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Near-UV sensitized 1.06 μm emission of Nd3+ ions via monovalent copper in phosphate glass
Materials Chemistry and Physics 162 (2015) 425e430
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Near-UV sensitized 1.06 mm emission of Nd3þ ions via monovalent copper in phosphate glass A. Jime nez a, *, Mariana Sendova b Jose a b
Department of Chemistry, University of North Florida, Jacksonville, FL 32224, USA Optical Spectroscopy & Nano-Materials Lab, New College of Florida, Sarasota, FL 34243, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Monovalent copper ions effectively stabilized in Nd3þ-containing phosphate glass. Enhanced Nd3þ near-infrared emission observed upon the Cuþ ions incorporation. Cuþ / Nd3þ non-radiative energy transfer efficiencies and likely energy transfer pathways evaluated. Potential for solid-state lasers and solar spectral conversion suggested.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2014 Received in revised form 3 June 2015 Accepted 7 June 2015 Available online 17 June 2015
Monovalent copper ions effectively incorporated in Nd-containing phosphate glass by a single-step meltquench method have been established as near-ultraviolet (UV) sensitizers of Nd3þ ions, resulting in a remarkable 4F3/2 / 4I11/2 emission at 1.06 mm. The spectroscopic data indicates an efficient energy conversion process. The Cuþ ions first absorb photons broadly around 360 nm, and subsequently transfer the energy from the Stokes-shifted emitting states to resonant Nd3þ energy levels in the visible. Ultimately, the Nd3þ electronic excited states decay and the upper lasing state 4F3/2 is populated, leading to the enhanced emission at 1.06 mm. The characteristic features of the Cuþ visible emission spectra and the reduced lifetime of the corresponding Cuþ donor states indicate an efficient non-radiative transfer. The Cuþ/Nd3þ co-doped phosphate glass appears suitable as solid-state laser material with enhanced pump range in the near-UV part of the spectrum and for solar spectral conversion in photovoltaic cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Glasses Optical materials Photoluminescence spectroscopy
1. Introduction Laser technology has revolutionized today's society in a variety of fields such as information processing, communications, medicine, and scientific research in general [1,2]. Particularly, Nd3þ solid-state lasers based on high-solubility phosphate glass matrices are much valued for high-power lasing applications [3,4]. This has
* Corresponding author. nez). E-mail address: [email protected] (J.A. Jime http://dx.doi.org/10.1016/j.matchemphys.2015.06.009 0254-0584/© 2015 Elsevier B.V. All rights reserved.
led to the current initiative of exploring the potential of such for laser fusion power plants to generate electricity (concept of Laser Inertial Fusion Energy e LIFE) [5]. Hence, in view of the technological relevance, which nowadays includes helping meet global energy demands, several research groups have investigated on the effects (e.g. sensitizing) of adding co-dopants such as Cr3þ [6], Sn2þ [7], and semiconductor [8] or metallic nanoparticles [9e11], to Nd3þ-containing glass matrices. More than three decades ago, Edwards and Gomulka [6] suggested that monovalent copper ions could sensitize Nd3þ emission in laser glass given the strong spectral overlap of Cuþ emission and
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Nd3þ absorption. However, the authors discounted copper as candidate since at the time Cuþ pumping would need to be performed rather deep in the ultraviolet (UV) below 320 nm, at which the glass matrices would eventually solarize. In addition, stabilizing considerable amounts of Cuþ ions in glass matrices is a non-trivial task given the predominant stability of the Cu2þ ion, especially for syntheses carried out in an ambient atmosphere [12]. Hence, intricate preparation procedures are often employed for doping glasses with Cuþ ions which may include controlled reducing [13], or inert [14] atmospheres. As an alternative, appropriate reducing agents such as tin (II) oxide or antimony (III) oxide can be included as part of the batch materials; yet, carrying out the preparation at high temperatures under an oxidizing air atmosphere still poses risks to the redox chemicals [15e17]. Consequently, studies on the influence of Cuþ ions on the luminescence of rare-earths in glass matrices are scarce [18,19]. Nevertheless, we have recently succeeded in our group [20] in fabricating luminescent Cuþ-doped glasses of a high-solubility bariumephosphate matrix relevant to the field of photonics [3,17]. The effective incorporation of relatively large amounts of monovalent copper has been realized using redox chemicals CuO and SnO, by simply melting batch materials under ambient atmosphere with sucrose as antioxidant agent protecting tin (II). Pertinent to the field of plasmonic materials, this approach also allows for the precipitation of Cu nanoparticles during subsequent thermal treatments [21]. Further, the optical absorption features of the high concentrations of Cuþ ions stabilized in the matrix appeared predominantly in the near-UV, with associated excitation bands around 345 nm [20,21]. Thus, we felt motivated to co-dope the phosphate matrix with Nd3þ ions, since in the event that an efficient Cuþ / Nd3þ energy transfer occurred, the glass system would further overcome the limitation earlier pointed out by Edwards and Gomulka [6]. Accordingly, we herein report on the spectroscopic properties of Cuþ and Nd3þ co-doped bariumephosphate glass attained by the single-step melt-quench method. It is demonstrated that indeed Cuþ ions act as sensitizers of Nd3þ activator ions, leading to a remarkable 1.06 mm emission. Hence, the codoping of the Nd3þ-containing phosphate glass with monovalent copper shows potential for lasing realizable under near-UV light pumping relevant to excitation of Cuþ. Moreover, the UV-to-visible Stokes shift in Cuþ luminescence together with the consequent near-infrared Nd3þ emission appears desirable for solar spectral conversion in photovoltaic cells [22e24], particularly Ge-based solar cells characterized by broad spectral (from the visible to the near-infrared) efficiency [24].
