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1.5 μm luminescence enhancement of Er3+ by local field surface plasmon resonance of Ag nanoparticles in silicate glasses
Journal of Non-Crystalline Solids 521 (2019) 119522
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1.5 μm luminescence enhancement of Er3+ by local field surface plasmon resonance of Ag nanoparticles in silicate glasses
T
⁎
R. Rajaramakrishnaa,b, Y. Ruangtaweepa,c, N. Sangwaranateed, , J. Kaewkhaoa,c a
Center of Excellence in Glass Technology and Materials Science, Nakhon Pathom Rajabhat University, Meuang, Nakhon Pathom 73000, Thailand Department of Post Graduate Studies and Research in Physics, The National College (Autonomous), 7th block Jayanagar, Bangalore 560070, India c Physics Program, Faculty of Science and Technology, Nakhon Pathom Rajabhat University, Meuang, Nakhon Pathom 73000, Thailand d Applied Physics, Faculty of Science and Technology, Suan Sunandha Rajabhat University, Bangkok, Thailand b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silver nanoparticle SPR Emission cross section Mc Cumber theory
Enhancement of the 1.5 μm luminescence intensity for (49-x)SiO2-20BaO-20ZnO-10Li2O -0.5Er2O3-xAgNO3 (SBZL0.5Er) (x = 0.0, 0.5, 1.0 coded as 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag) glasses were studied. Surface plasmon resonance (SPR) effect was observed for Ag nanoparticle doped glasses. Size of the silver Nanoparticles were calculated theoretically and found to be ~16 nm and ~5.7 nm for 0.5Er0.5Ag and 0.5Er1.0Ag respectively. Stimulated emission cross section calculated from the Judd-Ofelt theory shows 0.63 × 10−20 cm2, 0.74 × 10−20 cm2 and 0.95 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag respectively. Stimulated emission cross section calculated from the Mc Cumber theory shows 1.52 × 10−20 cm2, 1.03 × 10−20 cm2 and 1.43 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glasses, respectively. The obtained values are in good agreement to consider them for potential use in C and L band of optical communication window.
1. Introduction Oxide glasses which contain alkali metal ions have been topic of interest due to their optical properties and optical transparency. The addition of transition metal oxides provide such glasses imbibe with color and electronic conductors by polaron hopping. Transparent glasses can become colored by an entirely different mechanism when a much lower quantity of noble metal nanoparticles is introduced. The source of this absorption band is attributed to the electromagnetic field stimulate combined oscillations of the electrons engage states near the Fermi level in the conduction band (Surface plasmon's). The colors are formed as an outcome of Mie scattering and/or surface plasmon resonance absorption by metal nanoparticles embedded in the oxide matrix. Primarily, the cast glass remains colorless suggesting that gold or silver is dissolved in the ionic state. [1,2]. Glass doped with RE3+ ions containing silver or gold metal nanoparticles turn out to be attractive due to distinctive footprint of surface plasmons which enhances both photoluminescence efficiency and absorption cross-section as well as nonlinear optical properties of glasses [3–6]. The mechanism for the enhancement of the rare earth ions in optical properties may be elucidated as follows: whichever excitation incident light or material luminescence interact with nano particles, there will be coherent oscillation of the free electrons, collectively in the visible region, distinguished as
⁎
surface plasmon resonance (SPR) [7–9]. On the basis of this mechanism the present article focus on SPR interaction wavelength interaction with luminescence enhancement which is useful for optical communication window in C-band region. 2. Experiment Glass composition of (49.5-x)SiO2-20BaO-20ZnO-10Li2O -0.5Er2O3xAgNO3 (SBZL0.5Er) (x = 0.0, 0.5, 1.0 coded as 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag respectively) was synthesized by conventional melt quenching technique. AR grade (99.9%) Sigma Aldrich of SiO2, BaCO3, ZnO, Li2O, Er2O3 and AgNO3 are weighted and mixed thoroughly using mortar and pestle. The powder obtained from the mix then melted in a high purity alumina crucible at 1500 °C in air for 60 min. The molten liquid then melt quenched to get the glass samples which were shown in Fig. 1. Refractive index was determined by Abbe refractometer (ATAGO) with sodium vapor lamp that provide light of wavelength of 598.3 nm (D line) and mono bromo naphthalene as a contact liquid. In order to minimize the error every measurement was done at three times. UV-Vis-NIR spectrometer (Shimadzu,UV-3100) was used to measure the absorption spectra at room temperature in the range of 1800 nm to 200 nm wavelength. Photoluminescence spectra were measured using PTI-HORIBA, Canada using harmonic filter.
Corresponding author. E-mail address: [email protected] (N. Sangwaranatee).
https://doi.org/10.1016/j.jnoncrysol.2019.119522 Received 20 February 2019; Received in revised form 3 June 2019; Accepted 11 June 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 521 (2019) 119522
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Table 1 Physical parameters of SBZL0.5Er glass. Samples 3
Density (g/cm ) Molar Volume Refractive Index (n) Dielectric constant (ε) Er3+Rp (nm) Er3+Ri (nm) Field Strength of Er3+ (F × 1020 cm2) Ag NP Rp (nm) Ag NP Ri (nm) Field Strength of Ag NP (F × 1019 cm2) dEr3+-Er3+ (nm) dAg-Ag (nm) dEr3+-Ag (nm)
0.5Er0.0Ag
0.5Er0.5Ag
0.5Er1.0Ag
3.7654 21.506 1.6533 2.7334 1.090 2.657 3.217 – – – 2.657 – 2.657
3.7637 21.662 1.6531 2.7327 1.093 2.664 3.202 1.428 3.483 8.319 2.657 3.483 2.351
3.7476 21.821 1.6417 2.6951 1.095 2.670 3.186 1.134 2.765 13.20 2.657 2.765 2.150
Fig. 1. Photograph of the synthesized glass samples.
