5) CDA ¼ 4 2 σ Dem ðλÞσ Aabs ðλÞdλ 8π n
Fig. 5. Gaussian de-convolution fit for the 1.5 μm peak of Er3þ emis sion spectrum.
range for 4I15/2 and the 4I13/2 multiplets. The 4I15/2 level shows wider Stark splitting in comparison to 4I13/2 level for the present phosphatebased glass system indicating that the bandwidth firmly depends on the Stark splitting of the multiplets. The broad peaks at 6598.39 and 6198.30 cm 1 are assigned to electric-dipole and the sharp bands at 6465.95 and 6377.85 cm 1 are assigned to the magnetic-dipole component [39,40]. 3.6. Influence of OH group The FTIR absorption spectrum of host, 1.0 mol % Er3þ doped NZBFP glass in the 4000 cm 1 to 400 cm 1 range and also energy transfer be tween energy levels of Er3þ and OH vibrational modes is shown in Fig. S4(a and b). The absorption band noticed in the range of 4000–2200 cm 1 (Fig. S4a) is attributed to the OH group i.e., to the presence of water, because absorption band is characteristic of antisymmetric stretching vibrations of free OH groups or free H2O molecules. The ab sorption band centred at 2350 cm 1 is assigned to the P–O–H linkages, suggesting that OH is also bonded to P. The presences of OH group in the rare earth’s doped glasses have a deleterious effect on the optical properties, for instance, quenching of luminescence intensity through non-radiative energy transfer from excited Er3þ ions to OH centres. In Fig. S4(b) the mechanism of luminescence quenching for 1.54 μm emission as a result of non-radiative energy transfer followed by hy droxyl (OH ) quenching is shown. Two OH group vibrational quanta are neded to bridge the energy gap between the 4I13/2 and 4I15/2 states. The quenching effect due to OH is illustrated as; initially, Er3þ ions in the excited states exchange energy by the means of non-radiative transfer with the ions in the ground state. During this process, if any Er3þ ions were coupled to OH group then the excited energy would be lost resulting in luminescence quenching. From the FTIR spectrum it is observed that OH vibrations occur in 2500–4000 cm 1 range centred at 3202 cm 1 of Er3þ doped glasses. The first overtone (ν ¼ 1) of the stretching vibrations of OH group matches with the energy gap of the Er3þ 4I13/2 → 4I15/2 transition (NIR emission in Er3þ ion occurs at 1.54 μm and the first overtone of H–O–H band falls around 1.42 μm and therefore OH group plays an important role in the luminescence quenching of Er3þ. The OH ion concentration NOH (ions/cm3) in the Er3þ doped glass at 3202 cm 1 is estimated from the expression given below [41,42]: � � 1000 To NðOHÞ ¼ log (4) d T
R where σDem ðλÞσAabs ðλÞdλ, relates the overlap of integral spectral crosssections of sensitizer emission and activator absorption, c denotes the light velocity, and n refer to refractive index. On considering the spectral overlap of Yb3þ luminescence and Er3þ absorption cross-sections (as shown in Fig S5), the ET parameter CYb→Er is determined to be 1.26 � DA 10 39 cm6 S 1 for the glass system under investigation. In addition, the ET process within Yb3þ and Er3þ ions are discussed based on NIR pho toluminescence, partial energy-level scheme, PL decay rates and ET parameters of (yYb3þ/Er3þ) combinedly incorporated NZBFP glass matrices. Fig. 6(a) shows the NIR luminescence spectra for 1Er3þ-doped and yYb3þ/1Er3þ (y ¼ 1.0, 2.0, 3.0 mol%) co-doped NZBFP glasses on pumping with 980 nm LD. As seen from the PL spectrum, the emission intensity of yYb3þ/1Er3þ co-doped glass is far greater than the 1Er3þdoped glass. On raising the Yb3þ-doping level, the PL intensity of Er3þ has increased along with the broadening in the spectrum. In the studied range of Yb3þ concentration, no luminescence quenching is noticed, since Yb3þ possess broad absorption band and also larger absorption cross-section compared with Er3þ at 975 nm which resulted in trans ferring of its energy to Er3þ resonantly. Moreover, Yb3þ possess a twolevel system which is less affected by ion-ion interactions or nonradiative losses like OH group, multi-phonon, and cross-relaxations [47]; which could be the reason for the absence of luminescence quenching within the studied range of Yb3þ concentration. At low Yb2O3 doping level, the average gap within the Yb3þ (donor) and Er3þ (acceptor) is greater so, the interaction between them is weak. As the dopant level of Yb2O3 is increased, the average gap within Yb3þ and Er3þ ions decreased, as a result, the interaction between them increased leading to efficient ET within Yb3þ and Er3þ, due to firm overlap of PL and absorption cross-sections. The probability of radiation trapping 5
Optical Materials 100 (2020) 109616
C.P. Reddy et al.