Nd3þ luminescence properties, respectively, in comparison to the CuSneNd glass. All glasses were cut and polished in order to produce glass slabs for optical measurements with final thicknesses of about 1.0 mm. Optical absorption measurements were performed using a PerkineElmer 35 UV/Vis double-beam spectrophotometer. All absorption spectra were recorded with air as reference. Photoluminescence (PL) emission and excitation spectra were collected with a Horiba Jobin-Yvon Fluorolog-3 steady-state spectrofluorometer equipped with a solid-state N2-cooled InGaAs photodiode and a photomultiplier tube. Emission decay curves were recorded with a Photon Technology International QuantaMaster 30 spectrofluorometer equipped with a Xe flash lamp having a pulse width of about 2 ms. The flash lamp was kept operating at a frequency of 125 Hz with the time window for data acquisition set at 8000 ms. All measurements were carried out at room temperature with particular attention given to keep conditions constant during experiments. 3. Results and discussion Fig. 1 shows the optical absorption spectra for the Nd, CuSn and CuSneNd glasses. A significant redshift in the glass absorption edge is observed upon the CuO and SnO co-doping. Herein, a contribution is expected from twofold-coordinated tin centers [21,25]. Yet, the main species accountable for near-UV absorption are the Cuþ ions incorporated into the matrix, due to the 3d10 / 3d94s1 interconfiguration transitions [20,21]. An absorption band rises after 600 nm for the CuSn and CuSneNd glasses, due to the presence of Cu2þ ions which absorb broadly owing to 2E / 2T2 intraconfiguration (ded) transitions [16,20]. The Cu2þ absorption band can be employed for analytical purposes by use of absorption spectra of a set of CuO-containing reference glasses as reported in Ref. [20]. In such spectroscopic approach of chemical analysis, a plot of the peak optical density of the Cu2þ band around 850 nm vs. the amount of CuO added to the reference glasses is constructed. Such plot, based on reported data [20] is shown in the inset of Fig. 1, which can be treated as a calibration curve following Beer's Law. Thus, from the equation of the line the residual amounts of copper (II) in the CuSneNd glass can be estimated. The assessment has
2. Experimental The glasses were made with a 50P2O5:50BaO (mol%) composition, matrix which has been previously proposed by Uchida et al. [17] for nonlinear optical studies of metallic nanoparticles given its intrinsic high metal solubility. These were prepared from high purity compounds (P2O5 and BaCO3) by the melt-quenching technique [20]. In this simple procedure, batch materials are melted in porcelain crucibles at 1150 C for 15 min under normal atmospheric conditions and immediately quenched. In the event that reducing agent tin (II) oxide is present in the batch, sucrose is added as ~10 wt% on top of the well-mixed reagents to assist as an antioxidant preventing oxidation of tin (II) by atmospheric oxygen. Copper, tin, and/or neodymium doping was done by adding CuO, SnO, and/or Nd2O3 quantities in mol%, in relation to network former P2O5. Samples prepared for the present study are (referred to as): CuSn glass (10% of each CuO and SnO); CuSneNd (10% of each CuO and SnO, and 2% Nd2O3); and Nd glass (2% Nd2O3). The purpose of the CuSn and Nd glasses is to serve as references relative to Cuþ and
Fig. 1. Optical absorption spectra of the Nd, CuSn and CuSneNd glasses. The inset shows the plot of peak optical density of the Cu2þ band vs. concentration of CuO added to the reference glasses (squares), Ref. [20]; the solid line is the linear fit to the data. The hexagon is the data point obtained for the CuSneNd glass.
J.A. Jimenez, M. Sendova / Materials Chemistry and Physics 162 (2015) 425e430
been applied to the CuSn glass (10 mol% of each CuO and SnO) as carried out in our group and reported previously [20], yielding 0.20 mol% copper (II) after the melting. As observed in Fig. 1, apart from the Nd3þ bands overlaid, the Cu2þ absorption profile of the CuSneNd glass appears similar in intensity to that of the CuSn glass. By means of the equation of the line resulting from the regression analysis (Fig. 1, inset) and the experimental Cu2þ optical density at 850 nm, a corresponding amount of 0.17 mol% copper (II) oxide is estimated for the CuSneNd glass. The value is plotted together with the calibration curve in the inset of Fig. 1 (hexagonal symbol). Remarkably, the residual CuO quantities estimated for both the CuSn and CuSneNd glasses indicate a 98% reduction efficiency of copper (II). This is a significant result given the relative simplicity of the material preparation procedure carried out under ambient atmosphere. In terms of PL properties, the recent investigation on the CuSn glass has revealed two distinct emitting centers: Sn centers and the single Cuþ ions [20]. The excitation-wavelength dependence of the PL is such that under short-wavelength excitation (e.g. 260 nm), broadband emission is observed towards the blue having a significant contribution from tin (triplet-to-singlet transitions) [20]. However, with increasing excitation wavelength, emission shifts to the red, indicative of larger contributions from the Cuþ (3d94s1 / 3d10 transitions). Ultimately, under excitation at 360 nm and longer wavelengths, the emission spectra for the glass display single-band behavior in agreement with emission arising merely from the Cuþ ions [20]. The sensitization of neodymium ions luminescence by divalent tin was reported by Malashkevich et al. [7]. Henceforth, the focus is on assessing the effect of the Cuþ ions. Moreover, as previously established for relatively long excitation wavelengths (e.g. 360 nm) the contribution of Sn centers to the PL can be neglected [20,21]. Presented as trace (a) in Fig. 2 is the emission spectrum for the CuSn glass obtained under excitation at 360 nm. This spectrum is a reference (under the same excitation) regarding the influence of Nd3þ on Cuþ emission in the CuSneNd glass, trace (b) in Fig. 2. Certainly, the region for Cuþ emission in the glass system is relevant to Nd3þ absorption within the 500e650 nm range, consistent with the assumption by Edwards and Gomulka [6]. Consequently, the
Fig. 2. Spectral overlay of emission spectra obtained under excitation at 360 nm for (a) CuSn and (b) CuSneNd glasses, with (c) the absorption spectrum of the CuSneNd glass; notable Nd3þ ions absorption features are labeled in the latter in correspondence to electronic transitions from the 4I9/2 ground state to excited states specified.