of Er3+ ions, respectively as shown in Fig. 3 (a-b). And the attentiongrabbing part is an overlap of 4F5/2 + 4F3/2 and 2H9/2 around 428 nm, when doped with 1.0 mol% AgNO3 concentration which is due to surface plasmon resonance band. 2.3. Theoretical particle size using surface plasmon resonance band (SPRB) The particle size of metal nano-particles doped in glasses can be calculated by applying Mie scattering theory [11]. The size of the particle is given by [12]
d=
2ℏVf ΔE
(1)
where ℏ is Planck's constant and d is the average diameter of the silver nanoclusters. The broaden energy ΔE (in eV units) in Eq. (1) has been anticipated from the full width at half maximum (FWHM) of the optical absorption band. Eq. (1) is valid as long the size of the silver nanoparticle agglomeration is much smaller than the mean free path of the electrons in the bulk metal. The mean free path of the electrons is about 27 nm at room temperature for bulk silver. By using Eq. (1), the Gaussian profile from the UV-VIS spectra (Fig. 4) of the Ag nanoparticles were plotted, and estimated the average size of the Ag Nanoparticles is ~16 nm and ~5.7 nm for 0.5 mol% and 1.0 mol% AgNO3 content.
Fig. 2. Density Vs Molar Volume of SBZL0.5Er glass with various concentration of AgNO3.
2.1. Physical parameters discussions Fig. 2 shows the density and molar volume graph of the prepared SBZL0.5Er glasses doped with Ag nanoparticles which clearly shows that the density of the prepared glass samples decreases with increase in AgNO3 concentration. The decrease in glass density up to 1.0 mol% Ag nanoparticle is associated to the diminution of structural compactness due to the occupying of silver nanoparticles in the open space regions of the glass network structure. The molar volume value, increase from 21.506 to 21.821 cm3/mol, is ascribed to the bond length stretch or inter-atomic spaces which leads to increase in the glass free volume. The ionic radius of Ag (1.09 Å) is bigger than those of the other ions (Si4+ = 0.4 Å, Li1+ = 0.76 Å, Zn2+ = 0.74 Å and Er3+ = 1.004 Å) expect Ba2+ = 1.35 Å. The bigger ionic radius of Ag will create surplus volume and as a result, less ion packing density. The inter-particle distance between rare-earth ions and silver nanoparticles (dEr3+-Ag) were calculated using M. Reza Dousti et al. [10] which decreases with increase in concentration of AgNO3 content. Table 1 shows the data of polaron radius and inter-particle distance of Er3+ and Ag nanoparticles present in the glass. Hence the density of the present glass decreases and molar volume increases.
2.4. Structural studies FTIR spectra of the glasses are shown in Fig. 5 which exhibit three conventional bands originating from silicate groups. Er3+ and AgNO3 co-doped barium zinc silicate glasses show absorption in the region of 650 cm−1 - 1500 cm−1 wave numbers.
• Strong bands at 760 cm • •
−1 - 777 cm−1 was observed attributing to (Si-O-Si) symmetric stretching of bridging oxygen between tetrahedra. [13,14]. Weak band at 940 - 980 cm−1 are related to Si-O-Si anti-symmetric stretching of bridging oxygen within the tetrahedra [14] and also it could relate to the Si–O–Ba vibrations. The shoulder at 1100 cm−1 -1200 cm−1 are assigned at Si–O–Si bending mode [15]
2.5. Photoluminescence studies Photoluminescence studies were analyzed using Judd-Ofelt theory in which the absorption spectra plays vital role in analyzing the oscillator strength of electric dipole transitions. The theory and practice was considered from B. R. Judd [16] and G. S. Ofelt [17]. The data obtained in Table.2 shows oscillator strength is highest for the 4G11/2 transition and increases with increase in AgNO3 content suggesting that the
2.2. Absorption studies The absorption studies divulge eleven distinct bands at 378, 406, 450, 486, 522, 551, 652, 802, 975 and 1535 nm, which are assigned to the transitions from 4I15/2 ground state to 4G9/2, 4G11/2, 2H9/2, 4F3/ 4 4 2 4 4 4 4 4 2 + F5/2, F7/2, H11/2, S3/2, F9/2, I9/2, I11/2 and I13/2, exited states 2
Journal of Non-Crystalline Solids 521 (2019) 119522
R. Rajaramakrishna, et al.
Fig. 3. UV–Vis-NIR spectra of SBZL0.5Er glasses doped with Ag nanoparticles in (a) UV–Vis Range (b) NIR Range.
Fig. 4. Surface plasmon resonance of Ag Nanoparticle in SBZL0.5Er glasses.
