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Oxyfluoride silicate glasses and glass-ceramics doped with erbium and ytterbium - An examination of luminescence properties and up-conversion phenomena
Materials & Design 126 (2017) 174–182
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Oxyfluoride silicate glasses and glass-ceramics doped with erbium and ytterbium - An examination of luminescence properties and up-conversion phenomena
MARK
Radosław Lisieckia,⁎, Elwira Czerskab, Michał Żelechowerb, Radosław Swadźbac, Witold Ryba-Romanowskia a b c
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wrocław 50-422, Poland Department of Material Science, Silesian University of Technology, Katowice 40-019, Poland Institute for Iron Metallurgy, Gliwice 44-100, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxyfluoride glasses Glass-ceramics Up-conversion phenomena
Oxyfluoride glasses doped with erbium and ytterbium have been fabricated at T = 1460 °C. To prepare glassceramic systems, the precursor glasses were annealed at temperature 750 °C for 2 h. The ordered fluoride-like crystalline phases were recognized applying XRD, SAED and EDS methods. The spectroscopic properties of the glass-ceramic samples were significantly different as compared to those observed in the precursor glass. Particularly, the emission spectra of glass-ceramics are evidence that some of Er3 + and Yb3 + ions reside into the ordered environment. Up-conversion phenomena attributed to visible anti-stokes emission of Er3 + has been observed under excitation of Yb3 + ions by a 980 nm diode laser. Especially, the substantial enhancement of red up-converted luminescence was observed in the glass-ceramic system. Dissimilar spectral characteristic and relation of both aforementioned emissions examined in glass and glass-ceramic samples point at different contributions of processes relevant to up-conversion phenomena. The detailed analysis of the crucial Er3 + and Yb3 + excited states relaxation dynamics and their spectroscopic qualities has been performed to determine the mechanisms which lead to conversion of the infrared excitation into visible emission.
G RA P H I C A L AB S T R A C T
⁎
Corresponding author. E-mail address: [email protected] (R. Lisiecki).
http://dx.doi.org/10.1016/j.matdes.2017.04.046 Received 6 March 2017; Received in revised form 10 April 2017; Accepted 12 April 2017 Available online 13 April 2017 0264-1275/ © 2017 Elsevier Ltd. All rights reserved.
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1. Introduction Trivalent erbium ions in various crystalline and non-crystalline hosts are able to show efficient luminescence and attract much attention because of their practical applications as optical amplifiers, near-infrared solid-state laser media or NIR-to-visible up-conversion phosphors [1–5]. Inorganic glasses form an important class of host materials for Er3 + ions owing to the low cost and ease of fabrication combined with an excellent optical quality. It has been demonstrated that Er-doped glasses or glass fibers can operate as optical amplifiers in the standard telecommunication window [6–10]. Numerous published papers have been devoted to the preparation, structural and optical properties of different types of glass systems and results gathered indicate that erbium-doped silicate glasses offer favorable properties and are worth considering. In fact, it has been demonstrated that silicate glasses containing erbium ions are promising for active fibers fabrication [11]. Aluminosilicate glasses doped with erbium have been investigated as promising media for producing laser generation in the spectral range around 1.5 μm [12–15]. It has been reported that a higher SiO2 content in calcium aluminosilicate glasses brings about an increase of the erbium laser emission efficiency by 30% [16]. Also, the incorporation of heavy metal oxide and/or fluoride component to the silicate host significantly increases radiative parameters of Er3 + ions. Moreover, it has been observed that the efficiency of the NIR erbium emission can be enhanced markedly by the creation of CaF2 nanocrystals in aluminosilicate host co-doped with erbium and cerium [17]. This finding has stimulated the investigation of oxyfluoride glass-ceramics, materials which merge excellent qualities of glasses (easy shaping and low cost of fabrication) into some advantages of single crystals (narrow spectral lines and high efficiency of luminescent levels) and may be promising as laser active media. Oxyfluoride silicate glasses and glassceramics differing in chemical composition and type of fluoride nanocrystals have been prepared to accommodate luminescent rare earth ions. In particular, the precipitation of NaYF4 nanocrystals and their effect on the Yb-Er energy transfer in aluminosilicate glass ceramics has been considered [18]. It has been demonstrated that the creation of NaYF4 nanocrystals in multicomponent borosilicate glass containing Er3 + and Yb3 + ions brings about a significant increase of up-converted visible luminescence [19]. Investigation of structure and optical band gap of multicomponent aluminosilicate glass-ceramics has revealed that CaF2 was the only precipitated crystalline phase [20]. Upconversion luminescence excited at 980 nm was observed from Er3 +/ Yb3 +-doped ternary SiO2-Al2O3-PbF2 glass and glass-ceramic samples excited at 1550 nm [21]. Creation of BaLuF5 and their effect on Tb3 + luminescence have been reported for aluminosilicate glass-ceramic samples [22]. Quite recently a precipitation of CaF2 nanocrystals and energy transfer phenomena between Eu3 +-Tb3 +, Eu3 +-Dy3 +, Ce3 +Dy3 +, and Ce3 +-Eu3 + ion pairs in aluminosilicate glass-ceramics have been studied [23,24]. Precipitation of Na(Gd,Y)F4 and their advantageous effect on erbium luminescence in multicomponent aluminosilicate glass has been demonstrated [25]. It has been demonstrated also that a two-step thermal treatment of multicomponent aluminosilicate glass containing erbium and ytterbium results in enhancement of upconverted green emission [26]. In this work we report on structural peculiarity, morphology, and optical properties of oxyfluoride glass-ceramics obtained by thermal treatment of multicomponent lead aluminosilicate glass co-doped with erbium and ytterbium. Results obtained during of X-ray diffraction measurement, as well as images and electron diffraction patterns provided by a high resolution transmission electron microscope technique were analyzed thoroughly to infer the nature of crystalline precipitates. Decay curves of luminescence originated on excited states of erbium and ytterbium ions were recorded and analyzed to determine excited state relaxation dynamics in the system under study. Results of spectroscopic measurement were employed to assess spectral parameters relevant to near infrared laser operation and phenomena of up-
Fig. 1. X-ray diffraction patterns of as-melted glass, glass-ceramic sample (GC) and the fluorite (CaF2) reference pattern.
conversion of infrared radiation into visible luminescence. As far as we know results presented in the following are new and original. 2. Experimental 2.1. Sample preparation The starting reagents were melted to obtain the glass samples with the batch composition in mol%: 48%SiO2–11%Al2O3–7% Na2O–10%CaO–10%PbO–(14-x-y %)PbF2 where x = 0.2, 05, 1% ErF3 and y = 0.6, 1, 3% YbF3. The anhydrous Na2O, PbO, PbF2 and LnF3 (Ln = Er, Yb) were used. The PbF2 was applied with a small excess due to the fluorine loss that may occur in the preparation procedure. The aforementioned reagents were thoroughly mixed and introduced into the corundum crucible in dry box. The resistance furnace operated at 1460 °C was utilized to melt the batch for 1 h and 40 min in normal atmosphere. The liquid material was poured into brass mold. To obtain the glass-ceramics samples, the precursor (as-melted) ingots were cut into pieces and heat-treated at temperature 750 °C and for 2 h to activate crystallization process. 2.2. Measurement set-ups The structural properties of the materials have been determined by X-ray diffraction (XRD-X-Pert Philips diffractometer, curved graphite monochromator, CuKα radiation at 40 kV and 30 mA). The morphological features were measured with a transmission electron microscope (HRTEM JEOL JEM-3010 equipped with the GATAN CCD slow-scan camera at an accelerating voltage of 300 kV and FEI TITAN 80-300 at an accelerating voltage of 300 kV). Excitation and luminescence spectra were recorded employing an Optron Dong Woo fluorometer system containing an ozone-free Xe lamp, a primary monochromator with 150 mm focal length, an emission monochromator with 750 mm focal length and a Hamamatsu R-955 photomultiplier. Up-converted luminescence spectra were excited by an Apollo Instruments diode laser emitting continuous wave (CW) radiation at 980 nm. An InGaAs detector was applied to measure emission spectra within NIR spectral range. Luminescence decay curves were recorded upon selective excitation provided by a Continuum Surelite optical parametric oscillator (OPO) pumped with the third harmonic of Nd:YAG. The resulting transients were attained with R-955 photomultiplier and a cooled InSb Janson J10D detector and connected to a Tektronix Model MDO4054B3 digital oscilloscope. A part of luminescence spectra were excited by means of a Coherent Libra femtosecond laser system coupled to a Light Conversion OPerA Optical Parametric Amplifier. Influence of femtose175
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Fig. 2. Bright field TEM image of the glass-ceramics grain (a), electron diffraction pattern of the white circle area (b), simulated (the ELDYF software) electron diffraction pattern of the hexagonal PbF2 with indexed planes (c) and X-ray spectra (d).
