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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 11 — May. 21, 2012
  • pp: 12416–12421
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Coexistence of fast photodarkening and slow photobleaching in Ge19As21Se60 thin films

Pritam Khan, A. R. Barik, E. M. Vinod, K. S. Sangunni, H. Jain, and K. V. Adarsh  »View Author Affiliations


Optics Express, Vol. 20, Issue 11, pp. 12416-12421 (2012)
http://dx.doi.org/10.1364/OE.20.012416


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Abstract

We experimentally demonstrate the coexistence of two opposite photo-effects, viz. fast photodarkening (PD) and slow photobleaching (PB) in Ge19As21Se60 thin films, when illuminated with a laser of wavelength 671 nm. PD appears to begin instantaneously upon light illumination and saturates in tens of seconds. By comparison, PB is a slower process that starts only after PD has saturated. Both PD and PB follow stretched exponetial dependence on time. Modeling of overall change as a linear sum of two contributions suggests that the changes in As and Ge parts of glass network respond to light effectively indepndent of each other.

© 2012 OSA

1. Introduction

Chalcogenide glasses (ChG) exhibit numerous photoinduced effects with bandgap or sub-bandgap illumination. Among these, the most notable effects are photodarkening (PD) in As-based chalcogenides and photobleaching (PB) in Ge-based chalcogenides [1

1. B. J. Eggleton, B. L. Davies, and K. Richardson, “Chalcogenide photonics,” Nat. Photonics 5, 141–148 (2011).

,2

2. K. Shimakawa, A. Kolobov, and S. R. Elliott, “Photoinduced effects and metastability in amorphous semiconductors and insulators,” Adv. Phys. 44(6), 475–588 (1995). [CrossRef]

]. Besides the fundamental interest in PD/PB in ChG, these effects find useful technological applications in high bit rate waveguide writing, dense holographic recording, etc [3

3. K. E. Asatryan, S. Frédérick, T. Galstian, and R. Vallée, “Recording of polarization holograms in photodarkened amorphous chalcogenide films,” Appl. Phys. Lett. 84(10), 1626–1628 (2004). [CrossRef]

6

6. O. M. Efimov, L. B. Glebov, K. A. Richardson, E. Van Stryland, T. Cardinal, S. H. Park, M. Couzi, and J. L. Brunéel, “Waveguide writing in chalcogenide glasses by a train of femtosecond laser pulses,” Opt. Mater. 17(3), 379–386 (2001). [CrossRef]

].

PD in As-based ChG has been investigated for many years and is believed to originate from photoinduced structural transformations. Interestingly, PD comprises of a transient and a metastable part [7

7. A. Ganjoo, K. Shimakawa, K. Kitano, and E. A. Davis, “Transient photodarkening in amorphous chalcogenides,” J. Non-Cryst. Solids 299-302, 917–923 (2002). [CrossRef]

, 8

8. K. Antoine, H. Jain, M. Vlcek, S. D. Senanayake, and D. A. Drabold, “Chemical origin of polarization-dependent photoinduced changes in an As36Se64glass film via in situ synchrotron x-ray photoelectron spectroscopy,” Phys. Rev. B 79(5), 054204 (2009). [CrossRef]

], such that the former decays quickly once the illumination is switched off, leaving behind the metastable part that can only be reversed by annealing near the glass transition temperature [2

2. K. Shimakawa, A. Kolobov, and S. R. Elliott, “Photoinduced effects and metastability in amorphous semiconductors and insulators,” Adv. Phys. 44(6), 475–588 (1995). [CrossRef]

]. By contrast, light illumination of Ge-based ChG primarily results in PB, which appears to be irreversible [2

2. K. Shimakawa, A. Kolobov, and S. R. Elliott, “Photoinduced effects and metastability in amorphous semiconductors and insulators,” Adv. Phys. 44(6), 475–588 (1995). [CrossRef]

]. However, recent studies on GeSe2 thin films have shown that the irreversible PB is accompanied by a transient PD [9

9. E. Flaxer, M. Klebanov, D. Abrahamoff, S. Noah, and V. Lyubin, “Photodarkening of As50Se50 glassy films under μs light pulses,” Opt. Mater. 31(4), 688–690 (2009). [CrossRef]