Fig. 5. FTIR spectra of SBZL0.5Er glasses doped with Ag nanoparticles.
oscillator strength enhances due to the surface plasmon resonance (SPR) occurring at specific wavelength with the interaction of light wavelength. In the present glass the SPR band originating around 428 nm wavelength, consuming two transition 4F3/2+4F5/2 and 2H9/2 of Er3+ ions with clear robust absorption pursue by the interaction of light scattering, enrichment of the electromagnetic fields or surface plasmon resonance (SPR) due to the dielectric features of the glass host [18–20]. Due consuming of two transitions 1.0 mol% AgNO3 content glass show higher value in the quality of fitting (δRMS). When trivalent rare earth ions located in glass it directly influences the SPR, excitation and emission intensity is equally improved. This is confirmed from the emission spectra both in VIS region when excited at 288 nm and NIR range at 380 nm as shown in Fig. 6(a) and 6(b). Enhancement may take place when wavelength is very close to the surface plasmon wavelength [19–21]. JO parameters shows Ω2 > Ω6 > Ω4 indicating that Er3+ ions inside the glass is very sensitive to their surrounding environment. Same trend is followed with increase in AgNO3 content. Whereas the Ω2 increases with increase in AgNO3 content indicating that enhancement sensitivity of the rare-earth ions which is restrained to restricted electromagnetic fields or surface plasmon resonance (SPR) in the glass. The
Table 2 Oscillator strengths and JO parameters of SBZL0.5Er glass. Sample or Transition
4
I13/2 I11/2 4 I9/2 4 F9/2 4 S3/2 4 H11/2 4 F7/2 4 F3/+4F5/2 2 H9/2 4 G11/2 4 G9/2 Ω2 (×10−20) Ω4 (×10−20) Ω6 (×10−20) δRMS 4
3
0.5Er0.0Ag
0.5Er0.5Ag
0.5Er1.0Ag
fexp
fcal
fexp
fcal
fexp
fcal
2.857 1.098 0.581 4.287 1.292 10.32 3.019 1.467 1.077 13.23 3.753 4.113 2.538 2.940 0.91
2.694 1.169 0.680 4.301 1.049 8.565 4.215 1.280 1.572 15.10 2.872
3.246 1.201 0.823 3.142 0.957 7.597 2.370 3.337 3.303 14.89 2.237 4.765 1.141 3.069 0.98
3.067 1.407 0.290 3.176 1.269 8.116 4.356 1.547 1.786 14.33 1.0.836
4.312 1.515 0.952 2.627 0.525 7.567 3.691 – – 17.25 2.084 5.898 0.07 4.088 0.99
3.931 1.844 0.066 2.989 1.675 8.936 5.311 – – 15.79 1.451
Journal of Non-Crystalline Solids 521 (2019) 119522
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Fig. 6. Photoluminescence spectra of SBZL0.5Er glasses doped with Ag nanoparticles (a) Visible Region (λexi = 288 nm) (b) NIR Region (λexi = 380 nm).
Fig. 7. Energy transfer of SBZL0.5Er glasses doped with Ag nanoparticles.
interaction of the light with Er3+ ions in SBZL0.5Er glass containing silver NPs can be summarized to two divergent course; interaction of the light with (i) RE ions and (ii) metallic nano particles. Ag+ ions are excited under 288 nm and Er3+ ions can be initially excited under 380 nm from 4I15/2 ground state to 4G11/2 excited state as shown in Fig. 7. It is expected that in Ag+ could reduce partially to Ag0 and hence the homogeneity of such ions are not possible which is clearly observed from Fig. 1. The Ag+ broad emission band were observed in visible region ~435 nm due to lodged Ag+ pairs [22,23] when excited at 288 nm. The emission intensity in the visible region increases at 0.5 mol % of AgNO3 and then decreases at 1.0 mol% of AgNO3 but the trend reverses in NIR emission indicating partial energy transfer (ET) from Ag+ due to SPR phenomenon to Er3+ ions hence increase in emission intensity at NIR region. When excited to 380 nm the electron jumps to 4 G11/2 level of excited state of Er3+ ions and through non radiative transition (NRT) it relaxes to 4I13/2. The radiative emission occurs from 4 I13/2 to ground state of 4I15/2 (1.5 μm) NIR emission. In the present work, the overlapping of absorption band (4I15/2 → 4I13/2) and emission band (4I13/2 → 4I15/2) are presented in Fig. 8, the spectral region 1425–1625 nm indicates the possible RET (resonance energy transfer)
Fig. 8. Photoluminescence and absorption overlapping spectra of SBZL0.5Er glasses doped with Ag nanoparticles in NIR Region.
process among the Er3+ − Er3+ ions in NIR range (~1.535 μm). The stimulated emission cross section calculated using radiative properties of the Judd-Ofelt theory, which was found to be 0.63 × 10−20 cm2, 0.74 × 10−20 cm2 and 0.95 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glass samples respectively. The photo emission band at around 1.535 μm related to 4I13/2 / 4I15/2 transition is significant one as it is valuable for optical communication and IR laser applications. Er3+ emission at 1.53 μm has timely privileged and pleased a great deal of interest as an eye-safe laser for range finding applications [24]. A considerable broadening of emission with increased AgNO3 concentration is evidently pragmatic from Fig. 6(b). Such kind of broadening tendency is mainly due to the radiative entrap, which always happen in a distinctive 3-level system while the absorption and photoluminescence spectra overlap. The effect of radiative entrap is in relative with concentration, the intensity of the emission positioned at lower energies with respect to 1536 nm accordingly amplify, and then it 4
Journal of Non-Crystalline Solids 521 (2019) 119522
R. Rajaramakrishna, et al.
leads to a spectral broadening of the emission line. The FWHM of 1.5 μm emission is calculated and found to be 39.44 ± 0.2, 39.05 ± 0.2 and 43.17 ± 0.2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glasses respectively. The larger values of FWHM could be beneficial to the tunable lasers and wavelength division multiplexing (WDM) system applications [25]. Such broadening could be observed due to appearance of SPR absorption bands which mostly depend on several features such as the size variation, particle density, spatial distribution and refractive index [26]. It is recognized that appearance of plasmonic resonance in the visible spectrum provides an support for dielectric influence which predominantly signifies and enhances the rare-earth ions present to enhance its emission intensity due the support of energy transfer from nearby nano particle to rare-earth ions surroundings hence, the broadening of spatial distribution and increase in the intensity as observed [27,28]. 2.6. Mc Cumber theory According to Mc Cumber's theory [29] the absorption cross section, stimulated emission-cross section and radiative lifetime of an optical center in thermal equilibrium are interrelated to each other at each single frequency via a set of theoretical formulas [24]. The absorption cross-section, σabs(λ), for the 4I15/2 → 4I13/2 transition of Er3+ was estimated from transmission spectrum with Eq. (2) [24].