cond pulse on sample was measured exploiting a Hamamatsu C5650 streak camera coupled to an Acton 2500i monochromator.
melted sample reveals the pattern characteristic for an amorphous state. Intense and sharp diffraction peaks appear within 28–88° 2Θ in the XRD pattern of heat-treated glass co-doped with erbium and ytterbium. In some cases the lanthanide ions can be considered as nucleation agents that result in more efficient crystallization process [27]. The crystalline phase formed in glass-ceramic has been identified as the fluorite-like CaF2 belonging to cubic system with Fm-3m space group. High resolution transmission electron microscope technique was utilized to examine structural and morphological properties of the glass-ceramic samples. Fig. 2 presents TEM image of the glass-ceramic grain, electron diffraction pattern of the hexagonal PbF2 phase (space
3. Results and discussion 3.1. Morphological and structural characterization Fig. 1 displays X-ray diffraction patterns of Er, Yb double-doped asmelted glass and of glass-ceramic sample annealed at 750 °C for 2 h. It can be seen that the oxyfluoride glass undergoes structural modification as a result of the heat-treatment process. The X-ray diffraction of the as-
Fig. 3. Bright field TEM image of the glass-ceramic grain (a), high resolution TEM image showing nanocrystals embedded in the glassy host (b), the Fast Fourier Transform (FFT) of the image equivalent to the corresponding electron diffraction pattern, identified as the cubic fluorite-like CaF2 structure (c) and X-ray spectra (EDS) of the dark particle in the image (a) indicating high content of Pb, Yb, Er against their lower content in the glassy host (d).
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Fig. 4. Excitation spectra of Er3 + emission monitored at λ = 1536 nm (upper) and emission spectra excited at λ = 445 nm (lower).
oxyfluoride glass. Luminescence originating from the 4F7/2, 2H11/2, S3/2, 4I11/2 and 4I13/2 was measured. Because the NIR 4I13/2 → 4I15/2 erbium emission can be effectively excited employing commercial AlGaAs and InGaAs diode lasers the excitation spectra were recorded also at longer wavelengths in near infrared region. It can be seen in Fig. 4 that two bands centered at 802 and 974 nm appear in this region. The former band, related to the 4I15/2 → 4I9/2 transition is weakly affected by erbium concentration. The latter band is more prominent and its intensity depends on Yb3 + concentration since it is related to partly overlapping 4I15/2 → 4I11/2 (Er) and 2F7/2 → 2F5/2 (Yb) transitions. Actually, the 1.55 μm erbium emission can be effectively excited within 910–1010 nm spectral range by means of Yb-Er energy transfer. Lower-part of Fig. 4 presents visible and near-infrared luminescence spectrum excited at 445 nm in glass (0.2%Er, 0.6%Yb) sample. The visible luminescence spectrum consists of several emission bands. The emission centered around 495 nm can be assigned to 4F7/2 → 4I15/2 transition and the green erbium luminescence originates in thermally coupled 2H11/2, 4S3/2 excited states and terminates on erbium ground state. The blue excitation light activates also a near infrared emission in 964–1080 nm spectral range. Owing to the fact that the 4I11/2 multiplet of Er3 + and the 2F5/2 multiplet of Yb3 + have nearly the same energies the Er-Yb excitation energy transfer can be readily acquired. Accordingly, the observed NIR emission is superposition of the 4I11/2 → 4I15/2 (Er) and 2F5/2 → 2F7/2 (Yb) transitions. It is worth noticing that a red emission related to the 4F9/2 → 4I15/2 transition is efficiently suppressed in samples of oxyfluoride glass independent on Er3 + and Yb3 + concentrations. The contribution of a red band to luminescence spectra excited by femtosecond pulses at 380 nm can be discerned, however. Fig. 5 exemplifies streak camera images for 0.2%Er, 0.6%Yb glass and glass-ceramic emission spectra recorded upon femtosecond excitation at 380 nm. It can be seen that in addition to a strong green luminescence the 4F9/2 → 4I15/2 red emission, albeit weak, contributes to the spectra. The NIR emission of Er3 + related to the 4I13/2 → 4I15/2 transition in oxyfluoride (0.2%Er, 0.6%Yb) glass and glass-ceramic samples was registered under excitation with a laser diode at 980 nm and the adequate spectra are depicted on the left side of Fig. 6. The spectrum measured for a glass sample is poorly resolved and corresponds to
group P6-168; reference code 00-049-1518 PDF4 database) with indexed planes and X-ray spectra (EDS) of the white circle area indicating high content of Pb, Yb, Er against their lower content in the glassy host. Fig. 3 displays high resolution TEM image showing nanocrystals in the glassy host, electron diffraction pattern of the cubic CaF2 phase (space group Fm-3m-225; reference code 01-075-0097 PDF4 database) and X-ray spectra (EDS) of the dark particle in the adequate TEM image. It is worth noticing that M. Rezvani and L. Farahinia examined SiO2-Al2O3-CaO-CaF2 oxyfluoride glasses and glass-ceramics containing CaF2 nanocrystals [20]. It can be discerned in our materials that for glass-ceramic sample numerous crystalline species are embedded into amorphous phase. The TEM images exemplify the fairly monodisperse and spherical in shape crystalline components. Electron diffraction study confirms XRD results pointing that the ordered areas belong to cubic fluorite-like structure (CaF2) and hexagonal phase (PbF2). On the other hand the energy dispersive Xray spectroscopy analysis implied that crystalline particles comprise mainly lead, ytterbium and erbium with lower content of calcium and sodium. Accordingly, the presence of the crystallites characterized by isomorphic hexagonal PbF2 structure can be confirmed as well.
4
3.2. Spectra and down-converted emission The excitation spectrum of Er3 + emission monitored at λ = 1530 nm in oxyfluoride glass (0.2%Er,0.6%Yb) sample recorded at room temperature in the UV–visible NIR wavelength region is displayed in Fig. 4. Moderately resolved bands are attributed to the electronic transitions from the 4I15/2 ground state to the high-lying excited levels of Er3 +. Two bands located at 380 and 520 nm are especially prominent and correspond to 4I15/2 → 4G11/2 and 4I15/ 2 3+ transitions, respectively. These are usually called 2 → H11/2 Er “hypersensitive” transition and their positions and intensities are very sensitive to small changes of environment around rare earth ions. The observed bands are structureless due to fully amorphous character of the sample examined. The excitation spectrum was utilized to select the wavelengths that can be effectively applied to activate the erbium luminescence. Several excited state of erbium are luminescent in 177
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Fig. 5. Emission spectra recorded for as-melted and glass-ceramic samples doped with 0.2%Er and 0.6%Yb glasses upon femtosecond excitation at 380 nm. Images attained with a streak camera revealing details of spectral distribution of sample luminescence intensity in spectral region 500–700 nm.
overlapping transitions between crystal field levels of the excited multiplet and crystal field components of erbium ground state. In comparison to the precursor glass the spectral characteristic of the
glass-ceramic is clearly different. It can be seen that emission line-width distinctly decreased in the studied oxyfluoride glass-ceramic system. Especially, this effect is noticeable within long-wavelength wing of the
Fig. 6. The NIR emission spectrum attributed to 4I13/2-4I15/2 transition recorded for 0.2%Er, 0.6%Yb glass and 0.2%Er, 0.6%Yb glass-ceramic (750 °C_2h) excited at 980 nm (a). Absorption (4I15/2 → 4I13/2) and emission (4I13/2 → 4I15/2) cross section spectra (b) and estimated effective emission cross-section (4I13/2 → 4I15/2) for several P parameters in oxyfluoride glass (c).