427
broad Cuþ band emission in the CuSneNd glass shows dips in correspondence with some Nd3þ absorption peaks (main spectroscopic states [26] indicated in trace (c) of Fig. 2). Therefore, clear indication exists of a resonant Cuþ / Nd3þ energy transfer channel (e.g. cascade-type) proceeding from the emitting states in Cuþ ions. Additional information about the nature of the Cuþ / Nd3þ energy transfer is obtained from emission decay curves acquired for the CuSn and CuSneNd glasses. Fig. 3 compares PL decay curves obtained under 360 nm excitation for Cuþ emission monitored at 585 nm in correspondence with the prominent 4I9/2 / 4G5/2 absorption in Nd3þ (Fig. 2). The decay curves exhibit non-exponential behavior, where a much faster decay is evident for the CuSneNd glass. In an effort to investigate on the involvement of other Nd3þ states farther away from peak Cuþ emission, decay curves were also recorded (not shown) under the 360 nm excitation by monitoring emission at 525 nm (e.g. 4I9/2 / 2K13/2, 4G7/2 Nd3þ transitions). Curve fitting was achieved with a bi-exponential function, consistent with the fast (tf) and slow (ts) lifetime components characteristic of the Cuþ ions [27]. The determined decay times are summarized in Table 1. Both tf and ts components differ considerably among the two glasses for the two emission wavelengths. The bi-exponential decay in Cuþ emission can be interpreted in terms of a four-level system (schematic shown in Fig. 3, inset), where excitation immediately populates the singlet state of the 3d94s1 configuration (level 4) [27]. Subsequently, electrons undergo intersystem crossing to the triplet state (3Eg), split in two metastable states due to spineorbit coupling (upper level 2, T2g, and lower level 3, T1g) lying close to each other. After a relatively short time period, level 4 is emptied and the system behaves as a threelevel one in which relaxation from upper level T2g is fast, while that from lower level T1g is slower [27]. Accordingly, the tf and ts components deduced from the fits to the decay curves are likely related to Cuþ ions relaxation from T2g and T1g states, respectively, to the 1Ag ground state (level 1). The estimated lifetimes for the CuSn glass (Table 1) are in good agreement with those reported by Debnath and Das [28] and Borsella et al. [29] for Cuþ ions in tetragonally-distorted octahedral sites in glasses. This is in accord with Cuþ emission detected at relatively low energy (Fig. 2), which is expected for a Cuþ environment of octahedral coordination undergoing tetragonal distortion with the non-bridging oxygen ions
Fig. 3. PL decay curves for CuSn and CuSneNd glasses obtained under excitation at 360 nm by monitoring emission at 585 nm. The energy level diagram in the inset shows the states linked to the Cuþ luminescence process.
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Table 1 Fast (tf) and slow (ts) lifetimes of Cuþ ions in the CuSn and CuSneNd glasses determined from experimental decay curves along with estimated Cuþ / Nd3þ energy transfer efficiencies (h). Glass
CuSn CuSneNd
h
lem ¼ 585 nm
lem ¼ 525 nm
tf (ms)
ts (ms)
tf (ms)
ts (ms)
41.1 (±0.3) 23.8 (±0.3) 42.1%
136.1 (±1.1) 73.6 (±1.2) 45.9%
35.2 (±0.3) 22.1 (±0.6) 37.2%
120.5 (±1.4) 73.0 (±3.8) 39.4%
in the amorphous host [28,29]. The differences manifested between the two emission wavelengths (i.e. 585 and 525 nm) in the CuSn glass are expected in accord with the dissimilar Cuþ sites associated [13]. On the other hand, the decay times in the CuSneNd glass are significantly reduced relative to the CuSn glass. This result is consistent with the modification of the Cuþ PL (Fig. 2), and represents clear evidence of an effective non-radiative energy transfer from Cuþ donors to the Nd3þ activator ions [30]. Further, the energy transfer efficiency h can be estimated by [6]
h ¼ 1 ðtCuþ Nd3þ =tCuþ Þ
(1)
where tCuþ Nd3þ and tCuþ are the lifetimes of the sensitizer (Cuþ) in the presence and absence of the activator (Nd3þ), respectively. Such efficiencies have been estimated in association to both fast and slow decay time components in the Cuþ donors for the two emission wavelengths monitored. The results are presented together with the lifetimes in Table 1. Remarkably, efficiencies above 40% are estimated for both lifetime components for the 585 nm emission, with an average between the two of 44%. On the other hand, for emission at 525 nm, the energy transfer efficiencies appear somewhat diminished, with an average between the fast and slow decay time components estimates of 38%. Since the percent differences between the 585 nm and 525 nm efficiencies are 12% and 15% for the fast and slow components estimates, respectively, a higher energy transfer efficiency is indicated in relation to the longer wavelength. This is likely due to the fact that the 4I9/2 / 4G5/2 absorption in Nd3þ ions around 585 nm lies near the peak of the Cuþ broad PL band (Fig. 2), i.e., a larger degree of spectral overlap exists. Thus, a larger population of the monovalent copper ions participates in the non-radiative Cuþ(3Eg) / Nd3þ(4G5/2) energy transfer channel. Still, consistency amongst efficiencies of fast vs. slow decay time estimates suggest that the energy transfer proceeds similarly from the inhomogeneously broadened T2g and T1g emitting states in Cuþ ions to the resonant states in Nd3þ. In fact, comparable energy transfer efficiencies from Cuþ species to Sm3þ and Eu3þ ions in glasses have been recently reported by Wei et al. [18] and Guo et al. [19], respectively. Monovalent copper ions then appear as suitable candidates as energy donors to Nd3þ ions relative to other sensitizers proposed (e.g., Cr3þ, Sn2þ) [6,7]. Fig. 4 shows excitation spectra for the CuSneNd and Nd glasses acquired by monitoring emission from the 4F3/2 / 4I11/2 Nd3þ transition at 1058 nm. The Nd glass which can be taken as reference in this experiment shows the excitation peaks characteristic of Nd3þ ions (e.g., Nd3þ absorption features in Fig. 1). However, the CuSneNd glass exhibits an additional excitation band within 250e450 nm, which displays a broad maximum around 360 nm (spanning from about 340 to 390 nm). This band concurs with the reported features of the 3d10 / 3d94s1 transitions in Cuþ ions in the same glass matrix where associated excitation peaks around 345 nm have been indicated [20,21]. It is certainly unambiguous evidence of the donoreacceptor energy transfer process considered (vide supra). Furthermore, the rather symmetrical shape and nearUV character of the band points out to a predominant role of Cuþ
Fig. 4. Excitation spectra of CuSneNd and Nd glasses as recorded by monitoring emission of 4F3/2 / 4I11/2 transition in Nd3þ at 1058 nm.
ions as sensitizers over tin. This is because the excitation features of Sn centers are such that the excitation peaks appear deeper in the UV (e.g. near 290 nm) [20,21,25], not noticeable in Fig. 4. However, with the information available at present, it cannot be elucidated to what extent the presence of different valence states of tin can influence Nd3þ emission. It would be desirable in this respect to investigate in future studies on tin and neodymium co-doping of high-solubility phosphate glass matrices (e.g. in connection with Xray photoelectron spectroscopy characterization [25]) for complementing the earlier work by Malashkevich et al. [7] on Lacontaining glass. The Cuþ / Nd3þ energy transfer is further confirmed by the emission spectra presented in Fig. 5, recorded for the CuSneNd and Nd glasses under excitation at 380 nm. Such excitation wavelength is particularly appropriate for the assessment as it excites Cuþ ions (and not Sn centers) and lacks the resonance with main Nd3þ
Fig. 5. Near-infrared PL spectra obtained for the CuSneNd and Nd glasses under excitation at 380 nm.