2.303 log σabs (λ ) =
Fig. 9. Absorption and Emission cross-section of SBZL0.5Er glasses doped with Ag nanoparticles.
positive when the population inversion is larger than 0.4 for all these glasses meaning there by regular population inversion more than 40%, the samples has smooth gain band-width in the range of 1450–1650 nm which belongs to C (1530–1565 nm) and L (1565–1625 nm) optical communication bands.
( ) I°
I
(2)
CL
where log(I0/I) is the absorbance, “C” is the concentration of the Er3+ ions in the glass, “L” is sample thickness. The emission cross-section, σem(λ) of the 4I13/2 → 4I15/2 (Er3+) laser transition can be calculated from the absorption cross-section σabs(λ) which is related as
ε − hcλ−1 ⎞ σem (λ ) = σabs (λ ) exp ⎛ kT ⎝ ⎠ ⎜
3. Conclusions The Er3+-doped and Er3+–Ag nanoparticle co-doped lithium barium zinc silicate glasses was synthesized by melt quench technique. The optical properties were investigated by optical spectroscopy techniques (absorption, photo-luminescence spectra) as well as Judd–Ofelt theory and Mc Cumber theory were employed to understand the emission behavior in NIR region. Based on the analysis and results obtained the conclusions are made as follows:
⎟
(3)
where σabs(λ) is the absorption cross-section, “k” is the Boltzmann constant and “h” is the Planck's constant, “ɛ” is the free energy required to excite one Er3+ ion from the 4I15/2 state to 4I13/2 state at temperature T. The emission cross section (σemi(λ)) for the 4I13/2 → 4I15/2 transition of Er3+ ions are found to be 1.03 × 10−20 cm2, 1.43 × 10−20 cm2 and 1.52 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glasses, respectively. Similarly, the absorption cross-sections (σabs(λ)) are 8.89 × 10−21 cm2, 1.23 × 10−20 cm2 and 1.30 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glasses, respectively. The σabs(λ) and σemi(λ) of the 4I13/2 → 4I15/2 transition are shown in Fig. 9. Larger σemi(λ) exhibits low threshold and show higher gain laser operation, therefore, the present glasses could be potential competitor for laser applications in the NIR region. These values were considered to compare for NIR emission with other reported literatures; Li2B4O7:Er,Ag glass 1.05 × 10−20 cm2 [23], 1.0 mol% Er3+ doped PKAZF glass 5.19 × 10−2 cm2 [24], fluoride silicate 0.74 × 10−20 cm2 [30], zinc cadmium phosphate 0.47 × 10−20 cm2 [31], bismuth borate 0.70 × 10−20 cm2 [32], and strontium lithium bismuth borate 0.95 × 10−20 cm2 [33] were obtained for glasses, respectively.
• Optical absorption spectra of the Er
•
•
2.7. Optical gain coefficient The optical gain coefficient is another vital factor for investigating the performance of laser media. From the absorption cross-section (σabs(λ)) and emission cross-section (σemi(λ)) values, the room temperature gain coefficient is calculated by using the following Eq. (4).
G (λ ) = γσemi (λ ) − (1 − γ ) σabs (λ )
(4) Declarations of interest
The gain cross sections by means of wavelength for various values of γ = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 are as presented in the Fig. 10 for the present glass samples. From the Fig. 10 the gain is
None. 5
3+
ions doped (0.5Er0.0Ag) and Er + Ag nano particle co-doped (0.5Er0.5Ag and 0.5Er1.0Ag) glasses consist of numerous narrow bands in the visible and infrared spectral ranges, which attributed f – f transitions of the Er3+ ions. Strong absorption band of SPR of the silver metallic nanoparticles was observed in the as prepared glasses at 428 nm. FTIR spectra of Er3+-doped and Er3+ + Ag nano particle co-doped glasses reveal the signature peaks of silicate networks. Photoluminescence spectra of the Er-doped (0.5Er0.0Ag) and EreAg co-doped glasses (0.5Er0.5Ag and 0.5Er1.0Ag) reveal intense infrared (4I13/2 → 4I15/2) emission, such intense emission is due to energy transfer from Ag+ to Er3+ ions as well as local field effects induced by surface plasmon resonance of the silver metallic nanoparticles. The Judd–Ofelt theory was employed and calculated theoretical and experimental oscillator strengths. Judd–Ofelt intensity parameters (Ω2, Ω4, Ω6) have been calculated which shows Ω2 > Ω6 > Ω4 trend. Improvement of the Er3+ luminescence intensity in the 0.5Er0.5Ag and 0.5Er1.0Ag glasses has been observed as results of energy transfer from Ag nanoparticle to Er3+ ions as well as local field effects induced by surface plasmon resonance of the silver metallic nanoparticles. 3+
Journal of Non-Crystalline Solids 521 (2019) 119522
R. Rajaramakrishna, et al.
Fig. 10. Optical gain cross-section of SBZL0.5Er glasses doped with Ag nanoparticles.