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erbium emission in material under study should be expected particularly within 1565–1640 nm. Results presented above deserve some comments because they differ markedly from those obtained during investigation of erbium-doped oxyfluoride glasses and glass ceramics with similar chemical composition and reported in [30]. In contrast to our findings Authors of ref. [30] observed a very weak green luminescence related to the 2H11/ 4 4 2, S3/2 → I15/2 transition. Instead, they recorded a very intense band located in near infrared between ca 1600–1700 nm and assigned this band to the 4S3/2 → 4I9/2 transition. According to spectra shown in Ref. [30] its intensity is markedly higher than that of neighboring band related to the 4I13/2 → 4I15/2 transition. This finding implies very unusual set of branching ratios for the 4S3/2 luminescence of Er3 + differing strongly from that calculated within a framework of the JuddOfelt theory and reported in [31]. Our attempt to discern the 4S3/ 4 2 → I9/2 infrared luminescence in our samples were unsuccessful even when exciting by powerful femtosecond light pulses. Accordingly, we believe that experimental results gathered in the present work are consistent with calculated spectroscopic parameters given in [31].
spectra. Furthermore, the spectrum attributed to glass-ceramic sample contains numerous emission band components. It follows from different distribution of erbium crystal field environments in the oxyfluoride sample comprising ordered fluoride crystalline components. In respect to that it can be assumed that a part of the trivalent erbium was incorporated into the ordered crystalline phase. One of the most important parameters that affect laser performance of a luminescent material is the stimulated emission cross section σem. We calculated the σem(λ) applying the Füchtbauer-Ladenburg formula:
σem (λ) =
βλ5I(λ) 8πn2cτr
∫ λI(λ)dλ
(1)
where I(λ) represents the experimental emission intensity at the wavelength λ, c is the light velocity, n, β and τr are the refractive index, branching ratio and radiative lifetime, respectively. To estimate radiative lifetime of 4I13/2 level the experimental oscillator strength Pexp for the 4I15/2 → 4I13/2 absorption band was calculated employing the following formula:
Pexp =
2.303mc2 Nπ e2
∫ α(ν )dν
(2)
3.3. Relaxation of the excited states
where N denotes the concentration of dopant ions, m, e and c are the electron mass, electron charge and the velocity of the light respectively. The symbol α is the molar absorptivity at the energy ν (cm− 1). Subsequently, the rate Ar of the 4I13/2 → 4I15/2 radiative transition was evaluated making use of relations:
Ar =
8π 2 e2n2 2J′ + 1 × × Pexp mcλ2 2J + 1
The experimental lifetimes of erbium and ytterbium luminescent levels measured for glasses and glass-ceramic systems are gathered in Table 1. Additionally, Fig. 7 compares experimental luminescence decay curves recorded for an as-melted 0.2%Er, 0.6%Yb glass sample. The relatively short experimental lifetimes (2–3 μs) were measured for thermally coupled 2H11/2, 4S3/2 levels in glasses and glass ceramics. Comparably, slightly shorter (1–2 μs) experimental lifetimes were found for the 4F9/2 excited state. Relaxation processes of excited states of erbium ions consist of radiative transitions, nonradiative multiphonon relaxation and nonradiative decay by ion-ion interaction. Unlike radiative transitions which are weakly influenced by a host matrix the nonradiative processes are significantly host-dependent and play a crucial role in the decays of excited states. The highest energy phonons corresponding to Si-O vibrations (~ 1000–1100 cm− 1) have been found in silicate host [32]. Accordingly, impact of multiphonon relaxation on 2H11/2,4S3/2 and 4F9/2 excited states relaxation dynamic is significant because energy gaps between luminescen levels and next lower energy levels can be bridged by simultaneous emission of 2–3 phonons. The 2F5/2 level of ytterbium and 4I11/2 level of erbium are usually coupled together by non-resonant energy transfer and so they decay with a common rate. The decay curves measured within 10,230 cm− 1–10,319 cm− 1 spectral range were used to estimate 2F5/ 3+ 4 )/ I11/2 (Er3 +) experimental lifetimes. It follows from these 2 (Yb findings that the 2F5/2/4I11/2 luminescence is significantly affected by the quenching process especially in samples doped with higher concentration of erbium and ytterbium ions. To assess the radiative lifetime of 2F5/2 level the experimental oscillator strength Pexp and radiative transition rate Ar were calculated applying 2F7/2 → 2F5/2 absorption band and equations 2–3. It was found that Pexp = 3.99 × 10− 6, Ar = 818 s− 1 and τr(2F5/2) = 1.22 ms. It is worth noticing that the experimental lifetime of the 2F5/2 level for as-melted glass single doped with 3% Yb was estimated to be 1.05 ms. Impact of the erbium co-doping on the 2F5/2 relaxation dynamic is significant in
(3)
where n is the refractive index, the values of J′ and J are 13/2 and 15/2 respectively and λ denotes emission wavelength. In this way the Pexp = 1.91 × 10− 6, Ar = 167 s− 1 and finally τr(4I13/2) = 7.26 ms were determined. These data were used to estimate 4 I13/2 → 4I15/2 stimulated emission cross section σem. Fig. 6 (graph (b)) compares calculated emission cross section to experimental 4 I15/2 → 4I13/2 absorption cross section. The highest value of emission cross section σem = 0.57 × 10− 20 cm2 was found at λ = 1534 nm in oxyfluoride glass investigated. For a comparison the σem = 0.42 × 10− 20 cm2 was reported in the Er-doped ZBLAN glass [28] and σem = 0.55 × 10− 20 cm2 was documented for Er-doped silicate glass [29]. It is justified to assume that the adverse phenomenon associated with self-absorption phenomena can give rise to effective reduction of NIR emission originating in first Er3 + excited state. To determine quantitatively the influence of this adverse phenomenon a so-called effective emission cross sections σeff (λ) that takes into account absorption losses has been evaluated according to the relation (4)
σeff (λ) = Pσem (λ) − (1 − P)σabs (λ)
where P is the population inversion parameter denoted as the ratio of a number of erbium ions in the excited state to a total number of erbium ions in the material. Fig. 6 (graph c) shows results of calculation for several reasonable P values. It follows from this figure that the peak value of σeff (λ) = 0.4 × 10− 21 cm2 was found for P = 0.3 at λ = 1594 nm. These findings imply that the optical amplification of
Table 1 Experimental lifetimes of excited states of erbium and ytterbium ions measured for glass and glass-ceramic systems. Sample
4
0.2%Er, 0.6%Yb as-melted 0.2%Er, 0.6%Yb 750 °C_2h 0.5%Er, 1.5%Yb as-melted 0.5%Er, 1.5%Yb 750 °C_2h 1%Er, 3%Yb as-melted 1%Er, 3%Yb 750 °C_2h
3 3 2 2 2 2
S3/2 (Er) [μs] ( ± 0.22) ( ± 0.25) ( ± 0.16) ( ± 0.15) ( ± 0.15) ( ± 0.14)
4
F9/2 (Er) [μs]
1 1 2 2 1 1
( ± 0.07) ( ± 0.07) ( ± 0.08) ( ± 0.08) ( ± 0.07) ( ± 0.07)
179
2
F5/2 (Yb)/4I11/2 (Er) [μs]
4
554 ( ± 3.91) 538 ( ± 4.42) 157 ( ± 2.21) 143 ( ± 1.95) 48 ( ± 1.14) 47 ( ± 1.69)
4802 4471 4057 4123 4257 4263
I13/2 (Er) [μs] ( ± 67) ( ± 25) ( ± 103) ( ± 127) ( ± 104) ( ± 62)
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Fig. 7. Luminescence decay curves measured for luminescent Er3 + and Yb3 + levels in oxyfluoride glass systems.