Fig. 6. (a) NIR emission spectra for Er3þ & Yb3þ/Er3þ combinedly doped NZBFP glasses, (b) partial energy-level scheme of Yb3þ & Er3þ ions.
exhibited mono-exponential nature because of resonant ET within Yb3þ and Er3þ ions. Emission decay rates of the Yb3þ/Er3þ combinedly incorporated glass matrices enhanced on rising Yb3þ content. The esti mated lifetimes are 212.6, 267.3, 348.5 and 457.1 μs, respectively. This provides evidence that Yb3þ/Er3þ co-doped NZBFP glasses exhibited longer lifetimes in comparison with the Er3þ-doped glass suggesting a resonant ET from Yb3þ → Er3þ. The optical amplification and gain parameters such as peak wave length (λp), lifetimes (τErþYb), effective band-width (Δλeff), stimulated PL cross-section (σe (λ)), optical gain (G), and gain bandwidth (ΔG) are provided in Table 1. Stimulated PL cross-section (σ e (λ)) and FWHM or effective bandwidth (Δλeff) parameters play a significant part in optical amplifiers in view of achieving high and wideband gain amplification. Moreover, glass samples with larger effective bandwidth (Δλeff) could be potential candidates for WDM (wavelength-division multiplexing) ap plications. The stimulated PL cross-section σ e (λ) for the 4I13/2 → 4I15/2 transition is evaluated using the equation given below [50]:
tends to become dominant for higher donor concentration resulting in the constraining of donor (Yb3þ) excitation energy for a long time in the system enhancing the probability of ET to (Er3þ) acceptors. The ET mechanism is illustrated as on exciting with 980 nm Laser Diode, Yb3þ ions in the ground level are pumped to Yb3þ:2F5/2 and Er3þ ions to Er3þ:4I11/2, when the ions in both the levels come to resonance, some of the excited Yb3þ:2F5/2 ions transfer excitation energy to Er3þ:4I11/2 level. Then, Er3þ ions in excited level relax non-radiatively through populating 4 I13/2 level and finally cascade down to 4I15/2 level radiatively with the emission of photons at 1.54 μm. The ET mechanism from Yb3þ to Er3þ: 2 F5/2(Yb3þ)þ4I15/2(Er3þ)→2F7/2(Yb3þ)þ4I11/2(Er3þ) [48,49]. Fig. 6(b) the energy level scheme demonstrating the ET process within Yb3þ and Er3þ ions. The PL peak at 1545 nm attributed to the 4I15/2 transition is partially electric-dipole, which contributes to emission broadening and partially magnetic dipole which contributes to sharp central peak in emission satisfying the selection rules ΔS ¼ ΔL ¼ 0 and ΔJ ¼ þ1 [46]. In Fig. 6(a), spectral broadening of Er3þ luminescence bands is evidenced on increasing Yb3þ dopant level, because of radiation trapping, which is proportional to dopant content. The PL peak centred around 1.54 μm is extended along the entire low telecommunication window (S-, C-, and L-bands) in the measured region of 1425–1700 nm. The ET process within emission centres of Yb3þ and Er3þ in NZBFP glass matrix is further evaluated from their emission decay profiles. The decay lifetime curves have been recorded for yYb3þ/1Er3þ (y ¼ 1.0, 2.0, and 3.0 mol%) upon pumping with 980 nm LD for Er3þ: 4I13/2 → 4I15/2 transition (1545 nm) at room temperature as shown in Fig. 7. As seen from decay profiles, the lifetime decay curve recorded for the Er3þ singly doped glass exhibited dual exponential nature while Yb3þ-doped glasses
σ e ðλÞ ¼
8π
λ4p 2 cn Δλ
(6)
τ
eff m
, where n represents material’s refractive index, C represents the light velocity, λp refers to PL peak wavelength, τm represents the emission lifetime and Δλeff represents the effective band-width of the PL band. The effective bandwidth is estimated from the ratio of emission peak area to its average height using relation: � Z � Δλeff ¼ IðλÞdλ I λp (7)
Table 1 Peak wavelength (λP), Effective bandwidth (Δ λeff), PL lifetime (τm); Stimulated PL cross-section (σe), Gain bandwidth (σe x Δ λeff), and optical gain (σe x τm) of various concentrations of Er3þ-doped and Er3þ/Yb3þ combinedly doped NZBFP glass matrices assigned to 4I13/2 → 4I15/2 emission transition.