Acknowledgements
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1.5 μm luminescence enhancement of Er3+ by local field surface plasmon resonance of Ag nanoparticles in silicate glasses
T
⁎
R. Rajaramakrishnaa,b, Y. Ruangtaweepa,c, N. Sangwaranateed, , J. Kaewkhaoa,c a
Center of Excellence in Glass Technology and Materials Science, Nakhon Pathom Rajabhat University, Meuang, Nakhon Pathom 73000, Thailand Department of Post Graduate Studies and Research in Physics, The National College (Autonomous), 7th block Jayanagar, Bangalore 560070, India c Physics Program, Faculty of Science and Technology, Nakhon Pathom Rajabhat University, Meuang, Nakhon Pathom 73000, Thailand d Applied Physics, Faculty of Science and Technology, Suan Sunandha Rajabhat University, Bangkok, Thailand b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silver nanoparticle SPR Emission cross section Mc Cumber theory
Enhancement of the 1.5 μm luminescence intensity for (49-x)SiO2-20BaO-20ZnO-10Li2O -0.5Er2O3-xAgNO3 (SBZL0.5Er) (x = 0.0, 0.5, 1.0 coded as 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag) glasses were studied. Surface plasmon resonance (SPR) effect was observed for Ag nanoparticle doped glasses. Size of the silver Nanoparticles were calculated theoretically and found to be ~16 nm and ~5.7 nm for 0.5Er0.5Ag and 0.5Er1.0Ag respectively. Stimulated emission cross section calculated from the Judd-Ofelt theory shows 0.63 × 10−20 cm2, 0.74 × 10−20 cm2 and 0.95 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag respectively. Stimulated emission cross section calculated from the Mc Cumber theory shows 1.52 × 10−20 cm2, 1.03 × 10−20 cm2 and 1.43 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glasses, respectively. The obtained values are in good agreement to consider them for potential use in C and L band of optical communication window.
1. Introduction Oxide glasses which contain alkali metal ions have been topic of interest due to their optical properties and optical transparency. The addition of transition metal oxides provide such glasses imbibe with color and electronic conductors by polaron hopping. Transparent glasses can become colored by an entirely different mechanism when a much lower quantity of noble metal nanoparticles is introduced. The source of this absorption band is attributed to the electromagnetic field stimulate combined oscillations of the electrons engage states near the Fermi level in the conduction band (Surface plasmon's). The colors are formed as an outcome of Mie scattering and/or surface plasmon resonance absorption by metal nanoparticles embedded in the oxide matrix. Primarily, the cast glass remains colorless suggesting that gold or silver is dissolved in the ionic state. [1,2]. Glass doped with RE3+ ions containing silver or gold metal nanoparticles turn out to be attractive due to distinctive footprint of surface plasmons which enhances both photoluminescence efficiency and absorption cross-section as well as nonlinear optical properties of glasses [3–6]. The mechanism for the enhancement of the rare earth ions in optical properties may be elucidated as follows: whichever excitation incident light or material luminescence interact with nano particles, there will be coherent oscillation of the free electrons, collectively in the visible region, distinguished as
⁎
surface plasmon resonance (SPR) [7–9]. On the basis of this mechanism the present article focus on SPR interaction wavelength interaction with luminescence enhancement which is useful for optical communication window in C-band region. 2. Experiment Glass composition of (49.5-x)SiO2-20BaO-20ZnO-10Li2O -0.5Er2O3xAgNO3 (SBZL0.5Er) (x = 0.0, 0.5, 1.0 coded as 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag respectively) was synthesized by conventional melt quenching technique. AR grade (99.9%) Sigma Aldrich of SiO2, BaCO3, ZnO, Li2O, Er2O3 and AgNO3 are weighted and mixed thoroughly using mortar and pestle. The powder obtained from the mix then melted in a high purity alumina crucible at 1500 °C in air for 60 min. The molten liquid then melt quenched to get the glass samples which were shown in Fig. 1. Refractive index was determined by Abbe refractometer (ATAGO) with sodium vapor lamp that provide light of wavelength of 598.3 nm (D line) and mono bromo naphthalene as a contact liquid. In order to minimize the error every measurement was done at three times. UV-Vis-NIR spectrometer (Shimadzu,UV-3100) was used to measure the absorption spectra at room temperature in the range of 1800 nm to 200 nm wavelength. Photoluminescence spectra were measured using PTI-HORIBA, Canada using harmonic filter.
Corresponding author. E-mail address: [email protected] (N. Sangwaranatee).
https://doi.org/10.1016/j.jnoncrysol.2019.119522 Received 20 February 2019; Received in revised form 3 June 2019; Accepted 11 June 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Physical parameters of SBZL0.5Er glass. Samples 3
Density (g/cm ) Molar Volume Refractive Index (n) Dielectric constant (ε) Er3+Rp (nm) Er3+Ri (nm) Field Strength of Er3+ (F × 1020 cm2) Ag NP Rp (nm) Ag NP Ri (nm) Field Strength of Ag NP (F × 1019 cm2) dEr3+-Er3+ (nm) dAg-Ag (nm) dEr3+-Ag (nm)
0.5Er0.0Ag
0.5Er0.5Ag
0.5Er1.0Ag
3.7654 21.506 1.6533 2.7334 1.090 2.657 3.217 – – – 2.657 – 2.657
3.7637 21.662 1.6531 2.7327 1.093 2.664 3.202 1.428 3.483 8.319 2.657 3.483 2.351
3.7476 21.821 1.6417 2.6951 1.095 2.670 3.186 1.134 2.765 13.20 2.657 2.765 2.150
Fig. 1. Photograph of the synthesized glass samples.