glass and glass ceramics. Experimental lifetime of the 2F5/2/4I11/2 excited states is reduced to 554 μs in oxyfluoride glass doped with 0.2% Er and 0.6% Yb and drops to 48 μs in 1% Er, 3% Yb glass sample. The nature of this effect is quite complex because the variation of the 2 F5/2 relaxation is governed by the change of the intrinsic decay rate induced by the change of Yb-Er energy transfer rate. On the other hand, it can be seen in Table 1 that experimental lifetimes of the 4I13/2 multiplet exceed 4 ms and the highest value of 4.8 ms was found for a glass sample containing the lowest concentration of erbium ions. This value can be compared to the corresponding 4I13/2 radiative lifetime of 7.26 ms determined using Eq. (3). Accordingly, the quantum efficiency defined as the ratio of the experimental luminescence lifetime to the radiative lifetime is slightly lower than 70%. Actually, the erbium and ytterbium ions are partially incorporated into fluoride crystalline
phases in glass-ceramic samples. The impact of heat treatment process (750 °C, 2 h) on Er3 + and Yb3 + excited states relaxation dynamic is ineffective, however. 3.4. Up-conversion phenomena Observed up-conversion phenomena will be interpreted referring to energy levels scheme for Er3 + and Yb3 + in oxyfluoride glass constructed using spectroscopic data and presented in Fig. 8. The solid arrows in this scheme indicate radiative transitions related to absorption or emission of photons while the dashed lines denote nonradiative processes. It was previously described that Er3 + emission from 4F7/2, 2 H11/2, 4S3/2 and 4F9/2 levels can be excited at 380 nm or 445 nm. Consequently, lower-lying erbium emitting levels 4I11/2 and 4I13/2 are
Fig. 8. Er3 + and Yb3 + energy levels scheme (a). Upconversion spectra excited at 980 nm measured for as-melted (b) and glass-ceramic (c) samples containing 0.2%Er,0.6%Yb, 0.5% Er,1.5%Yb and 1%Er,3%Yb.
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glass-ceramic samples doped with different concentrations of the optically active ions, namely 0.2%Er, 0.6%Yb; 0.5%Er, 1.5%Yb and 1%Er, 3%Yb. The band centered at about 550 nm is related to the 2H11/ 4 4 2, S3/2 → I15/2 transition and the one centered at about 650 nm is associated with the 4F9/2 → 4I15/2 transition of Er3 +. It can be inferred from these spectra that up-converted emission is enhanced for samples containing higher concentrations of the erbium and ytterbium ions. On the other hand the impact of Er3 + and Yb3 + content on intensity ratio of the green to red luminescence is significantly effective. In the case of the 0.2%Er, 0.6%Yb sample the ratio of integrated intensities of the green to red emission is roughly 1:0.8 for as-melted glass and 1:2 for glass-ceramic. For the 0.5%Er, 1.5%Yb and 1%Er, 3%Yb glass samples these ratios are 1:1.7 and 1:2.7, respectively. In respect to that the I550/ I660 emission intensity ratios increases to 1:2.5 and 1:3.2 for the glassceramic counterparts, respectively. Fig. 9 compares up-converted emission spectra in the visible region for as-melted glass and glassceramic containing 0.2% of Er and 0.6% of Yb excited at 980 nm. It can be seen that these spectra are substantially different. The luminescence spectrum measured for a heat-treated sample shows fine structures i.e. maxima of the band components at 522, 528, 539, 544 and 553 nm for 2 H11/2, 4S3/2 → 4I15/2 transition and at 652, 656 and 668 nm for 4F9/ 4 2 → I15/2 transition, respectively. It has been previously observed that in optical materials co-coped with Er3 + and Yb3 + an increase of ytterbium concentration enhances the 4F9/2 → 4I15/2 up-converted emission intensity as compared to the 2H11/2,4S3/2 → 4I15/2 one when the λ = 980 nm excitation wavelength is applied [33]. The dependences of integrated up-converted emission on the excitation power at 980 nm is presented in Fig. 10. The slope of the linear fit is close to 2 for both up-converted emissions in glass samples and (0.2%Er, 0.6%Yb/ 750 °C 2 h) glass-ceramic implying that the thermally coupled 2H11/ 4 4 2, S3/2 levels and F9/2 level are populated as a result of two-photon excitation process. The up-conversion process may involve an excited state absorption (ESA) and/or a two-step Yb–Er energy transfer (ETU).
Fig. 9. Up-converted emission spectra related to the (2H11/2,4S3/2)-4I15/2 and 4F9/2-4I15/2 transitions in 0.2%Er, 0.6%Yb glass and glass-ceramic samples excited at 980 nm.
populated as result of radiative and mainly nonradiative transitions. More efficient 4I13/2 → 4I15/2 Er3 + emission may be achieved when the optical excitation at about 800 nm through the 4I15/2 → 4I9/2 pump transition or at about 980 nm through the 4I15/2–4I11/2 pump transition of erbium is employed. The absorption intensity of the latter transition is frequently higher and more useful in practice. Moreover, ytterbium ions are usually characterized by broad and intense 2F7/2 → 2F5/2 absorption band and eventually the excitation energy (λ ≈ 980 nm) can be conveniently transferred to erbium ions in Er,Yb co-doped optical systems. Actually, at high excitation power the contribution of additional losses corresponding to up-converted emission in the Yb–Er system should be considered. Fig. 8 shows room temperature upconverted spectra excited at 980 nm and recorded for the glass and
Fig. 10. Integrated up-converted emission intensity versus power of incident infrared radiation at 980 nm for glass and glass-ceramic systems.
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The 4I11/2 erbium excited state is in energy resonance with 2F5/2 state of Yb3 +. As a consequence an efficient Yb-Er energy transfer occurs. The green up-converted emission is excited as result of a second energy transfer (ETU) and/or excited state absorption (ESA) from 4I11/2 Er3 + excited state. On the other hand those processes originating in 4I13/2 level are responsible for effectiveness of erbium red up-converted emission when the first excited state of erbium is populated mainly by nonradiative relaxation from 4I11/2 level. Since, the radiative effectiveness of 2F5/2/4I11/2 levels decreases at higher Er3 + and Yb3 + concentrations in the materials under study (see Table 1), the efficient nonradiative 4I11/2 → 4I13/2 energy flow occurs. The experimental lifetime of 4I13/2 is quite long (> 4 ms) for investigated glasses independently on activator content and its quantum efficiency was found to be ~70%. In respect to that the 4F9/2 → 4I15/2 red upconverted erbium emission is efficiently excited as result of effective YbeEr energy transfer. With reference to that the enhancement of the red up-converted emission efficiency in these systems doped with higher ytterbium concentration is due to an additional two consecutive energy transfers from Yb3 + to Er3 +, namely:
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(1) Yb3 +(2F5/2) + Er3 +(4I15/2) → Yb3 +(2F7/2) + Er3 +(4I11/2) (2) Yb3 +(2F5/2) + Er3 +(4I13/2) → Yb3 +(2F7/2) + Er3 +(4F9/2) 4. Conclusions The silica-based oxyfluoride multicomponent glasses co-doped with different concentration of erbium and ytterbium were fabricated. The heat-treatment of the as-melted glasses resulted in precipitation of the fluoride crystalline phases and finally glass-ceramics have been manufactured. Examinations performed reveal that optically active ions can be conveniently incorporated into ordered fluoride components in glass-ceramics. The spectroscopic properties of as-melted glasses and glass-ceramics are clearly different especially when the up-conversion phenomena is considered. Relaxation dynamic of 2F5/2/4I11/2 excited states is strongly determined by erbium and ytterbium concentrations. As result of that the higher content of luminescent activators and heattreatment process give rise to enhancement particularly of 4F9/2 → 4I15/ 2 erbium red up-converted luminescence. In the case of the heavily doped materials this unfavorable effect can affect the efficiency of the potential optical NIR 4I13/2 → 4I15/2 optical amplification that is expected within 1565–1640 nm spectral range. Acknowledgment The National Science Centre (Poland) supported this work under research project 2014/13/B/ST8/04041.