J.A. Jimenez, M. Sendova / Materials Chemistry and Physics 162 (2015) 425e430
excitation peaks. Consequently, Nd3þ emission from the Nd glass is weak. On the other hand, a remarkable emission from the 4F3/ 4 4 4 3þ is observed for the 2 / I9/2, I11/2, I13/2 transitions in Nd CuSneNd glass, consistent with an energy transfer from Cuþ to Nd3þ which results in populating the 4F3/2 emitting state. Especially notable is the enhanced 4F3/2 / 4I11/2 emission at about 1.06 mm in the CuSneNd glass, suggestive of the significance of the monovalent copper ions as sensitizers of Nd3þ for lasing achievable under broad near-UV light pumping. Herein, at least ~34-fold Nd3þ PL enhancement is estimated for the CuSneNd glass relative to the Nd reference. The data considered allows for representing the Cuþ / Nd3þ energy transfer process schematically as shown in Fig. 6. Herein, following optical excitation of Cuþ ions (e.g., 380 nm) populating the singlet state of the 3d94s1 configuration, electrons pass to the 3 Eg state, further split into metastable states T2g and T1g. The Cuþ ions can subsequently emit broadly (inhomogeneous broadening) in the visible or else the associated energy can be transferred to relevant Nd3þ states. Besides the 4G7/2, 2K13/2, and 4G5/2 states for which the Cuþ / Nd3þ non-radiative transfer efficiency was estimated, other resonant states in Nd3þ likely involved are 4G11/2, 2G9/ 2 2 2 4 2 2 4 2, D3/2, P3/2, K15/2, G9/2, G7/2, H11/2, and F9/2 [26]. In this scheme, the upper states being initially populated would subsequently decay non-radiatively to the 4F3/2 metastable state which in turn undergoes the radiative relaxation. This explains the obtained results regarding the role of the Cuþ ions as UV sensitizers of the near-infrared Nd3þ emission. Dipoleedipole interactions have been generally considered to be at the origin of energy transfer processes between Nd3þ ions [31,32], and other species such as transition metals ions [33]. Thus, the radiationless energy transfer from Cuþ donors to Nd3þ acceptors in the present work likely proceeds via interactions of the electric dipoleedipole type. The mean distance between the
429
CuþeNd3þ donoreacceptor pair can be estimated by
R¼
1 ðnD þ nA Þ1=3
(2)
where nD and nA are the number of donor and acceptor species per cm3, respectively [34]. Taking into account the concentration of Nd3þ ions in the CuSneNd glass, together with the estimated Cuþ concentration considering a chemical reduction efficiency of Cu2þ to Cuþ of about 98% (Fig. 1 inset, hexagonal symbol), a mean CuþeNd3þ distance R of about 8.8 Å is estimated by Eq. (2). This can be considered appropriate within the regime of electric dipole interactions [31,32]. Hence, as considerable spectral overlap has been realized (Fig. 2), favorable conditions have been achieved for the resonant non-radiative transfer. As a final remark, since the presence of Cu2þ ions is expected to decrease the efficiency in the Nd3þ lasing emission [35], further optimization of material synthesis is desirable in order to reduce copper (II) below 2%. 4. Conclusions A spectroscopic investigation was carried out aimed for elucidating the near-UV sensitizing effects of Cuþ ions stabilized in phosphate glass on the near-infrared emission of Nd3þ ions coexisting in the matrix. A comparative assessment of material optical properties including emission decay dynamics indicated an efficient resonant energy transfer from monovalent copper ions (donors) to neodymium ions (acceptors) which results in populating the 4F3/2 emitting state in Nd3þ. As a result, a remarkable 1.06 mm emission from the 4F3/2 / 4I11/2 transition in Nd3þ ions was realized under near-UV excitation of the Cuþ ions acting as sensitizers. Accordingly, the effective co-doping of Nd-containing phosphate glass with monovalent copper shows potential for
Fig. 6. Simplified schematic of energy transfer (ET) from emitting states in monovalent copper ions to Nd3þ ions in the phosphate glass matrix (the dashed horizontal arrows represent the non-radiative channels for which the transfer efficiency was estimated). Vertical solid arrows represent radiative transitions (radiationless relaxations omitted).