of Er3+ ions, respectively as shown in Fig. 3 (a-b). And the attentiongrabbing part is an overlap of 4F5/2 + 4F3/2 and 2H9/2 around 428 nm, when doped with 1.0 mol% AgNO3 concentration which is due to surface plasmon resonance band. 2.3. Theoretical particle size using surface plasmon resonance band (SPRB) The particle size of metal nano-particles doped in glasses can be calculated by applying Mie scattering theory [11]. The size of the particle is given by [12]
d=
2ℏVf ΔE
(1)
where ℏ is Planck's constant and d is the average diameter of the silver nanoclusters. The broaden energy ΔE (in eV units) in Eq. (1) has been anticipated from the full width at half maximum (FWHM) of the optical absorption band. Eq. (1) is valid as long the size of the silver nanoparticle agglomeration is much smaller than the mean free path of the electrons in the bulk metal. The mean free path of the electrons is about 27 nm at room temperature for bulk silver. By using Eq. (1), the Gaussian profile from the UV-VIS spectra (Fig. 4) of the Ag nanoparticles were plotted, and estimated the average size of the Ag Nanoparticles is ~16 nm and ~5.7 nm for 0.5 mol% and 1.0 mol% AgNO3 content.
Fig. 2. Density Vs Molar Volume of SBZL0.5Er glass with various concentration of AgNO3.
2.1. Physical parameters discussions Fig. 2 shows the density and molar volume graph of the prepared SBZL0.5Er glasses doped with Ag nanoparticles which clearly shows that the density of the prepared glass samples decreases with increase in AgNO3 concentration. The decrease in glass density up to 1.0 mol% Ag nanoparticle is associated to the diminution of structural compactness due to the occupying of silver nanoparticles in the open space regions of the glass network structure. The molar volume value, increase from 21.506 to 21.821 cm3/mol, is ascribed to the bond length stretch or inter-atomic spaces which leads to increase in the glass free volume. The ionic radius of Ag (1.09 Å) is bigger than those of the other ions (Si4+ = 0.4 Å, Li1+ = 0.76 Å, Zn2+ = 0.74 Å and Er3+ = 1.004 Å) expect Ba2+ = 1.35 Å. The bigger ionic radius of Ag will create surplus volume and as a result, less ion packing density. The inter-particle distance between rare-earth ions and silver nanoparticles (dEr3+-Ag) were calculated using M. Reza Dousti et al. [10] which decreases with increase in concentration of AgNO3 content. Table 1 shows the data of polaron radius and inter-particle distance of Er3+ and Ag nanoparticles present in the glass. Hence the density of the present glass decreases and molar volume increases.
2.4. Structural studies FTIR spectra of the glasses are shown in Fig. 5 which exhibit three conventional bands originating from silicate groups. Er3+ and AgNO3 co-doped barium zinc silicate glasses show absorption in the region of 650 cm−1 - 1500 cm−1 wave numbers.
• Strong bands at 760 cm • •
−1 - 777 cm−1 was observed attributing to (Si-O-Si) symmetric stretching of bridging oxygen between tetrahedra. [13,14]. Weak band at 940 - 980 cm−1 are related to Si-O-Si anti-symmetric stretching of bridging oxygen within the tetrahedra [14] and also it could relate to the Si–O–Ba vibrations. The shoulder at 1100 cm−1 -1200 cm−1 are assigned at Si–O–Si bending mode [15]
2.5. Photoluminescence studies Photoluminescence studies were analyzed using Judd-Ofelt theory in which the absorption spectra plays vital role in analyzing the oscillator strength of electric dipole transitions. The theory and practice was considered from B. R. Judd [16] and G. S. Ofelt [17]. The data obtained in Table.2 shows oscillator strength is highest for the 4G11/2 transition and increases with increase in AgNO3 content suggesting that the
2.2. Absorption studies The absorption studies divulge eleven distinct bands at 378, 406, 450, 486, 522, 551, 652, 802, 975 and 1535 nm, which are assigned to the transitions from 4I15/2 ground state to 4G9/2, 4G11/2, 2H9/2, 4F3/ 4 4 2 4 4 4 4 4 2 + F5/2, F7/2, H11/2, S3/2, F9/2, I9/2, I11/2 and I13/2, exited states 2
Journal of Non-Crystalline Solids 521 (2019) 119522
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Fig. 3. UV–Vis-NIR spectra of SBZL0.5Er glasses doped with Ag nanoparticles in (a) UV–Vis Range (b) NIR Range.
Fig. 4. Surface plasmon resonance of Ag Nanoparticle in SBZL0.5Er glasses.