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Oxyfluoride silicate glasses and glass-ceramics doped with erbium and ytterbium - An examination of luminescence properties and up-conversion phenomena
MARK
Radosław Lisieckia,⁎, Elwira Czerskab, Michał Żelechowerb, Radosław Swadźbac, Witold Ryba-Romanowskia a b c
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wrocław 50-422, Poland Department of Material Science, Silesian University of Technology, Katowice 40-019, Poland Institute for Iron Metallurgy, Gliwice 44-100, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxyfluoride glasses Glass-ceramics Up-conversion phenomena
Oxyfluoride glasses doped with erbium and ytterbium have been fabricated at T = 1460 °C. To prepare glassceramic systems, the precursor glasses were annealed at temperature 750 °C for 2 h. The ordered fluoride-like crystalline phases were recognized applying XRD, SAED and EDS methods. The spectroscopic properties of the glass-ceramic samples were significantly different as compared to those observed in the precursor glass. Particularly, the emission spectra of glass-ceramics are evidence that some of Er3 + and Yb3 + ions reside into the ordered environment. Up-conversion phenomena attributed to visible anti-stokes emission of Er3 + has been observed under excitation of Yb3 + ions by a 980 nm diode laser. Especially, the substantial enhancement of red up-converted luminescence was observed in the glass-ceramic system. Dissimilar spectral characteristic and relation of both aforementioned emissions examined in glass and glass-ceramic samples point at different contributions of processes relevant to up-conversion phenomena. The detailed analysis of the crucial Er3 + and Yb3 + excited states relaxation dynamics and their spectroscopic qualities has been performed to determine the mechanisms which lead to conversion of the infrared excitation into visible emission.
G RA P H I C A L AB S T R A C T
⁎
Corresponding author. E-mail address: [email protected] (R. Lisiecki).
http://dx.doi.org/10.1016/j.matdes.2017.04.046 Received 6 March 2017; Received in revised form 10 April 2017; Accepted 12 April 2017 Available online 13 April 2017 0264-1275/ © 2017 Elsevier Ltd. All rights reserved.
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1. Introduction Trivalent erbium ions in various crystalline and non-crystalline hosts are able to show efficient luminescence and attract much attention because of their practical applications as optical amplifiers, near-infrared solid-state laser media or NIR-to-visible up-conversion phosphors [1–5]. Inorganic glasses form an important class of host materials for Er3 + ions owing to the low cost and ease of fabrication combined with an excellent optical quality. It has been demonstrated that Er-doped glasses or glass fibers can operate as optical amplifiers in the standard telecommunication window [6–10]. Numerous published papers have been devoted to the preparation, structural and optical properties of different types of glass systems and results gathered indicate that erbium-doped silicate glasses offer favorable properties and are worth considering. In fact, it has been demonstrated that silicate glasses containing erbium ions are promising for active fibers fabrication [11]. Aluminosilicate glasses doped with erbium have been investigated as promising media for producing laser generation in the spectral range around 1.5 μm [12–15]. It has been reported that a higher SiO2 content in calcium aluminosilicate glasses brings about an increase of the erbium laser emission efficiency by 30% [16]. Also, the incorporation of heavy metal oxide and/or fluoride component to the silicate host significantly increases radiative parameters of Er3 + ions. Moreover, it has been observed that the efficiency of the NIR erbium emission can be enhanced markedly by the creation of CaF2 nanocrystals in aluminosilicate host co-doped with erbium and cerium [17]. This finding has stimulated the investigation of oxyfluoride glass-ceramics, materials which merge excellent qualities of glasses (easy shaping and low cost of fabrication) into some advantages of single crystals (narrow spectral lines and high efficiency of luminescent levels) and may be promising as laser active media. Oxyfluoride silicate glasses and glassceramics differing in chemical composition and type of fluoride nanocrystals have been prepared to accommodate luminescent rare earth ions. In particular, the precipitation of NaYF4 nanocrystals and their effect on the Yb-Er energy transfer in aluminosilicate glass ceramics has been considered [18]. It has been demonstrated that the creation of NaYF4 nanocrystals in multicomponent borosilicate glass containing Er3 + and Yb3 + ions brings about a significant increase of up-converted visible luminescence [19]. Investigation of structure and optical band gap of multicomponent aluminosilicate glass-ceramics has revealed that CaF2 was the only precipitated crystalline phase [20]. Upconversion luminescence excited at 980 nm was observed from Er3 +/ Yb3 +-doped ternary SiO2-Al2O3-PbF2 glass and glass-ceramic samples excited at 1550 nm [21]. Creation of BaLuF5 and their effect on Tb3 + luminescence have been reported for aluminosilicate glass-ceramic samples [22]. Quite recently a precipitation of CaF2 nanocrystals and energy transfer phenomena between Eu3 +-Tb3 +, Eu3 +-Dy3 +, Ce3 +Dy3 +, and Ce3 +-Eu3 + ion pairs in aluminosilicate glass-ceramics have been studied [23,24]. Precipitation of Na(Gd,Y)F4 and their advantageous effect on erbium luminescence in multicomponent aluminosilicate glass has been demonstrated [25]. It has been demonstrated also that a two-step thermal treatment of multicomponent aluminosilicate glass containing erbium and ytterbium results in enhancement of upconverted green emission [26]. In this work we report on structural peculiarity, morphology, and optical properties of oxyfluoride glass-ceramics obtained by thermal treatment of multicomponent lead aluminosilicate glass co-doped with erbium and ytterbium. Results obtained during of X-ray diffraction measurement, as well as images and electron diffraction patterns provided by a high resolution transmission electron microscope technique were analyzed thoroughly to infer the nature of crystalline precipitates. Decay curves of luminescence originated on excited states of erbium and ytterbium ions were recorded and analyzed to determine excited state relaxation dynamics in the system under study. Results of spectroscopic measurement were employed to assess spectral parameters relevant to near infrared laser operation and phenomena of up-
Fig. 1. X-ray diffraction patterns of as-melted glass, glass-ceramic sample (GC) and the fluorite (CaF2) reference pattern.
conversion of infrared radiation into visible luminescence. As far as we know results presented in the following are new and original. 2. Experimental 2.1. Sample preparation The starting reagents were melted to obtain the glass samples with the batch composition in mol%: 48%SiO2–11%Al2O3–7% Na2O–10%CaO–10%PbO–(14-x-y %)PbF2 where x = 0.2, 05, 1% ErF3 and y = 0.6, 1, 3% YbF3. The anhydrous Na2O, PbO, PbF2 and LnF3 (Ln = Er, Yb) were used. The PbF2 was applied with a small excess due to the fluorine loss that may occur in the preparation procedure. The aforementioned reagents were thoroughly mixed and introduced into the corundum crucible in dry box. The resistance furnace operated at 1460 °C was utilized to melt the batch for 1 h and 40 min in normal atmosphere. The liquid material was poured into brass mold. To obtain the glass-ceramics samples, the precursor (as-melted) ingots were cut into pieces and heat-treated at temperature 750 °C and for 2 h to activate crystallization process. 2.2. Measurement set-ups The structural properties of the materials have been determined by X-ray diffraction (XRD-X-Pert Philips diffractometer, curved graphite monochromator, CuKα radiation at 40 kV and 30 mA). The morphological features were measured with a transmission electron microscope (HRTEM JEOL JEM-3010 equipped with the GATAN CCD slow-scan camera at an accelerating voltage of 300 kV and FEI TITAN 80-300 at an accelerating voltage of 300 kV). Excitation and luminescence spectra were recorded employing an Optron Dong Woo fluorometer system containing an ozone-free Xe lamp, a primary monochromator with 150 mm focal length, an emission monochromator with 750 mm focal length and a Hamamatsu R-955 photomultiplier. Up-converted luminescence spectra were excited by an Apollo Instruments diode laser emitting continuous wave (CW) radiation at 980 nm. An InGaAs detector was applied to measure emission spectra within NIR spectral range. Luminescence decay curves were recorded upon selective excitation provided by a Continuum Surelite optical parametric oscillator (OPO) pumped with the third harmonic of Nd:YAG. The resulting transients were attained with R-955 photomultiplier and a cooled InSb Janson J10D detector and connected to a Tektronix Model MDO4054B3 digital oscilloscope. A part of luminescence spectra were excited by means of a Coherent Libra femtosecond laser system coupled to a Light Conversion OPerA Optical Parametric Amplifier. Influence of femtose175
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Fig. 2. Bright field TEM image of the glass-ceramics grain (a), electron diffraction pattern of the white circle area (b), simulated (the ELDYF software) electron diffraction pattern of the hexagonal PbF2 with indexed planes (c) and X-ray spectra (d).