, 10

10. Q. Yan, H. Jain, J. Ren, D. Zhao, and G. Chen, “Effect of photo-oxidation on photobleaching of GeSe2 and Ge2Se3 films,” J. Phys. Chem. C 115(43), 21390–21395 (2011). [CrossRef]

]. Many models have been proposed to explain the observed PB in Ge-based ChG, but none of them appears to be applicable to all compositions [11

11. L. Tichý, H. Ticha, P. Nagels, and E. Sleeckx, “A review of the specific role of oxygen in irreversible photo- and thermally induced changes of the optical properties of thin film amorphous chalcogenides,” Opt. Mater. 4(6), 771–779 (1995). [CrossRef]

, 12

12. C. A. Spence and S. R. Elliott, “Light-induced oxidation and band-edge shifts in thermally evaporated films of germanium chalcogenide glasses,” Phys. Rev. B Condens. Matter 39(8), 5452–5463 (1989). [CrossRef] [PubMed]

]. In general, it is believed that intrinsic structural changes and photo-oxidation are responsible for the observed PB in Ge-based ChG [10

10. Q. Yan, H. Jain, J. Ren, D. Zhao, and G. Chen, “Effect of photo-oxidation on photobleaching of GeSe2 and Ge2Se3 films,” J. Phys. Chem. C 115(43), 21390–21395 (2011). [CrossRef]

]. Naturally, PD in As based and PB in Ge based ChG glasses calls for experiments to establish the light-induced behaviour of a glassy system consisting of Ge, As and Se as major constituents and to determine how the associated As-Se and Ge-Se parts respond to light illumination jointly. Such information will provide new insight in understanding PD/PB in ChG. With this motivation, we have investigated photoinduced changes in the optical transmittance of Ge19As21Se60 thin films. Our results clearly demonstrate an unusual coexistence of fast photodarkening (PD) and slow photobleaching (PB) in Ge19As21Se60 thin films, when illuminated with a laser of wavelength 671 nm and 2W/cm2 intensity.

2. Experimental details

Bulk Ge19As21Se60 glass was prepared by melt-quench method using 99.999% pure As, Ge and Se powders. The cast sample was used as the source material for depositing thin films of average thickness ~1.0 μm by thermal evaporation in a vacuum of about 1 × 10−6 Torr. PD/PB in these films was studied by a pump-probe optical absorption method using the experimental set up described previously [13

13. R. Naik, K. V. Adarsh, R. Ganesan, and K. S. Sangunni, “Giant photoinduced optical bleaching at room and liquid helium temperatures in Sb/As2S3 multilayered films,” Opt. Commun. 284(22), 5319–5322 (2011). [CrossRef]

]. We have chosen the wavelength of the pump beam as λ = 671nm (from a diode pumped solid state laser, DPSSL) and kept its intensity at 2 W/cm2. The probe beam was a low intensity white light in the wavelength regime of 450-1000 nm. During illumination, transmission of the sample was recorded using Ocean Optics high resolution composite grating spectrometer (HR 4000 CG), which has the capability to collect the entire optical pectrum in 2ms. In our experiments, full optical spectrum was collected in real time of 100 ms/spectrum

3. Results and discussions

To estimate the effect of pump beam illumination, first we recorded the transmission spectrum of as-prepared sample in dark condition and denoted it as Ti. Next, we turned on the pump beam and continuously recorded the transmission spectrum as a function of time (Tf) until the whole effect saturated, which took nearly an hour. Figure 1(a)
Fig. 1 Contour plot of Tf/Ti as a function of time at selected wavelengths close to the optical bandgap of Ge19As21Se60 thin film when irradiated with a 671nm laser of intensity: (a) 2 W/cm2 and (b) 0.2 W/cm2. The color bars indicate the value of Tf/Ti, where Tf and Ti are transmission spectra of the sample at a particular time after the laser was turned on and for as-prepared sample in dark condition, respectively. At indicated wavelengths, transmission decreases initially, followed by a sharp increase in transmission. Time evolution of Tf/Ti at λ = 590nm for the pump beam intensity (c) 2W/cm2 and (d) 0.2W/cm2. Here the black and red lines represent the experimental data and theoretical fit (using Eq. (5), respectively. Figures clearly demonstrate the coexistence of PD and PB with the former being a significantly faster process.
and 1(b) shows the contour plot of Tf/Ti as a function of time for selected wavelengths close to the optical bandgap of the sample. It is clear from the fig. that PD begins almost instantaneously after turning on the pump beam and saturates within a few tens of seconds. To our great surprise, after complete saturation of PD, PB starts to develop and grows, showing a remarkable shift in the transmission. Finally, it also saturates at a value which is well above the initial value, producing overall bleached film. To understand the role of intensity on the unusual coexistence of PD and PB, we have performed the same experiment at a moderately low intensity of 0.2 W/cm2. Basically, the whole effect is reproduced even at such low intensities. These experimental results clearly demonstrate the unusual coexistence of PD and PB in Ge19As21Se60 thin films.