Fig. 7. Emission decay curves for Er3þ: 4I13/2 → 4I15/2 transition (1545 nm) at room temperature. 6
Glass samples
λP (nm) (�1.0)
Δ λeff (nm) (�1.0)
τm
σEP
σEP x Δ λeff
σEP x τm
NZBFP: 1 Er3þ NZBFP: 1 Er3þ/1 Yb3þ NZBFP: 1 Er3þ/2 Yb3þ NZBFP: 1 Er3þ/3 Yb3þ
1545
53.8
2.12
5.71
307.19
12.10
1545
62.16
2.67
7.44
462.47
19.86
1545
59.74
3.48
6.38
381.14
22.20
1545
60.59
4.57
5.12
316.28
23.39
(ms) (�0.2)
(10 21 cm2) (�0.1)
(10 21 cm2 x nm) (�0.2)
(10 21 cm2 x ms) (�0.2)
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
, where I(λp) represents the intensity of the peak wavelength, I(λ) rep resents the PL intensity at the wavelength (λ). The effective band-width (Δλeff) has been chosen over FWHM, owing to the asymmetric PL band at 1.54 μm of Er3þ. In the current NZBFP glass system, on co-doping Yb3þ at different concentrations to 1 mol% Er3þ, effective line width (Δλeff) found to increase which could be because of the spectral overlapping of absorption (Er3þ) and PL bands (Yb3þ) ions and could also be in the wake of energy migration from Yb3þ→Er3þ. Furthermore, gain band width which is the product of stimulated PL cross-section (σe) and effective band-width (Δλeff) are important parameters to characterize laser materials operating in NIR region at/around 1.54 μm emission. Inokuti-Hirayama (I–H) model has been used to determine the coupling factor (CYbþEr) and critical distance (Ro) from the decay life time curve 1 Yb3þ/1Er3þ: NZBFP shown in Fig. 8. The 1 Yb3þ/1Er3þ decay lifetime curve was fitted to the Inokuti-Hirayama relation [51,52]: 2 3 � � � �3S 7 6 IðtÞ ¼ Io exp4 t=t Q t=t (8) 5 =
o
� � 4π Γ 1 3
Fig. 8. Inokuti-Hirayama fit model for NZBFP: 1 Yb3þ/1Er3þ glass.
�� 3S
=
Q¼
o
Na R3o
� CDA ¼ R6o
τYb
reviewing and editing.
(9)
Declaration of competing interest
(10)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
, where I(t) represents PL intensity at time t, to denotes donor lifetimes in the absence of Er3þ ions. Γ(1–3/S) is the Euler’s gamma function, which relates to the values depending on the interaction type. Γ(1–3/S) is equal to 1.77 (dipole-dipole interaction (S ¼ 6)), 1.43 (dipole-quadru pole interaction (S ¼ 8)) and 1.3 (quadrupole-quadrupole interaction (S ¼ 10)), Na presents concentration of acceptor, Q denotes energy transfer parameter, CDA refers to donor-acceptor coupling constant, Ro refers to distance within the Er3þ and Yb3þ ions. From decay lifetime curve of 1 Yb3þ/1Er3þ doped glass, I–H fitting suits better for S ¼ 6 with the experimental values Q ¼ 2.46, Ro ¼ 9.54 Å and CDA is 1.48 � 10 39 cm6 S 1 realizing the nature of interaction between Er3þ and Yb3þ is electric dipole-dipole type. Therefore, the non-radiative ET process between Yb3þ and Er3þ ions is governed by electric dipole-dipole interaction.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109616. References [1] P.J. Winzer, D.T. Neilson, A.R. Chraplyvy, Fiber-optic transmission and networking: the previous 20 and the next 20 years, Opt. Express 26 (2018) 24190–24239. [2] S. Tanabe, Rare-earth-doped glasses for fiber amplifiers in broadband telecommunication, Compt. Rendus Chem. 5 (2002) 815–824. [3] S. Addanki, I.S. Amiri, P. Yupapin, Review of optical fibers-introduction and applications in fiber lasers, Results Phys. 10 (2018) 743–750. [4] P.D. Dragic, M. Cavillon, J. Ballato, Materials for optical