cond pulse on sample was measured exploiting a Hamamatsu C5650 streak camera coupled to an Acton 2500i monochromator.
melted sample reveals the pattern characteristic for an amorphous state. Intense and sharp diffraction peaks appear within 28–88° 2Θ in the XRD pattern of heat-treated glass co-doped with erbium and ytterbium. In some cases the lanthanide ions can be considered as nucleation agents that result in more efficient crystallization process [27]. The crystalline phase formed in glass-ceramic has been identified as the fluorite-like CaF2 belonging to cubic system with Fm-3m space group. High resolution transmission electron microscope technique was utilized to examine structural and morphological properties of the glass-ceramic samples. Fig. 2 presents TEM image of the glass-ceramic grain, electron diffraction pattern of the hexagonal PbF2 phase (space
3. Results and discussion 3.1. Morphological and structural characterization Fig. 1 displays X-ray diffraction patterns of Er, Yb double-doped asmelted glass and of glass-ceramic sample annealed at 750 °C for 2 h. It can be seen that the oxyfluoride glass undergoes structural modification as a result of the heat-treatment process. The X-ray diffraction of the as-
Fig. 3. Bright field TEM image of the glass-ceramic grain (a), high resolution TEM image showing nanocrystals embedded in the glassy host (b), the Fast Fourier Transform (FFT) of the image equivalent to the corresponding electron diffraction pattern, identified as the cubic fluorite-like CaF2 structure (c) and X-ray spectra (EDS) of the dark particle in the image (a) indicating high content of Pb, Yb, Er against their lower content in the glassy host (d).
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Fig. 4. Excitation spectra of Er3 + emission monitored at λ = 1536 nm (upper) and emission spectra excited at λ = 445 nm (lower).
oxyfluoride glass. Luminescence originating from the 4F7/2, 2H11/2, S3/2, 4I11/2 and 4I13/2 was measured. Because the NIR 4I13/2 → 4I15/2 erbium emission can be effectively excited employing commercial AlGaAs and InGaAs diode lasers the excitation spectra were recorded also at longer wavelengths in near infrared region. It can be seen in Fig. 4 that two bands centered at 802 and 974 nm appear in this region. The former band, related to the 4I15/2 → 4I9/2 transition is weakly affected by erbium concentration. The latter band is more prominent and its intensity depends on Yb3 + concentration since it is related to partly overlapping 4I15/2 → 4I11/2 (Er) and 2F7/2 → 2F5/2 (Yb) transitions. Actually, the 1.55 μm erbium emission can be effectively excited within 910–1010 nm spectral range by means of Yb-Er energy transfer. Lower-part of Fig. 4 presents visible and near-infrared luminescence spectrum excited at 445 nm in glass (0.2%Er, 0.6%Yb) sample. The visible luminescence spectrum consists of several emission bands. The emission centered around 495 nm can be assigned to 4F7/2 → 4I15/2 transition and the green erbium luminescence originates in thermally coupled 2H11/2, 4S3/2 excited states and terminates on erbium ground state. The blue excitation light activates also a near infrared emission in 964–1080 nm spectral range. Owing to the fact that the 4I11/2 multiplet of Er3 + and the 2F5/2 multiplet of Yb3 + have nearly the same energies the Er-Yb excitation energy transfer can be readily acquired. Accordingly, the observed NIR emission is superposition of the 4I11/2 → 4I15/2 (Er) and 2F5/2 → 2F7/2 (Yb) transitions. It is worth noticing that a red emission related to the 4F9/2 → 4I15/2 transition is efficiently suppressed in samples of oxyfluoride glass independent on Er3 + and Yb3 + concentrations. The contribution of a red band to luminescence spectra excited by femtosecond pulses at 380 nm can be discerned, however. Fig. 5 exemplifies streak camera images for 0.2%Er, 0.6%Yb glass and glass-ceramic emission spectra recorded upon femtosecond excitation at 380 nm. It can be seen that in addition to a strong green luminescence the 4F9/2 → 4I15/2 red emission, albeit weak, contributes to the spectra. The NIR emission of Er3 + related to the 4I13/2 → 4I15/2 transition in oxyfluoride (0.2%Er, 0.6%Yb) glass and glass-ceramic samples was registered under excitation with a laser diode at 980 nm and the adequate spectra are depicted on the left side of Fig. 6. The spectrum measured for a glass sample is poorly resolved and corresponds to
group P6-168; reference code 00-049-1518 PDF4 database) with indexed planes and X-ray spectra (EDS) of the white circle area indicating high content of Pb, Yb, Er against their lower content in the glassy host. Fig. 3 displays high resolution TEM image showing nanocrystals in the glassy host, electron diffraction pattern of the cubic CaF2 phase (space group Fm-3m-225; reference code 01-075-0097 PDF4 database) and X-ray spectra (EDS) of the dark particle in the adequate TEM image. It is worth noticing that M. Rezvani and L. Farahinia examined SiO2-Al2O3-CaO-CaF2 oxyfluoride glasses and glass-ceramics containing CaF2 nanocrystals [20]. It can be discerned in our materials that for glass-ceramic sample numerous crystalline species are embedded into amorphous phase. The TEM images exemplify the fairly monodisperse and spherical in shape crystalline components. Electron diffraction study confirms XRD results pointing that the ordered areas belong to cubic fluorite-like structure (CaF2) and hexagonal phase (PbF2). On the other hand the energy dispersive Xray spectroscopy analysis implied that crystalline particles comprise mainly lead, ytterbium and erbium with lower content of calcium and sodium. Accordingly, the presence of the crystallites characterized by isomorphic hexagonal PbF2 structure can be confirmed as well.
4
3.2. Spectra and down-converted emission The excitation spectrum of Er3 + emission monitored at λ = 1530 nm in oxyfluoride glass (0.2%Er,0.6%Yb) sample recorded at room temperature in the UV–visible NIR wavelength region is displayed in Fig. 4. Moderately resolved bands are attributed to the electronic transitions from the 4I15/2 ground state to the high-lying excited levels of Er3 +. Two bands located at 380 and 520 nm are especially prominent and correspond to 4I15/2 → 4G11/2 and 4I15/ 2 3+ transitions, respectively. These are usually called 2 → H11/2 Er “hypersensitive” transition and their positions and intensities are very sensitive to small changes of environment around rare earth ions. The observed bands are structureless due to fully amorphous character of the sample examined. The excitation spectrum was utilized to select the wavelengths that can be effectively applied to activate the erbium luminescence. Several excited state of erbium are luminescent in 177
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Fig. 5. Emission spectra recorded for as-melted and glass-ceramic samples doped with 0.2%Er and 0.6%Yb glasses upon femtosecond excitation at 380 nm. Images attained with a streak camera revealing details of spectral distribution of sample luminescence intensity in spectral region 500–700 nm.
overlapping transitions between crystal field levels of the excited multiplet and crystal field components of erbium ground state. In comparison to the precursor glass the spectral characteristic of the
glass-ceramic is clearly different. It can be seen that emission line-width distinctly decreased in the studied oxyfluoride glass-ceramic system. Especially, this effect is noticeable within long-wavelength wing of the
Fig. 6. The NIR emission spectrum attributed to 4I13/2-4I15/2 transition recorded for 0.2%Er, 0.6%Yb glass and 0.2%Er, 0.6%Yb glass-ceramic (750 °C_2h) excited at 980 nm (a). Absorption (4I15/2 → 4I13/2) and emission (4I13/2 → 4I15/2) cross section spectra (b) and estimated effective emission cross-section (4I13/2 → 4I15/2) for several P parameters in oxyfluoride glass (c).