To explain the unusual coexistence of PB and PD within the same sample, we assume that the compositional units exist (As-As, Ge-Ge etc) in the film, which were created from the vapour containing various atomic fragments [14

14. A. R. Barik, K. V. Adarsh, R. Naik, C. S. S. Sandeep, R. Philip, D. Zhao, and H. Jain, “Photoinduced transparency of effective three-photon absorption coefficient for femtosecond laser pulses in Ge16As29Se55 thin films,” Appl. Phys. Lett. 98(20), 201111 (2011). [CrossRef]

,15

15. A. R. Barik, K. V. Adarsh, R. Naik, R. Ganesan, G. Yang, D. Zhao, H. Jain, and K. Shimakawa, “Role of rigidity and temperature in the kinetics of photodarkening in Ge(x)As(45-x)Se55 thin films,” Opt. Express 19(14), 13158–13163 (2011). [CrossRef] [PubMed]

]. When such a film is illuminated with λ = 671nm light, the structural units comprising of As-Se and Ge-Se respond rather independently and a considerable fraction of metastable homopolar bonds present in the atomic fragments are broken and subsequently converted into energetically favored heteropolar bonds. The reduction in As homopolar bonds presumably present in As-rich clusters results in PD [16

16. G. Yang, H. Jain, A. Ganjoo, D. Zhao, Y. Xu, H. Zeng, and G. Chen, “A photo-stable chalcogenide glass,” Opt. Express 16(14), 10565–10571 (2008). [CrossRef] [PubMed]

], i.e.

As(xy)SeZ+ySe+hυAsxSez
(1)

In the case of Ge atoms, surface oxidation and intrinsic structural changes described by the following chemical reaction lead to photobleaching [10

10. Q. Yan, H. Jain, J. Ren, D. Zhao, and G. Chen, “Effect of photo-oxidation on photobleaching of GeSe2 and Ge2Se3 films,” J. Phys. Chem. C 115(43), 21390–21395 (2011). [CrossRef]

]:

GeGe+SeSe+hυ2GeSe
(2)

Thus there exist two parallel mechanisms of PD and PB in these samples with light illumination. Between them PD is the faster process which saturates rapidly. It is then followed by the slower PB, which requires prolonged illumination to saturate. As a result, we observed an unusual coexistence of PD and PB. To model their reaction kinetics, we used a combination of stretched exponential functions that describe PD and PB separately. Rate equation for PD can be written as:
ΔT=C[exp{(tτd)βd}]+ΔTsd
(3)
and that for PB:
ΔT=ΔTsb[1exp{(tτd)βd}]
(4)
where the subscripts ‘d’ and ‘b’ correspond to PD and PB, respectively. ΔTS, τ, β, t and C are metastable part, effective time constant, dispersion parameter, illumination time and temperature dependent constant which is equal to maximum transient changes respectively. The net rate equation for the whole process is a summation of respective PD and PB components:

ΔT=C[exp{(tτd)βd}]+ΔTsd+ΔTsb[1exp{(tτd)βd}]
(5)

The experimental data fit very well to the stretched exponential functional forms as described in Eq. (5)—see Fig. 1(c) and 1(d). Fitting parameters calculated from theoretical fit are listed in Table 1