fiber lasers: a Review, Appl. Phys. Rev. 5 (2018), 041301. [5] M. Yamane, Y. Asahara, Glasses for Photonics, Cambridge Univ. Press, Cambridge, 2000. [6] G.C. Righini, Passive and active glasses for integrated optics, in: P. Mazzoldi (Ed.), From Galileo’s Occhialino to Optoelectronics, World Scientific, Singapore, 1993, pp. 272–294. [7] J.C.G. Bunzli, Lanthanide luminescence for biomedical analyses and imaging, Chem. Rev. 110 (2010) 2729. [8] J. Cao, X. Li, Z. Wang, Y. Wei, L. Chen, H. Guo, Optical thermometry based on upconversion luminescence behaviour of self-crystallized K3YF6: Er3þ glass ceramics, Sens. Actuators B Chem. 224 (2016) 507–513. [9] D.Y. Wang, P.C. Ma, J.C. Zhang, Y.H. Wang, Efficient down- and up-conversion luminescence in Er3þ Yb3þ co-doped Y7O6F9 for photovoltaics, ACS Appl. Energy Mater. 1 (2018) 447–454. [10] V.A.G. Rivera, F.A. Ferri, L.A.O. Nunes, E. Marega Jr., White light generation via up-conversion and blue tone in Er3þ/Tm3þ/Yb3þ-doped zinc-tellurite glasses, Opt. Mater. 67 (2017) 25–31. [11] L.M. Marcondes, C.R. da Cunha, B.P. de Sousa, S. Maestri, G.Y. Poirier, Thermal and spectroscopic properties studies of Er3þ-doped and Er3þ/Yb3þ-codoped niobium germanate glasses for optical applications, J. Lumin. 205 (2019) 487–494. [12] S. Tabanli, G. Eryurek, Optical investigation of Er3þ and Er3þ/Yb3þ doped zinctellurite glass for solid-state lighting and optical thermometry, Sens. Actuators A Phys. 285 (2019) 448–455. [13] Ch Basavapoornima, T. Maheswari, Shobha Rani Depuru, C.K. Jayasankar, Sensitizing effect of Yb3þ ions on photoluminescence properties of Er3þ ions in lead phosphate glasses: optical fiber amplifiers, Opt. Mater. 86 (2018) 256–269. [14] Y. Zhang, M. Li, J. Li, J. Tang, Z. Wu, Optical properties of Er3þ/Yb3þ codoped phosphate glass system for NIR lasers and fiber amplifiers, Ceram. Int. 44 (2018) 22467–22472.
4. Conclusion It is summarised that the NZBFP glasses with Er2O3, Yb2O3 in single and dual combination were successfully synthesized via melt quench technique and their optical and luminescence characteristics were measured and systematically analysed. The TGA-DTA traces suggested the host glass is thermal stable without devitrification. The Raman profile revealed the functional groups and phonon energy of the host glass. The absorption and stimulated PL cross-sections are calculated to be 7.74 � 10 21 cm2, 7.86 � 10 21 cm2 for 0⋅97 μm and 7.69 � 10 21 cm2, 8.15 � 10 21 cm2 for 1⋅54 μm in 1 Yb3þ/1Er3þ co-doped glass. Pumping at 980 nm Er3þ and yYb3þ/Er3þ doped glasses have exhibited NIR emission bands around 1545 nm. The increment in the NIR PL in tensity of Er3þ with the addition of Yb3þ because of resonant ET from Yb3þ →Er3þ has been discussed depending on the energy level diagram and increment in Er3þ lifetimes. I–H fitting confirms that the ET mech anism between Yb3þ and Er3þ governed by electric dipole-dipole interaction. The computed absorption, PL cross-sections and the gain at 1.54 μm of Yb3þ/Er3þ could suggest these materials useful in the progress of laser systems and fibre optical communications. Author contributions C. Parthasaradhi Reddy: experiment, data collection, manuscript drafting, data plotting; R. Ramaraghavulu: data collection and manu script drafting; T. Kalpana: software, reviewing; J. Gajendiran: 7
C.P. Reddy et al.