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Nd3þ lasing under broad band near-UV optical pumping. Further, opportunities come forward for exploring Cuþ/Nd3þ co-doped glasses as solar spectral converters for photovoltaic applications. Acknowledgments nez thanks undergraduate student Thomas Dubien from J.A. Jime the Chemistry Department at UNF for the experimental assistance. M. Sendova is grateful for the assistance of Dr. Brian Hosterman from the Optical Spectroscopy and Nanomaterials Lab at NCF. Research was partially sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-09-2-0004. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation heron. References [1] G. Boulon, Opt. Mater. 34 (2012) 499. [2] J. Ballato, P. Dragic, J. Am. Ceram. Soc. 96 (2013) 2675. [3] M. Yamane, Y. Asahara, Glasses for Photonics, Cambridge University Press, UK, 2000. [4] J.H. Campbell, J.S. Hayden, A. Marker, Int. J. Appl. Glass Sci. 2 (2011) 3. [5] A.C. Erlandson, S.M. Aceves, A.J. Bayramian, A.L. Bullington, R.J. Beach, C.D. Boley, J.A. Caird, R.J. Deri, A.M. Dunne, D.L. Flowers, M.A. Henesian, K.R. Manes, E.I. Moses, S.I. Rana, K.I. Schaffers, M.L. Spaeth, C.J. Stolz, S.J. Telford, Opt. Mater. Express 1 (2011) 1341. [6] J.G. Edwards, S. Gomulka, J. Phys. D Appl. Phys. 12 (1979) 187. [7] G.E. Malashkevich, A.G. Bazylev, A.L. Blinov, M.A. Borik, A.P. Voĭtovich, ush, Sov. J. Quantum Electron. 21 (1991) 601. V.S. Kalinov, V.N. Tade
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Near-UV sensitized 1.06 mm emission of Nd3þ ions via monovalent copper in phosphate glass A. Jime nez a, *, Mariana Sendova b Jose a b
Department of Chemistry, University of North Florida, Jacksonville, FL 32224, USA Optical Spectroscopy & Nano-Materials Lab, New College of Florida, Sarasota, FL 34243, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Monovalent copper ions effectively stabilized in Nd3þ-containing phosphate glass. Enhanced Nd3þ near-infrared emission observed upon the Cuþ ions incorporation. Cuþ / Nd3þ non-radiative energy transfer efficiencies and likely energy transfer pathways evaluated. Potential for solid-state lasers and solar spectral conversion suggested.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2014 Received in revised form 3 June 2015 Accepted 7 June 2015 Available online 17 June 2015
Monovalent copper ions effectively incorporated in Nd-containing phosphate glass by a single-step meltquench method have been established as near-ultraviolet (UV) sensitizers of Nd3þ ions, resulting in a remarkable 4F3/2 / 4I11/2 emission at 1.06 mm. The spectroscopic data indicates an efficient energy conversion process. The Cuþ ions first absorb photons broadly around 360 nm, and subsequently transfer the energy from the Stokes-shifted emitting states to resonant Nd3þ energy levels in the visible. Ultimately, the Nd3þ electronic excited states decay and the upper lasing state 4F3/2 is populated, leading to the enhanced emission at 1.06 mm. The characteristic features of the Cuþ visible emission spectra and the reduced lifetime of the corresponding Cuþ donor states indicate an efficient non-radiative transfer. The Cuþ/Nd3þ co-doped phosphate glass appears suitable as solid-state laser material with enhanced pump range in the near-UV part of the spectrum and for solar spectral conversion in photovoltaic cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Glasses Optical materials Photoluminescence spectroscopy
1. Introduction Laser technology has revolutionized today's society in a variety of fields such as information processing, communications, medicine, and scientific research in general [1,2]. Particularly, Nd3þ solid-state lasers based on high-solubility phosphate glass matrices are much valued for high-power lasing applications [3,4]. This has
* Corresponding author. nez). E-mail address: [email protected] (J.A. Jime http://dx.doi.org/10.1016/j.matchemphys.2015.06.009 0254-0584/© 2015 Elsevier B.V. All rights reserved.
led to the current initiative of exploring the potential of such for laser fusion power plants to generate electricity (concept of Laser Inertial Fusion Energy e LIFE) [5]. Hence, in view of the technological relevance, which nowadays includes helping meet global energy demands, several research groups have investigated on the effects (e.g. sensitizing) of adding co-dopants such as Cr3þ [6], Sn2þ [7], and semiconductor [8] or metallic nanoparticles [9e11], to Nd3þ-containing glass matrices. More than three decades ago, Edwards and Gomulka [6] suggested that monovalent copper ions could sensitize Nd3þ emission in laser glass given the strong spectral overlap of Cuþ emission and
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Nd3þ absorption. However, the authors discounted copper as candidate since at the time Cuþ pumping would need to be performed rather deep in the ultraviolet (UV) below 320 nm, at which the glass matrices would eventually solarize. In addition, stabilizing considerable amounts of Cuþ ions in glass matrices is a non-trivial task given the predominant stability of the Cu2þ ion, especially for syntheses carried out in an ambient atmosphere [12]. Hence, intricate preparation procedures are often employed for doping glasses with Cuþ ions which may include controlled reducing [13], or inert [14] atmospheres. As an alternative, appropriate reducing agents such as tin (II) oxide or antimony (III) oxide can be included as part of the batch materials; yet, carrying out the preparation at high temperatures under an oxidizing air atmosphere still poses risks to the redox chemicals [15e17]. Consequently, studies on the influence of Cuþ ions on the luminescence of rare-earths in glass matrices are scarce [18,19]. Nevertheless, we have recently succeeded in our group [20] in fabricating luminescent Cuþ-doped glasses of a high-solubility bariumephosphate matrix relevant to the field of photonics [3,17]. The effective incorporation of relatively large amounts of monovalent copper has been realized using redox chemicals CuO and SnO, by simply melting batch materials under ambient atmosphere with sucrose as antioxidant agent protecting tin (II). Pertinent to the field of plasmonic materials, this approach also allows for the precipitation of Cu nanoparticles during subsequent thermal treatments [21]. Further, the optical absorption features of the high concentrations of Cuþ ions stabilized in the matrix appeared predominantly in the near-UV, with associated excitation bands around 345 nm [20,21]. Thus, we felt motivated to co-dope the phosphate matrix with Nd3þ ions, since in the event that an efficient Cuþ / Nd3þ energy transfer occurred, the glass system would further overcome the limitation earlier pointed out by Edwards and Gomulka [6]. Accordingly, we herein report on the spectroscopic properties of Cuþ and Nd3þ co-doped bariumephosphate glass attained by the single-step melt-quench method. It is demonstrated that indeed Cuþ ions act as sensitizers of Nd3þ activator ions, leading to a remarkable 1.06 mm emission. Hence, the codoping of the Nd3þ-containing phosphate glass with monovalent copper shows potential for lasing realizable under near-UV light pumping relevant to excitation of Cuþ. Moreover, the UV-to-visible Stokes shift in Cuþ luminescence together with the consequent near-infrared Nd3þ emission appears desirable for solar spectral conversion in photovoltaic cells [22e24], particularly Ge-based solar cells characterized by broad spectral (from the visible to the near-infrared) efficiency [24].