Fig. 5. FTIR spectra of SBZL0.5Er glasses doped with Ag nanoparticles.
oscillator strength enhances due to the surface plasmon resonance (SPR) occurring at specific wavelength with the interaction of light wavelength. In the present glass the SPR band originating around 428 nm wavelength, consuming two transition 4F3/2+4F5/2 and 2H9/2 of Er3+ ions with clear robust absorption pursue by the interaction of light scattering, enrichment of the electromagnetic fields or surface plasmon resonance (SPR) due to the dielectric features of the glass host [18–20]. Due consuming of two transitions 1.0 mol% AgNO3 content glass show higher value in the quality of fitting (δRMS). When trivalent rare earth ions located in glass it directly influences the SPR, excitation and emission intensity is equally improved. This is confirmed from the emission spectra both in VIS region when excited at 288 nm and NIR range at 380 nm as shown in Fig. 6(a) and 6(b). Enhancement may take place when wavelength is very close to the surface plasmon wavelength [19–21]. JO parameters shows Ω2 > Ω6 > Ω4 indicating that Er3+ ions inside the glass is very sensitive to their surrounding environment. Same trend is followed with increase in AgNO3 content. Whereas the Ω2 increases with increase in AgNO3 content indicating that enhancement sensitivity of the rare-earth ions which is restrained to restricted electromagnetic fields or surface plasmon resonance (SPR) in the glass. The
Table 2 Oscillator strengths and JO parameters of SBZL0.5Er glass. Sample or Transition
4
I13/2 I11/2 4 I9/2 4 F9/2 4 S3/2 4 H11/2 4 F7/2 4 F3/+4F5/2 2 H9/2 4 G11/2 4 G9/2 Ω2 (×10−20) Ω4 (×10−20) Ω6 (×10−20) δRMS 4
3
0.5Er0.0Ag
0.5Er0.5Ag
0.5Er1.0Ag
fexp
fcal
fexp
fcal
fexp
fcal
2.857 1.098 0.581 4.287 1.292 10.32 3.019 1.467 1.077 13.23 3.753 4.113 2.538 2.940 0.91
2.694 1.169 0.680 4.301 1.049 8.565 4.215 1.280 1.572 15.10 2.872
3.246 1.201 0.823 3.142 0.957 7.597 2.370 3.337 3.303 14.89 2.237 4.765 1.141 3.069 0.98
3.067 1.407 0.290 3.176 1.269 8.116 4.356 1.547 1.786 14.33 1.0.836
4.312 1.515 0.952 2.627 0.525 7.567 3.691 – – 17.25 2.084 5.898 0.07 4.088 0.99
3.931 1.844 0.066 2.989 1.675 8.936 5.311 – – 15.79 1.451
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Fig. 6. Photoluminescence spectra of SBZL0.5Er glasses doped with Ag nanoparticles (a) Visible Region (λexi = 288 nm) (b) NIR Region (λexi = 380 nm).
Fig. 7. Energy transfer of SBZL0.5Er glasses doped with Ag nanoparticles.
interaction of the light with Er3+ ions in SBZL0.5Er glass containing silver NPs can be summarized to two divergent course; interaction of the light with (i) RE ions and (ii) metallic nano particles. Ag+ ions are excited under 288 nm and Er3+ ions can be initially excited under 380 nm from 4I15/2 ground state to 4G11/2 excited state as shown in Fig. 7. It is expected that in Ag+ could reduce partially to Ag0 and hence the homogeneity of such ions are not possible which is clearly observed from Fig. 1. The Ag+ broad emission band were observed in visible region ~435 nm due to lodged Ag+ pairs [22,23] when excited at 288 nm. The emission intensity in the visible region increases at 0.5 mol % of AgNO3 and then decreases at 1.0 mol% of AgNO3 but the trend reverses in NIR emission indicating partial energy transfer (ET) from Ag+ due to SPR phenomenon to Er3+ ions hence increase in emission intensity at NIR region. When excited to 380 nm the electron jumps to 4 G11/2 level of excited state of Er3+ ions and through non radiative transition (NRT) it relaxes to 4I13/2. The radiative emission occurs from 4 I13/2 to ground state of 4I15/2 (1.5 μm) NIR emission. In the present work, the overlapping of absorption band (4I15/2 → 4I13/2) and emission band (4I13/2 → 4I15/2) are presented in Fig. 8, the spectral region 1425–1625 nm indicates the possible RET (resonance energy transfer)
Fig. 8. Photoluminescence and absorption overlapping spectra of SBZL0.5Er glasses doped with Ag nanoparticles in NIR Region.
process among the Er3+ − Er3+ ions in NIR range (~1.535 μm). The stimulated emission cross section calculated using radiative properties of the Judd-Ofelt theory, which was found to be 0.63 × 10−20 cm2, 0.74 × 10−20 cm2 and 0.95 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glass samples respectively. The photo emission band at around 1.535 μm related to 4I13/2 / 4I15/2 transition is significant one as it is valuable for optical communication and IR laser applications. Er3+ emission at 1.53 μm has timely privileged and pleased a great deal of interest as an eye-safe laser for range finding applications [24]. A considerable broadening of emission with increased AgNO3 concentration is evidently pragmatic from Fig. 6(b). Such kind of broadening tendency is mainly due to the radiative entrap, which always happen in a distinctive 3-level system while the absorption and photoluminescence spectra overlap. The effect of radiative entrap is in relative with concentration, the intensity of the emission positioned at lower energies with respect to 1536 nm accordingly amplify, and then it 4
Journal of Non-Crystalline Solids 521 (2019) 119522
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leads to a spectral broadening of the emission line. The FWHM of 1.5 μm emission is calculated and found to be 39.44 ± 0.2, 39.05 ± 0.2 and 43.17 ± 0.2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glasses respectively. The larger values of FWHM could be beneficial to the tunable lasers and wavelength division multiplexing (WDM) system applications [25]. Such broadening could be observed due to appearance of SPR absorption bands which mostly depend on several features such as the size variation, particle density, spatial distribution and refractive index [26]. It is recognized that appearance of plasmonic resonance in the visible spectrum provides an support for dielectric influence which predominantly signifies and enhances the rare-earth ions present to enhance its emission intensity due the support of energy transfer from nearby nano particle to rare-earth ions surroundings hence, the broadening of spatial distribution and increase in the intensity as observed [27,28]. 2.6. Mc Cumber theory According to Mc Cumber's theory [29] the absorption cross section, stimulated emission-cross section and radiative lifetime of an optical center in thermal equilibrium are interrelated to each other at each single frequency via a set of theoretical formulas [24]. The absorption cross-section, σabs(λ), for the 4I15/2 → 4I13/2 transition of Er3+ was estimated from transmission spectrum with Eq. (2) [24].