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erbium emission in material under study should be expected particularly within 1565–1640 nm. Results presented above deserve some comments because they differ markedly from those obtained during investigation of erbium-doped oxyfluoride glasses and glass ceramics with similar chemical composition and reported in [30]. In contrast to our findings Authors of ref. [30] observed a very weak green luminescence related to the 2H11/ 4 4 2, S3/2 → I15/2 transition. Instead, they recorded a very intense band located in near infrared between ca 1600–1700 nm and assigned this band to the 4S3/2 → 4I9/2 transition. According to spectra shown in Ref. [30] its intensity is markedly higher than that of neighboring band related to the 4I13/2 → 4I15/2 transition. This finding implies very unusual set of branching ratios for the 4S3/2 luminescence of Er3 + differing strongly from that calculated within a framework of the JuddOfelt theory and reported in [31]. Our attempt to discern the 4S3/ 4 2 → I9/2 infrared luminescence in our samples were unsuccessful even when exciting by powerful femtosecond light pulses. Accordingly, we believe that experimental results gathered in the present work are consistent with calculated spectroscopic parameters given in [31].
spectra. Furthermore, the spectrum attributed to glass-ceramic sample contains numerous emission band components. It follows from different distribution of erbium crystal field environments in the oxyfluoride sample comprising ordered fluoride crystalline components. In respect to that it can be assumed that a part of the trivalent erbium was incorporated into the ordered crystalline phase. One of the most important parameters that affect laser performance of a luminescent material is the stimulated emission cross section σem. We calculated the σem(λ) applying the Füchtbauer-Ladenburg formula:
σem (λ) =
βλ5I(λ) 8πn2cτr
∫ λI(λ)dλ
(1)
where I(λ) represents the experimental emission intensity at the wavelength λ, c is the light velocity, n, β and τr are the refractive index, branching ratio and radiative lifetime, respectively. To estimate radiative lifetime of 4I13/2 level the experimental oscillator strength Pexp for the 4I15/2 → 4I13/2 absorption band was calculated employing the following formula:
Pexp =
2.303mc2 Nπ e2
∫ α(ν )dν
(2)
3.3. Relaxation of the excited states
where N denotes the concentration of dopant ions, m, e and c are the electron mass, electron charge and the velocity of the light respectively. The symbol α is the molar absorptivity at the energy ν (cm− 1). Subsequently, the rate Ar of the 4I13/2 → 4I15/2 radiative transition was evaluated making use of relations:
Ar =
8π 2 e2n2 2J′ + 1 × × Pexp mcλ2 2J + 1
The experimental lifetimes of erbium and ytterbium luminescent levels measured for glasses and glass-ceramic systems are gathered in Table 1. Additionally, Fig. 7 compares experimental luminescence decay curves recorded for an as-melted 0.2%Er, 0.6%Yb glass sample. The relatively short experimental lifetimes (2–3 μs) were measured for thermally coupled 2H11/2, 4S3/2 levels in glasses and glass ceramics. Comparably, slightly shorter (1–2 μs) experimental lifetimes were found for the 4F9/2 excited state. Relaxation processes of excited states of erbium ions consist of radiative transitions, nonradiative multiphonon relaxation and nonradiative decay by ion-ion interaction. Unlike radiative transitions which are weakly influenced by a host matrix the nonradiative processes are significantly host-dependent and play a crucial role in the decays of excited states. The highest energy phonons corresponding to Si-O vibrations (~ 1000–1100 cm− 1) have been found in silicate host [32]. Accordingly, impact of multiphonon relaxation on 2H11/2,4S3/2 and 4F9/2 excited states relaxation dynamic is significant because energy gaps between luminescen levels and next lower energy levels can be bridged by simultaneous emission of 2–3 phonons. The 2F5/2 level of ytterbium and 4I11/2 level of erbium are usually coupled together by non-resonant energy transfer and so they decay with a common rate. The decay curves measured within 10,230 cm− 1–10,319 cm− 1 spectral range were used to estimate 2F5/ 3+ 4 )/ I11/2 (Er3 +) experimental lifetimes. It follows from these 2 (Yb findings that the 2F5/2/4I11/2 luminescence is significantly affected by the quenching process especially in samples doped with higher concentration of erbium and ytterbium ions. To assess the radiative lifetime of 2F5/2 level the experimental oscillator strength Pexp and radiative transition rate Ar were calculated applying 2F7/2 → 2F5/2 absorption band and equations 2–3. It was found that Pexp = 3.99 × 10− 6, Ar = 818 s− 1 and τr(2F5/2) = 1.22 ms. It is worth noticing that the experimental lifetime of the 2F5/2 level for as-melted glass single doped with 3% Yb was estimated to be 1.05 ms. Impact of the erbium co-doping on the 2F5/2 relaxation dynamic is significant in
(3)
where n is the refractive index, the values of J′ and J are 13/2 and 15/2 respectively and λ denotes emission wavelength. In this way the Pexp = 1.91 × 10− 6, Ar = 167 s− 1 and finally τr(4I13/2) = 7.26 ms were determined. These data were used to estimate 4 I13/2 → 4I15/2 stimulated emission cross section σem. Fig. 6 (graph (b)) compares calculated emission cross section to experimental 4 I15/2 → 4I13/2 absorption cross section. The highest value of emission cross section σem = 0.57 × 10− 20 cm2 was found at λ = 1534 nm in oxyfluoride glass investigated. For a comparison the σem = 0.42 × 10− 20 cm2 was reported in the Er-doped ZBLAN glass [28] and σem = 0.55 × 10− 20 cm2 was documented for Er-doped silicate glass [29]. It is justified to assume that the adverse phenomenon associated with self-absorption phenomena can give rise to effective reduction of NIR emission originating in first Er3 + excited state. To determine quantitatively the influence of this adverse phenomenon a so-called effective emission cross sections σeff (λ) that takes into account absorption losses has been evaluated according to the relation (4)
σeff (λ) = Pσem (λ) − (1 − P)σabs (λ)
where P is the population inversion parameter denoted as the ratio of a number of erbium ions in the excited state to a total number of erbium ions in the material. Fig. 6 (graph c) shows results of calculation for several reasonable P values. It follows from this figure that the peak value of σeff (λ) = 0.4 × 10− 21 cm2 was found for P = 0.3 at λ = 1594 nm. These findings imply that the optical amplification of
Table 1 Experimental lifetimes of excited states of erbium and ytterbium ions measured for glass and glass-ceramic systems. Sample
4
0.2%Er, 0.6%Yb as-melted 0.2%Er, 0.6%Yb 750 °C_2h 0.5%Er, 1.5%Yb as-melted 0.5%Er, 1.5%Yb 750 °C_2h 1%Er, 3%Yb as-melted 1%Er, 3%Yb 750 °C_2h
3 3 2 2 2 2
S3/2 (Er) [μs] ( ± 0.22) ( ± 0.25) ( ± 0.16) ( ± 0.15) ( ± 0.15) ( ± 0.14)
4
F9/2 (Er) [μs]
1 1 2 2 1 1
( ± 0.07) ( ± 0.07) ( ± 0.08) ( ± 0.08) ( ± 0.07) ( ± 0.07)
179
2
F5/2 (Yb)/4I11/2 (Er) [μs]
4
554 ( ± 3.91) 538 ( ± 4.42) 157 ( ± 2.21) 143 ( ± 1.95) 48 ( ± 1.14) 47 ( ± 1.69)
4802 4471 4057 4123 4257 4263
I13/2 (Er) [μs] ( ± 67) ( ± 25) ( ± 103) ( ± 127) ( ± 104) ( ± 62)
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Fig. 7. Luminescence decay curves measured for luminescent Er3 + and Yb3 + levels in oxyfluoride glass systems.