Nd3þ luminescence properties, respectively, in comparison to the CuSneNd glass. All glasses were cut and polished in order to produce glass slabs for optical measurements with final thicknesses of about 1.0 mm. Optical absorption measurements were performed using a PerkineElmer 35 UV/Vis double-beam spectrophotometer. All absorption spectra were recorded with air as reference. Photoluminescence (PL) emission and excitation spectra were collected with a Horiba Jobin-Yvon Fluorolog-3 steady-state spectrofluorometer equipped with a solid-state N2-cooled InGaAs photodiode and a photomultiplier tube. Emission decay curves were recorded with a Photon Technology International QuantaMaster 30 spectrofluorometer equipped with a Xe flash lamp having a pulse width of about 2 ms. The flash lamp was kept operating at a frequency of 125 Hz with the time window for data acquisition set at 8000 ms. All measurements were carried out at room temperature with particular attention given to keep conditions constant during experiments. 3. Results and discussion Fig. 1 shows the optical absorption spectra for the Nd, CuSn and CuSneNd glasses. A significant redshift in the glass absorption edge is observed upon the CuO and SnO co-doping. Herein, a contribution is expected from twofold-coordinated tin centers [21,25]. Yet, the main species accountable for near-UV absorption are the Cuþ ions incorporated into the matrix, due to the 3d10 / 3d94s1 interconfiguration transitions [20,21]. An absorption band rises after 600 nm for the CuSn and CuSneNd glasses, due to the presence of Cu2þ ions which absorb broadly owing to 2E / 2T2 intraconfiguration (ded) transitions [16,20]. The Cu2þ absorption band can be employed for analytical purposes by use of absorption spectra of a set of CuO-containing reference glasses as reported in Ref. [20]. In such spectroscopic approach of chemical analysis, a plot of the peak optical density of the Cu2þ band around 850 nm vs. the amount of CuO added to the reference glasses is constructed. Such plot, based on reported data [20] is shown in the inset of Fig. 1, which can be treated as a calibration curve following Beer's Law. Thus, from the equation of the line the residual amounts of copper (II) in the CuSneNd glass can be estimated. The assessment has
2. Experimental The glasses were made with a 50P2O5:50BaO (mol%) composition, matrix which has been previously proposed by Uchida et al. [17] for nonlinear optical studies of metallic nanoparticles given its intrinsic high metal solubility. These were prepared from high purity compounds (P2O5 and BaCO3) by the melt-quenching technique [20]. In this simple procedure, batch materials are melted in porcelain crucibles at 1150 C for 15 min under normal atmospheric conditions and immediately quenched. In the event that reducing agent tin (II) oxide is present in the batch, sucrose is added as ~10 wt% on top of the well-mixed reagents to assist as an antioxidant preventing oxidation of tin (II) by atmospheric oxygen. Copper, tin, and/or neodymium doping was done by adding CuO, SnO, and/or Nd2O3 quantities in mol%, in relation to network former P2O5. Samples prepared for the present study are (referred to as): CuSn glass (10% of each CuO and SnO); CuSneNd (10% of each CuO and SnO, and 2% Nd2O3); and Nd glass (2% Nd2O3). The purpose of the CuSn and Nd glasses is to serve as references relative to Cuþ and
Fig. 1. Optical absorption spectra of the Nd, CuSn and CuSneNd glasses. The inset shows the plot of peak optical density of the Cu2þ band vs. concentration of CuO added to the reference glasses (squares), Ref. [20]; the solid line is the linear fit to the data. The hexagon is the data point obtained for the CuSneNd glass.
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been applied to the CuSn glass (10 mol% of each CuO and SnO) as carried out in our group and reported previously [20], yielding 0.20 mol% copper (II) after the melting. As observed in Fig. 1, apart from the Nd3þ bands overlaid, the Cu2þ absorption profile of the CuSneNd glass appears similar in intensity to that of the CuSn glass. By means of the equation of the line resulting from the regression analysis (Fig. 1, inset) and the experimental Cu2þ optical density at 850 nm, a corresponding amount of 0.17 mol% copper (II) oxide is estimated for the CuSneNd glass. The value is plotted together with the calibration curve in the inset of Fig. 1 (hexagonal symbol). Remarkably, the residual CuO quantities estimated for both the CuSn and CuSneNd glasses indicate a 98% reduction efficiency of copper (II). This is a significant result given the relative simplicity of the material preparation procedure carried out under ambient atmosphere. In terms of PL properties, the recent investigation on the CuSn glass has revealed two distinct emitting centers: Sn centers and the single Cuþ ions [20]. The excitation-wavelength dependence of the PL is such that under short-wavelength excitation (e.g. 260 nm), broadband emission is observed towards the blue having a significant contribution from tin (triplet-to-singlet transitions) [20]. However, with increasing excitation wavelength, emission shifts to the red, indicative of larger contributions from the Cuþ (3d94s1 / 3d10 transitions). Ultimately, under excitation at 360 nm and longer wavelengths, the emission spectra for the glass display single-band behavior in agreement with emission arising merely from the Cuþ ions [20]. The sensitization of neodymium ions luminescence by divalent tin was reported by Malashkevich et al. [7]. Henceforth, the focus is on assessing the effect of the Cuþ ions. Moreover, as previously established for relatively long excitation wavelengths (e.g. 360 nm) the contribution of Sn centers to the PL can be neglected [20,21]. Presented as trace (a) in Fig. 2 is the emission spectrum for the CuSn glass obtained under excitation at 360 nm. This spectrum is a reference (under the same excitation) regarding the influence of Nd3þ on Cuþ emission in the CuSneNd glass, trace (b) in Fig. 2. Certainly, the region for Cuþ emission in the glass system is relevant to Nd3þ absorption within the 500e650 nm range, consistent with the assumption by Edwards and Gomulka [6]. Consequently, the
Fig. 2. Spectral overlay of emission spectra obtained under excitation at 360 nm for (a) CuSn and (b) CuSneNd glasses, with (c) the absorption spectrum of the CuSneNd glass; notable Nd3þ ions absorption features are labeled in the latter in correspondence to electronic transitions from the 4I9/2 ground state to excited states specified.