2.303 log σabs (λ ) =
Fig. 9. Absorption and Emission cross-section of SBZL0.5Er glasses doped with Ag nanoparticles.
positive when the population inversion is larger than 0.4 for all these glasses meaning there by regular population inversion more than 40%, the samples has smooth gain band-width in the range of 1450–1650 nm which belongs to C (1530–1565 nm) and L (1565–1625 nm) optical communication bands.
( ) I°
I
(2)
CL
where log(I0/I) is the absorbance, “C” is the concentration of the Er3+ ions in the glass, “L” is sample thickness. The emission cross-section, σem(λ) of the 4I13/2 → 4I15/2 (Er3+) laser transition can be calculated from the absorption cross-section σabs(λ) which is related as
ε − hcλ−1 ⎞ σem (λ ) = σabs (λ ) exp ⎛ kT ⎝ ⎠ ⎜
3. Conclusions The Er3+-doped and Er3+–Ag nanoparticle co-doped lithium barium zinc silicate glasses was synthesized by melt quench technique. The optical properties were investigated by optical spectroscopy techniques (absorption, photo-luminescence spectra) as well as Judd–Ofelt theory and Mc Cumber theory were employed to understand the emission behavior in NIR region. Based on the analysis and results obtained the conclusions are made as follows:
⎟
(3)
where σabs(λ) is the absorption cross-section, “k” is the Boltzmann constant and “h” is the Planck's constant, “ɛ” is the free energy required to excite one Er3+ ion from the 4I15/2 state to 4I13/2 state at temperature T. The emission cross section (σemi(λ)) for the 4I13/2 → 4I15/2 transition of Er3+ ions are found to be 1.03 × 10−20 cm2, 1.43 × 10−20 cm2 and 1.52 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glasses, respectively. Similarly, the absorption cross-sections (σabs(λ)) are 8.89 × 10−21 cm2, 1.23 × 10−20 cm2 and 1.30 × 10−20 cm2 for 0.5Er0.0Ag, 0.5Er0.5Ag and 0.5Er1.0Ag glasses, respectively. The σabs(λ) and σemi(λ) of the 4I13/2 → 4I15/2 transition are shown in Fig. 9. Larger σemi(λ) exhibits low threshold and show higher gain laser operation, therefore, the present glasses could be potential competitor for laser applications in the NIR region. These values were considered to compare for NIR emission with other reported literatures; Li2B4O7:Er,Ag glass 1.05 × 10−20 cm2 [23], 1.0 mol% Er3+ doped PKAZF glass 5.19 × 10−2 cm2 [24], fluoride silicate 0.74 × 10−20 cm2 [30], zinc cadmium phosphate 0.47 × 10−20 cm2 [31], bismuth borate 0.70 × 10−20 cm2 [32], and strontium lithium bismuth borate 0.95 × 10−20 cm2 [33] were obtained for glasses, respectively.
• Optical absorption spectra of the Er
•
•
2.7. Optical gain coefficient The optical gain coefficient is another vital factor for investigating the performance of laser media. From the absorption cross-section (σabs(λ)) and emission cross-section (σemi(λ)) values, the room temperature gain coefficient is calculated by using the following Eq. (4).
G (λ ) = γσemi (λ ) − (1 − γ ) σabs (λ )
(4) Declarations of interest
The gain cross sections by means of wavelength for various values of γ = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 are as presented in the Fig. 10 for the present glass samples. From the Fig. 10 the gain is
None. 5
3+
ions doped (0.5Er0.0Ag) and Er + Ag nano particle co-doped (0.5Er0.5Ag and 0.5Er1.0Ag) glasses consist of numerous narrow bands in the visible and infrared spectral ranges, which attributed f – f transitions of the Er3+ ions. Strong absorption band of SPR of the silver metallic nanoparticles was observed in the as prepared glasses at 428 nm. FTIR spectra of Er3+-doped and Er3+ + Ag nano particle co-doped glasses reveal the signature peaks of silicate networks. Photoluminescence spectra of the Er-doped (0.5Er0.0Ag) and EreAg co-doped glasses (0.5Er0.5Ag and 0.5Er1.0Ag) reveal intense infrared (4I13/2 → 4I15/2) emission, such intense emission is due to energy transfer from Ag+ to Er3+ ions as well as local field effects induced by surface plasmon resonance of the silver metallic nanoparticles. The Judd–Ofelt theory was employed and calculated theoretical and experimental oscillator strengths. Judd–Ofelt intensity parameters (Ω2, Ω4, Ω6) have been calculated which shows Ω2 > Ω6 > Ω4 trend. Improvement of the Er3+ luminescence intensity in the 0.5Er0.5Ag and 0.5Er1.0Ag glasses has been observed as results of energy transfer from Ag nanoparticle to Er3+ ions as well as local field effects induced by surface plasmon resonance of the silver metallic nanoparticles. 3+
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Fig. 10. Optical gain cross-section of SBZL0.5Er glasses doped with Ag nanoparticles.
Acknowledgements
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