glass and glass ceramics. Experimental lifetime of the 2F5/2/4I11/2 excited states is reduced to 554 μs in oxyfluoride glass doped with 0.2% Er and 0.6% Yb and drops to 48 μs in 1% Er, 3% Yb glass sample. The nature of this effect is quite complex because the variation of the 2 F5/2 relaxation is governed by the change of the intrinsic decay rate induced by the change of Yb-Er energy transfer rate. On the other hand, it can be seen in Table 1 that experimental lifetimes of the 4I13/2 multiplet exceed 4 ms and the highest value of 4.8 ms was found for a glass sample containing the lowest concentration of erbium ions. This value can be compared to the corresponding 4I13/2 radiative lifetime of 7.26 ms determined using Eq. (3). Accordingly, the quantum efficiency defined as the ratio of the experimental luminescence lifetime to the radiative lifetime is slightly lower than 70%. Actually, the erbium and ytterbium ions are partially incorporated into fluoride crystalline
phases in glass-ceramic samples. The impact of heat treatment process (750 °C, 2 h) on Er3 + and Yb3 + excited states relaxation dynamic is ineffective, however. 3.4. Up-conversion phenomena Observed up-conversion phenomena will be interpreted referring to energy levels scheme for Er3 + and Yb3 + in oxyfluoride glass constructed using spectroscopic data and presented in Fig. 8. The solid arrows in this scheme indicate radiative transitions related to absorption or emission of photons while the dashed lines denote nonradiative processes. It was previously described that Er3 + emission from 4F7/2, 2 H11/2, 4S3/2 and 4F9/2 levels can be excited at 380 nm or 445 nm. Consequently, lower-lying erbium emitting levels 4I11/2 and 4I13/2 are
Fig. 8. Er3 + and Yb3 + energy levels scheme (a). Upconversion spectra excited at 980 nm measured for as-melted (b) and glass-ceramic (c) samples containing 0.2%Er,0.6%Yb, 0.5% Er,1.5%Yb and 1%Er,3%Yb.
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glass-ceramic samples doped with different concentrations of the optically active ions, namely 0.2%Er, 0.6%Yb; 0.5%Er, 1.5%Yb and 1%Er, 3%Yb. The band centered at about 550 nm is related to the 2H11/ 4 4 2, S3/2 → I15/2 transition and the one centered at about 650 nm is associated with the 4F9/2 → 4I15/2 transition of Er3 +. It can be inferred from these spectra that up-converted emission is enhanced for samples containing higher concentrations of the erbium and ytterbium ions. On the other hand the impact of Er3 + and Yb3 + content on intensity ratio of the green to red luminescence is significantly effective. In the case of the 0.2%Er, 0.6%Yb sample the ratio of integrated intensities of the green to red emission is roughly 1:0.8 for as-melted glass and 1:2 for glass-ceramic. For the 0.5%Er, 1.5%Yb and 1%Er, 3%Yb glass samples these ratios are 1:1.7 and 1:2.7, respectively. In respect to that the I550/ I660 emission intensity ratios increases to 1:2.5 and 1:3.2 for the glassceramic counterparts, respectively. Fig. 9 compares up-converted emission spectra in the visible region for as-melted glass and glassceramic containing 0.2% of Er and 0.6% of Yb excited at 980 nm. It can be seen that these spectra are substantially different. The luminescence spectrum measured for a heat-treated sample shows fine structures i.e. maxima of the band components at 522, 528, 539, 544 and 553 nm for 2 H11/2, 4S3/2 → 4I15/2 transition and at 652, 656 and 668 nm for 4F9/ 4 2 → I15/2 transition, respectively. It has been previously observed that in optical materials co-coped with Er3 + and Yb3 + an increase of ytterbium concentration enhances the 4F9/2 → 4I15/2 up-converted emission intensity as compared to the 2H11/2,4S3/2 → 4I15/2 one when the λ = 980 nm excitation wavelength is applied [33]. The dependences of integrated up-converted emission on the excitation power at 980 nm is presented in Fig. 10. The slope of the linear fit is close to 2 for both up-converted emissions in glass samples and (0.2%Er, 0.6%Yb/ 750 °C 2 h) glass-ceramic implying that the thermally coupled 2H11/ 4 4 2, S3/2 levels and F9/2 level are populated as a result of two-photon excitation process. The up-conversion process may involve an excited state absorption (ESA) and/or a two-step Yb–Er energy transfer (ETU).
Fig. 9. Up-converted emission spectra related to the (2H11/2,4S3/2)-4I15/2 and 4F9/2-4I15/2 transitions in 0.2%Er, 0.6%Yb glass and glass-ceramic samples excited at 980 nm.
populated as result of radiative and mainly nonradiative transitions. More efficient 4I13/2 → 4I15/2 Er3 + emission may be achieved when the optical excitation at about 800 nm through the 4I15/2 → 4I9/2 pump transition or at about 980 nm through the 4I15/2–4I11/2 pump transition of erbium is employed. The absorption intensity of the latter transition is frequently higher and more useful in practice. Moreover, ytterbium ions are usually characterized by broad and intense 2F7/2 → 2F5/2 absorption band and eventually the excitation energy (λ ≈ 980 nm) can be conveniently transferred to erbium ions in Er,Yb co-doped optical systems. Actually, at high excitation power the contribution of additional losses corresponding to up-converted emission in the Yb–Er system should be considered. Fig. 8 shows room temperature upconverted spectra excited at 980 nm and recorded for the glass and
Fig. 10. Integrated up-converted emission intensity versus power of incident infrared radiation at 980 nm for glass and glass-ceramic systems.
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The 4I11/2 erbium excited state is in energy resonance with 2F5/2 state of Yb3 +. As a consequence an efficient Yb-Er energy transfer occurs. The green up-converted emission is excited as result of a second energy transfer (ETU) and/or excited state absorption (ESA) from 4I11/2 Er3 + excited state. On the other hand those processes originating in 4I13/2 level are responsible for effectiveness of erbium red up-converted emission when the first excited state of erbium is populated mainly by nonradiative relaxation from 4I11/2 level. Since, the radiative effectiveness of 2F5/2/4I11/2 levels decreases at higher Er3 + and Yb3 + concentrations in the materials under study (see Table 1), the efficient nonradiative 4I11/2 → 4I13/2 energy flow occurs. The experimental lifetime of 4I13/2 is quite long (> 4 ms) for investigated glasses independently on activator content and its quantum efficiency was found to be ~70%. In respect to that the 4F9/2 → 4I15/2 red upconverted erbium emission is efficiently excited as result of effective YbeEr energy transfer. With reference to that the enhancement of the red up-converted emission efficiency in these systems doped with higher ytterbium concentration is due to an additional two consecutive energy transfers from Yb3 + to Er3 +, namely:
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(1) Yb3 +(2F5/2) + Er3 +(4I15/2) → Yb3 +(2F7/2) + Er3 +(4I11/2) (2) Yb3 +(2F5/2) + Er3 +(4I13/2) → Yb3 +(2F7/2) + Er3 +(4F9/2) 4. Conclusions The silica-based oxyfluoride multicomponent glasses co-doped with different concentration of erbium and ytterbium were fabricated. The heat-treatment of the as-melted glasses resulted in precipitation of the fluoride crystalline phases and finally glass-ceramics have been manufactured. Examinations performed reveal that optically active ions can be conveniently incorporated into ordered fluoride components in glass-ceramics. The spectroscopic properties of as-melted glasses and glass-ceramics are clearly different especially when the up-conversion phenomena is considered. Relaxation dynamic of 2F5/2/4I11/2 excited states is strongly determined by erbium and ytterbium concentrations. As result of that the higher content of luminescent activators and heattreatment process give rise to enhancement particularly of 4F9/2 → 4I15/ 2 erbium red up-converted luminescence. In the case of the heavily doped materials this unfavorable effect can affect the efficiency of the potential optical NIR 4I13/2 → 4I15/2 optical amplification that is expected within 1565–1640 nm spectral range. Acknowledgment The National Science Centre (Poland) supported this work under research project 2014/13/B/ST8/04041.
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