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broad Cuþ band emission in the CuSneNd glass shows dips in correspondence with some Nd3þ absorption peaks (main spectroscopic states [26] indicated in trace (c) of Fig. 2). Therefore, clear indication exists of a resonant Cuþ / Nd3þ energy transfer channel (e.g. cascade-type) proceeding from the emitting states in Cuþ ions. Additional information about the nature of the Cuþ / Nd3þ energy transfer is obtained from emission decay curves acquired for the CuSn and CuSneNd glasses. Fig. 3 compares PL decay curves obtained under 360 nm excitation for Cuþ emission monitored at 585 nm in correspondence with the prominent 4I9/2 / 4G5/2 absorption in Nd3þ (Fig. 2). The decay curves exhibit non-exponential behavior, where a much faster decay is evident for the CuSneNd glass. In an effort to investigate on the involvement of other Nd3þ states farther away from peak Cuþ emission, decay curves were also recorded (not shown) under the 360 nm excitation by monitoring emission at 525 nm (e.g. 4I9/2 / 2K13/2, 4G7/2 Nd3þ transitions). Curve fitting was achieved with a bi-exponential function, consistent with the fast (tf) and slow (ts) lifetime components characteristic of the Cuþ ions [27]. The determined decay times are summarized in Table 1. Both tf and ts components differ considerably among the two glasses for the two emission wavelengths. The bi-exponential decay in Cuþ emission can be interpreted in terms of a four-level system (schematic shown in Fig. 3, inset), where excitation immediately populates the singlet state of the 3d94s1 configuration (level 4) [27]. Subsequently, electrons undergo intersystem crossing to the triplet state (3Eg), split in two metastable states due to spineorbit coupling (upper level 2, T2g, and lower level 3, T1g) lying close to each other. After a relatively short time period, level 4 is emptied and the system behaves as a threelevel one in which relaxation from upper level T2g is fast, while that from lower level T1g is slower [27]. Accordingly, the tf and ts components deduced from the fits to the decay curves are likely related to Cuþ ions relaxation from T2g and T1g states, respectively, to the 1Ag ground state (level 1). The estimated lifetimes for the CuSn glass (Table 1) are in good agreement with those reported by Debnath and Das [28] and Borsella et al. [29] for Cuþ ions in tetragonally-distorted octahedral sites in glasses. This is in accord with Cuþ emission detected at relatively low energy (Fig. 2), which is expected for a Cuþ environment of octahedral coordination undergoing tetragonal distortion with the non-bridging oxygen ions
Fig. 3. PL decay curves for CuSn and CuSneNd glasses obtained under excitation at 360 nm by monitoring emission at 585 nm. The energy level diagram in the inset shows the states linked to the Cuþ luminescence process.
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Table 1 Fast (tf) and slow (ts) lifetimes of Cuþ ions in the CuSn and CuSneNd glasses determined from experimental decay curves along with estimated Cuþ / Nd3þ energy transfer efficiencies (h). Glass
CuSn CuSneNd
h
lem ¼ 585 nm
lem ¼ 525 nm
tf (ms)
ts (ms)
tf (ms)
ts (ms)
41.1 (±0.3) 23.8 (±0.3) 42.1%
136.1 (±1.1) 73.6 (±1.2) 45.9%
35.2 (±0.3) 22.1 (±0.6) 37.2%
120.5 (±1.4) 73.0 (±3.8) 39.4%
in the amorphous host [28,29]. The differences manifested between the two emission wavelengths (i.e. 585 and 525 nm) in the CuSn glass are expected in accord with the dissimilar Cuþ sites associated [13]. On the other hand, the decay times in the CuSneNd glass are significantly reduced relative to the CuSn glass. This result is consistent with the modification of the Cuþ PL (Fig. 2), and represents clear evidence of an effective non-radiative energy transfer from Cuþ donors to the Nd3þ activator ions [30]. Further, the energy transfer efficiency h can be estimated by [6]
h ¼ 1 ðtCuþ Nd3þ =tCuþ Þ
(1)
where tCuþ Nd3þ and tCuþ are the lifetimes of the sensitizer (Cuþ) in the presence and absence of the activator (Nd3þ), respectively. Such efficiencies have been estimated in association to both fast and slow decay time components in the Cuþ donors for the two emission wavelengths monitored. The results are presented together with the lifetimes in Table 1. Remarkably, efficiencies above 40% are estimated for both lifetime components for the 585 nm emission, with an average between the two of 44%. On the other hand, for emission at 525 nm, the energy transfer efficiencies appear somewhat diminished, with an average between the fast and slow decay time components estimates of 38%. Since the percent differences between the 585 nm and 525 nm efficiencies are 12% and 15% for the fast and slow components estimates, respectively, a higher energy transfer efficiency is indicated in relation to the longer wavelength. This is likely due to the fact that the 4I9/2 / 4G5/2 absorption in Nd3þ ions around 585 nm lies near the peak of the Cuþ broad PL band (Fig. 2), i.e., a larger degree of spectral overlap exists. Thus, a larger population of the monovalent copper ions participates in the non-radiative Cuþ(3Eg) / Nd3þ(4G5/2) energy transfer channel. Still, consistency amongst efficiencies of fast vs. slow decay time estimates suggest that the energy transfer proceeds similarly from the inhomogeneously broadened T2g and T1g emitting states in Cuþ ions to the resonant states in Nd3þ. In fact, comparable energy transfer efficiencies from Cuþ species to Sm3þ and Eu3þ ions in glasses have been recently reported by Wei et al. [18] and Guo et al. [19], respectively. Monovalent copper ions then appear as suitable candidates as energy donors to Nd3þ ions relative to other sensitizers proposed (e.g., Cr3þ, Sn2þ) [6,7]. Fig. 4 shows excitation spectra for the CuSneNd and Nd glasses acquired by monitoring emission from the 4F3/2 / 4I11/2 Nd3þ transition at 1058 nm. The Nd glass which can be taken as reference in this experiment shows the excitation peaks characteristic of Nd3þ ions (e.g., Nd3þ absorption features in Fig. 1). However, the CuSneNd glass exhibits an additional excitation band within 250e450 nm, which displays a broad maximum around 360 nm (spanning from about 340 to 390 nm). This band concurs with the reported features of the 3d10 / 3d94s1 transitions in Cuþ ions in the same glass matrix where associated excitation peaks around 345 nm have been indicated [20,21]. It is certainly unambiguous evidence of the donoreacceptor energy transfer process considered (vide supra). Furthermore, the rather symmetrical shape and nearUV character of the band points out to a predominant role of Cuþ
Fig. 4. Excitation spectra of CuSneNd and Nd glasses as recorded by monitoring emission of 4F3/2 / 4I11/2 transition in Nd3þ at 1058 nm.
ions as sensitizers over tin. This is because the excitation features of Sn centers are such that the excitation peaks appear deeper in the UV (e.g. near 290 nm) [20,21,25], not noticeable in Fig. 4. However, with the information available at present, it cannot be elucidated to what extent the presence of different valence states of tin can influence Nd3þ emission. It would be desirable in this respect to investigate in future studies on tin and neodymium co-doping of high-solubility phosphate glass matrices (e.g. in connection with Xray photoelectron spectroscopy characterization [25]) for complementing the earlier work by Malashkevich et al. [7] on Lacontaining glass. The Cuþ / Nd3þ energy transfer is further confirmed by the emission spectra presented in Fig. 5, recorded for the CuSneNd and Nd glasses under excitation at 380 nm. Such excitation wavelength is particularly appropriate for the assessment as it excites Cuþ ions (and not Sn centers) and lacks the resonance with main Nd3þ
Fig. 5. Near-infrared PL spectra obtained for the CuSneNd and Nd glasses under excitation at 380 nm.