Optical Materials 100 (2020) 109616
[15] S. Nian, Y. Zhang, W. Cao, Z. Wu, Y. Shu, Optical properties of Er3þ/Yb3þ co-doped bismuth calcium borate glass system for NIR lasers and fiber amplifiers, J. Lumin. 194 (2018) 440–445. [16] R.J. Amjad, A. Sattar, M.R. Dousti, Upconversion and 1.53 μm near-infrared luminescence study of the Er3þ Yb3þ co-doped novel phosphate glasses, Optik 200 (2020), 163426. [17] M.Dh Hassib, K.M. Kaky, A. Kumar, E. S¸akar, M.A. Mahdi, Boro-silicate glasses codoped Er3þ/Yb3þ for optical amplifier and gamma radiation shielding applications, Phys. B Condens. Matter 567 (2019) 37–44. [18] A. Maaoui, F.B. Slimen, M. Haouari, A. Bulou, H.B. Ouad, Upconversion and near infrared emission properties of a novel Er3þ/Yb3þ codoped fluoro-tellurite glass, J. Alloy. Comp. 682 (2016) 115–123. [19] A. Miguel, M.A. Arriandiaga, R. Morea, J. Fernandez, R. Balda, Down- and upconversion emissions in Er3þ–Yb3þ codoped TeO2–ZnO–ZnF2 glasses, J. Lumin. 158 (2015) 142–148. [20] A. Langar, C. Bouzidi, H. Elhouichet, M. Ferid, Er–Yb co-doped phosphate glasses with improved gain characteristics for an efficient 1.55 μm broadband optical amplifiers, J. Lumin. 148 (2014) 249–255. [21] J. Zhou, Q. Liu, W. Feng, Y. Sun, F. Li, Up-conversion luminescent materials: advances and applications, Chem. Rev. 115 (2015) 395–465. [22] V. Naresh, B.S. Ham, Influence of multiphonon and cross relaxations on 3P0 and 1 D2 emission levels of Pr3þ doped borosilicate glasses for broad band signal amplification, J. Alloy. Comp. 664 (2016) 321–330. [23] M. Deepa, R. Doddoji, C.S.D. Viswanath, A.V. Chandrasekhar, Optical and NIR luminescence spectral studies: Nd3þ-doped borosilicate glasses, J. Lumin. 213 (2019) 191–196. [24] S. Damodaraiah, R.P. Vijaya Lakshmi, Y.C. Ratnakaram, Role of bismuth content on structural and luminescence properties of Ho3þ doped phosphate glasses, J. Mol. Struct. 1200 (2020), 127157. [25] A. Amarnath Reddy, S. Surendra Babu, G. Vijaya Prakash, Er3þ-doped phosphate glasses with improved gain characteristics for broadband optical amplifiers, Opt. Commun. 285 (2012) 5364–5367. [26] T. Xu, X. Zhang, G. Li, S. Dai, Q. Nie, X. Shen, X. Zhang, Spectroscopic properties and thermal stability of Er3þ-doped TeO2–B2O3–Nb2O5–ZnO glass for potential WDM amplifier, Spectrochim. Acta 67 (2007) 559–563. [27] S.K. Taherunnisa, D.V. Krishna Reddy, T. Sambasiva Rao, K.S. Rudramamba, Y. A. Zhydachevskyy, A. Suchocki, M. Piasecki, M. Rami Reddy, Effect of upconversion luminescence in Er3þ doped phosphate glasses for developing ErbiumDoped Fibre Amplifiers (EDFA) and G-LED’s,, Opt. Mater. 3 (2019) 100034. [28] Ch Basavapoornima, T. Maheswari, S.R. Depuru, C.K. Jayasankar, Sensitizing effect of Yb3þ ions on photoluminescence properties of Er3þ ions in lead phosphate glasses: optical fiber amplifiers, Opt. Mater. 86 (2018) 256–269. [29] S. Meruva, B.L. Carlos, F. Julio Jr., A. Peres, Optical characterization, luminescence properties of Er3þ and Er3þ/Yb3þ co-doped tellurite glasses for broadband amplification, Proc. SPIE 8961 Fiber Lasers XI Technol. Syst. Appl. (2014) 896132, https://doi.org/10.1117/12.2037000. [30] H. Sun, L. Hu, C. Yu, G. Zhou, Z. Duan, J. Zhang, Z. Jiang, Investigation of the effect of fluoride ions introduction on structural, OH- content and up-conversion luminescence properties in Er3þ-doped heavy metal oxide glasses, Chem. Phys. Lett. 408 (2005) 179–185. [31] G.L. Agawane, K. Linganna, J.H. In, J.H. Choi, High emission cross-section Er3þdoped fluorophosphate glasses for active device application, Optik 198 (2019), 163228.