Table 1. Fitting parameters obtained from Eq. (5) that corresponds to PD and PB at two intensities. The subscript b and d refer to bleaching and darkening, respectively.

table-icon
View This Table
. Note that the effective reaction time for PD is relatively short, a few seconds for the laser intensity of 2 W/cm2. By contrast, PB is a slower process compared to PD, with much longer reaction times. Interestingly, as we decrease the intensity by a factor of ten, the magnitude of PD and PB remain about the same, but the kinetics change remarkably. In our experiments, we observe a ten times decrease in kinetics, when the pump beam intensity is reduced from 2 to 0.2 W/cm2. Thus the intensity of the pump beam predominantly determines the kinetics of PD and PB.

To observe the reversibility of combined transient effects in Ge19As21Se60 film we turned off and on the pump beam after the complete saturation of PB. On turning the pump beam off, the transmission increased further and saturated quickly (Fig. 2
Fig. 2 The observation of reversible, transient photodarkening (TPD). When the pump beam was turned off, the relative transparency of the film (Tf/Ti) increases and saturates quickly. However, when the pump beam is turned on, it decreases and quickly returns to the pre-switching value.
). When the illumination was switched on subsequently, transmission decreased and reached the values before the illumination was switched off. Subsequent on-off cycles of the pump beam showed reversible transient photodarkening (TPD) every time. The TPD component in Ge19As21Se60 is probably of the same origin (in bond switching and atom movement) that was observed in As chalcogenide films and more recently in Ge2Se3 and GeSe2 [9

9. E. Flaxer, M. Klebanov, D. Abrahamoff, S. Noah, and V. Lyubin, “Photodarkening of As50Se50 glassy films under μs light pulses,” Opt. Mater. 31(4), 688–690 (2009). [CrossRef]

, 10

10. Q. Yan, H. Jain, J. Ren, D. Zhao, and G. Chen, “Effect of photo-oxidation on photobleaching of GeSe2 and Ge2Se3 films,” J. Phys. Chem. C 115(43), 21390–21395 (2011). [CrossRef]

].

Thus our experiments clearly demonstrate that reversible TPD is an instantaneous process similar to PD. However, when such on-off cycles were repeated with prolonged pump beam illumination, the dynamics changed appreciably as the balance between PD (both metastable and transient) and PB changed with exposure time. At short times, PB is very weak and therefore not observable in the presence of large PD. At long times PD has saturated (although it includes reversible transient component) and PB begins to increase eventually dominating the response. All our data clearly indicate PD is an almost instantaneous process with light illumination and PB is a relatively much slower process.

In conclusion, we have demonstrated experimentally the unusual coexistence of fast PD and slow PB in Ge19As21Se60 thin films, when illuminated with a laser beam of λ = 671nm. The observed effect is explained by assuming that the structural fragments containing homopolar As-As or Ge-Ge, and heteropolar As-Se and Ge-Se bonds respond to laser illumination rather independently, and give rise to PD and PB, respectively. Notably, the kinetic curves of both PD and PB follow stretched exponential response. Here PD is a fast process that appears almost instantaneously with light illumination and saturates in tens of seconds. On the other hand, PB is a slower process and saturates only after prolonged illumination. Apart from PD/PB, the sample also shows transient/reversible effects similar to those observed in As-based ChG and GeSe2 thin films.

Acknowledgments

The authors thank Department of Science and Technology of India (Project no: SR/S2/LOP-003/2010) for financial support. They also thank the US National Science Foundation for supporting our international collaboration through International Materials Institute for New Functionality in Glass (DMR-0844014).

References and links

1.

B. J. Eggleton, B. L. Davies, and K. Richardson, “Chalcogenide photonics,” Nat. Photonics 5, 141–148 (2011).

2.

K. Shimakawa, A. Kolobov, and S. R. Elliott, “Photoinduced effects and metastability in amorphous semiconductors and insulators,” Adv. Phys. 44(6), 475–588 (1995). [CrossRef]

3.

K. E. Asatryan, S. Frédérick, T. Galstian, and R. Vallée, “Recording of polarization holograms in photodarkened amorphous chalcogenide films,” Appl. Phys. Lett. 84(10), 1626–1628 (2004). [CrossRef]

4.