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excitation peaks. Consequently, Nd3þ emission from the Nd glass is weak. On the other hand, a remarkable emission from the 4F3/ 4 4 4 3þ is observed for the 2 / I9/2, I11/2, I13/2 transitions in Nd CuSneNd glass, consistent with an energy transfer from Cuþ to Nd3þ which results in populating the 4F3/2 emitting state. Especially notable is the enhanced 4F3/2 / 4I11/2 emission at about 1.06 mm in the CuSneNd glass, suggestive of the significance of the monovalent copper ions as sensitizers of Nd3þ for lasing achievable under broad near-UV light pumping. Herein, at least ~34-fold Nd3þ PL enhancement is estimated for the CuSneNd glass relative to the Nd reference. The data considered allows for representing the Cuþ / Nd3þ energy transfer process schematically as shown in Fig. 6. Herein, following optical excitation of Cuþ ions (e.g., 380 nm) populating the singlet state of the 3d94s1 configuration, electrons pass to the 3 Eg state, further split into metastable states T2g and T1g. The Cuþ ions can subsequently emit broadly (inhomogeneous broadening) in the visible or else the associated energy can be transferred to relevant Nd3þ states. Besides the 4G7/2, 2K13/2, and 4G5/2 states for which the Cuþ / Nd3þ non-radiative transfer efficiency was estimated, other resonant states in Nd3þ likely involved are 4G11/2, 2G9/ 2 2 2 4 2 2 4 2, D3/2, P3/2, K15/2, G9/2, G7/2, H11/2, and F9/2 [26]. In this scheme, the upper states being initially populated would subsequently decay non-radiatively to the 4F3/2 metastable state which in turn undergoes the radiative relaxation. This explains the obtained results regarding the role of the Cuþ ions as UV sensitizers of the near-infrared Nd3þ emission. Dipoleedipole interactions have been generally considered to be at the origin of energy transfer processes between Nd3þ ions [31,32], and other species such as transition metals ions [33]. Thus, the radiationless energy transfer from Cuþ donors to Nd3þ acceptors in the present work likely proceeds via interactions of the electric dipoleedipole type. The mean distance between the
429
CuþeNd3þ donoreacceptor pair can be estimated by
R¼
1 ðnD þ nA Þ1=3
(2)
where nD and nA are the number of donor and acceptor species per cm3, respectively [34]. Taking into account the concentration of Nd3þ ions in the CuSneNd glass, together with the estimated Cuþ concentration considering a chemical reduction efficiency of Cu2þ to Cuþ of about 98% (Fig. 1 inset, hexagonal symbol), a mean CuþeNd3þ distance R of about 8.8 Å is estimated by Eq. (2). This can be considered appropriate within the regime of electric dipole interactions [31,32]. Hence, as considerable spectral overlap has been realized (Fig. 2), favorable conditions have been achieved for the resonant non-radiative transfer. As a final remark, since the presence of Cu2þ ions is expected to decrease the efficiency in the Nd3þ lasing emission [35], further optimization of material synthesis is desirable in order to reduce copper (II) below 2%. 4. Conclusions A spectroscopic investigation was carried out aimed for elucidating the near-UV sensitizing effects of Cuþ ions stabilized in phosphate glass on the near-infrared emission of Nd3þ ions coexisting in the matrix. A comparative assessment of material optical properties including emission decay dynamics indicated an efficient resonant energy transfer from monovalent copper ions (donors) to neodymium ions (acceptors) which results in populating the 4F3/2 emitting state in Nd3þ. As a result, a remarkable 1.06 mm emission from the 4F3/2 / 4I11/2 transition in Nd3þ ions was realized under near-UV excitation of the Cuþ ions acting as sensitizers. Accordingly, the effective co-doping of Nd-containing phosphate glass with monovalent copper shows potential for
Fig. 6. Simplified schematic of energy transfer (ET) from emitting states in monovalent copper ions to Nd3þ ions in the phosphate glass matrix (the dashed horizontal arrows represent the non-radiative channels for which the transfer efficiency was estimated). Vertical solid arrows represent radiative transitions (radiationless relaxations omitted).
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Nd3þ lasing under broad band near-UV optical pumping. Further, opportunities come forward for exploring Cuþ/Nd3þ co-doped glasses as solar spectral converters for photovoltaic applications. Acknowledgments nez thanks undergraduate student Thomas Dubien from J.A. Jime the Chemistry Department at UNF for the experimental assistance. M. Sendova is grateful for the assistance of Dr. Brian Hosterman from the Optical Spectroscopy and Nanomaterials Lab at NCF. Research was partially sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-09-2-0004. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation heron. References [1] G. Boulon, Opt. Mater. 34 (2012) 499. [2] J. Ballato, P. Dragic, J. Am. Ceram. Soc. 96 (2013) 2675. [3] M. Yamane, Y. Asahara, Glasses for Photonics, Cambridge University Press, UK, 2000. [4] J.H. Campbell, J.S. Hayden, A. Marker, Int. J. Appl. Glass Sci. 2 (2011) 3. [5] A.C. Erlandson, S.M. Aceves, A.J. Bayramian, A.L. Bullington, R.J. Beach, C.D. Boley, J.A. Caird, R.J. Deri, A.M. Dunne, D.L. Flowers, M.A. Henesian, K.R. Manes, E.I. Moses, S.I. Rana, K.I. Schaffers, M.L. Spaeth, C.J. Stolz, S.J. Telford, Opt. Mater. Express 1 (2011) 1341. [6] J.G. Edwards, S. Gomulka, J. Phys. D Appl. Phys. 12 (1979) 187. [7] G.E. Malashkevich, A.G. Bazylev, A.L. Blinov, M.A. Borik, A.P. Voĭtovich, ush, Sov. J. Quantum Electron. 21 (1991) 601. V.S. Kalinov, V.N. Tade
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