[32] C. Ivascu, A.T. Gabor, O. Cozar, L. Daraban, I. Ardelean, FTIR-Raman and thermoluminescence investigation of P2O5-BaO-Li2O glass system, J. Mol. Struct. 993 (2011) 249–253. [33] I. Jlassi, N. Sdiri, H. Elhouichet, M. Ferid, Raman and impedance spectroscopy methods of P2O5-Li2O-Al2O3 glass system doped with MgO, J. Alloy. Comp. 645 (2015) 125–130. [34] F. Wang, F. Song, S. An, W. Wan, H. Guo, S. Liu, J. Tian, Er3þ/Yb3þ-codoped phosphate glass for short-length high-gain fibre lasers and amplifiers, Appl. Opt. 54 (2015) 1198–1205. [35] A.D. Sontakke, K. Annapurna, Energy transfer based efficient infrared emission at 1.57 lm from Yb3þ–Er3þ codoped Zinc-borobismuthate glasses, Opt. Mater. 35 (2013) 472–478. [36] D.E. McCumber, Theory of phonon-terminated optical masers, Phys. Rev. A 134 (1964) A299. [37] V. Naresh, S. Buddhudu, Analysis of visible-NIR emission and photoluminescence quenching in Er3þ:Bi2O3–AlF3–TeO2–B2O3 glasses, Phys. Chem. Glasses Eur. J. Glass Sci. Technol. B 56 (2015) 255–262. [38] J.H. Choi, F.G. Shi, A. Margaryan, A. Margaryan, W. van der Veer, Novel alkalinefree Er3þ-doped fluorophosphate glasses for broadband optical fiber lasers and amplifiers, J. Alloy. Comp. 450 (2008) 540–545. [39] F.R. Lopez, P. Babu, L. Jyothi, U.R. Rodriguez-Mendoza, I.R. Martin, C. K. Jayasankar, Er3þ-Yb3þ codoped phosphate glasses used for an efficient 1.5 μm broadband gain medium, Opt. Mater. 34 (2012) 1235–1240. [40] K. Damak, E. Yousef, S. AlFaify, C. Russel, R. Maalej, Raman, green and infrared emission cross sections of Er3þ doped TZPPN tellurite glass, Opt. Mater. Express 4 (2014) 597–612. [41] R. Zheng, Z. Wang, P. Lv, Y. Yuan, Y. Zhang, J. Zheng, W. Wei, Novel synthesis of low hydroxyl content Yb3þ-doped fluorophosphate glasses with long fluorescence lifetimes, J. Am. Ceram. Soc. (2014) 1–6. [42] F.F. Sene, J.R. Martinelli, L. Gomes, Optical and structural characterization of rare earth doped niobium phosphate glasses, J. Non-Cryst. Solids 348 (2004) 63–71. [43] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals, WileyInterscience Publishers, New York, 1968. [44] B.G. Wybourne, Spectroscopic Properties of Rare Earths, Wiley-Interscience, New York, 1965. [45] T. Forster, Intermolecular energy migration and fluorescence, Ann. Phys. 2 (1948) 55. [46] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 836. [47] P.D. Dragic, M. Cavillon, J. Ballato, Materials for optical fiber lasers: a review, Appl. Phys. Rev. 5 (2018), 041301. [48] L. Zhang, H. Hu, C. Qi, F. Lin, Spectroscopic properties and energy transfer in Yb3 þ /Er3þ-doped phosphate glasses, Opt. Mater. 17 (2001) 371–377. [49] B.P. Kore, A. Kumar, L. Erasmus, R.E. Kroon, J.J. Terblans, S.J. Dhoble, H.C. Swart, Energy transfer mechanisms and optical thermometry of BaMgF4: Yb3þ, Er3þ phosphor, Inorg. Chem. 57 (2018) 288–299. [50] K. Linganna, K. Suresh, S. Ju, W.T. Han, C.K. Jayasankar, V. Venkatramu, Optical properties of Er3þ-doped K-Ca-Al fluorophosphate glasses for optical amplification at 1.53 μm, Opt. Mater. Express 5 (2015) 1689. [51] B. Xu, B. Yang, Y. Zhang, H. Xia, J. Wang, Cooperative energy transfer in Tm3þ and Yb3þ co-doped phosphate glasses, J. Rare Earth 31 (2013) 164–168. [52] T. Wei, Y. Tian, C. Tian, X.F. Jing, J. Zhang, L. Zhang, S. Xu, Optical spectroscopy and population behavior between 4 I11/2 and 4 I13/2 levels of erbium doped germanate glass, Opt. Mater. Express 4 (2014) 2150.
8