M. L. Trunov, P. M. Lytvyn, S. N. Yannopoulos, I. A. Szabo, and S. Kokenyesi, “Photoinduced mass-transport based holographic recording of surface relief gratings in amorphous selenium films,” Appl. Phys. Lett. 99(5), 051906 (2011). [CrossRef]

5.

M. Hughes, W. Yang, and D. Hewak, “Fabrication and characterization of femtosecond laser written waveguides in chalcogenide glass,” Appl. Phys. Lett. 90(13), 131113 (2007). [CrossRef]

6.

O. M. Efimov, L. B. Glebov, K. A. Richardson, E. Van Stryland, T. Cardinal, S. H. Park, M. Couzi, and J. L. Brunéel, “Waveguide writing in chalcogenide glasses by a train of femtosecond laser pulses,” Opt. Mater. 17(3), 379–386 (2001). [CrossRef]

7.

A. Ganjoo, K. Shimakawa, K. Kitano, and E. A. Davis, “Transient photodarkening in amorphous chalcogenides,” J. Non-Cryst. Solids 299-302, 917–923 (2002). [CrossRef]

8.

K. Antoine, H. Jain, M. Vlcek, S. D. Senanayake, and D. A. Drabold, “Chemical origin of polarization-dependent photoinduced changes in an As36Se64glass film via in situ synchrotron x-ray photoelectron spectroscopy,” Phys. Rev. B 79(5), 054204 (2009). [CrossRef]

9.

E. Flaxer, M. Klebanov, D. Abrahamoff, S. Noah, and V. Lyubin, “Photodarkening of As50Se50 glassy films under μs light pulses,” Opt. Mater. 31(4), 688–690 (2009). [CrossRef]

10.

Q. Yan, H. Jain, J. Ren, D. Zhao, and G. Chen, “Effect of photo-oxidation on photobleaching of GeSe2 and Ge2Se3 films,” J. Phys. Chem. C 115(43), 21390–21395 (2011). [CrossRef]

11.

L. Tichý, H. Ticha, P. Nagels, and E. Sleeckx, “A review of the specific role of oxygen in irreversible photo- and thermally induced changes of the optical properties of thin film amorphous chalcogenides,” Opt. Mater. 4(6), 771–779 (1995). [CrossRef]

12.

C. A. Spence and S. R. Elliott, “Light-induced oxidation and band-edge shifts in thermally evaporated films of germanium chalcogenide glasses,” Phys. Rev. B Condens. Matter 39(8), 5452–5463 (1989). [CrossRef] [PubMed]

13.

R. Naik, K. V. Adarsh, R. Ganesan, and K. S. Sangunni, “Giant photoinduced optical bleaching at room and liquid helium temperatures in Sb/As2S3 multilayered films,” Opt. Commun. 284(22), 5319–5322 (2011). [CrossRef]

14.

A. R. Barik, K. V. Adarsh, R. Naik, C. S. S. Sandeep, R. Philip, D. Zhao, and H. Jain, “Photoinduced transparency of effective three-photon absorption coefficient for femtosecond laser pulses in Ge16As29Se55 thin films,” Appl. Phys. Lett. 98(20), 201111 (2011). [CrossRef]

15.

A. R. Barik, K. V. Adarsh, R. Naik, R. Ganesan, G. Yang, D. Zhao, H. Jain, and K. Shimakawa, “Role of rigidity and temperature in the kinetics of photodarkening in Ge(x)As(45-x)Se55 thin films,” Opt. Express 19(14), 13158–13163 (2011). [CrossRef] [PubMed]

16.

G. Yang, H. Jain, A. Ganjoo, D. Zhao, Y. Xu, H. Zeng, and G. Chen, “A photo-stable chalcogenide glass,” Opt. Express 16(14), 10565–10571 (2008). [CrossRef] [PubMed]

OCIS Codes
(160.2750) Materials : Glass and other amorphous materials
(310.6860) Thin films : Thin films, optical properties
(160.5335) Materials : Photosensitive materials

ToC Category:
Thin Films

History
Original Manuscript: April 18, 2012
Revised Manuscript: May 10, 2012
Manuscript Accepted: May 11, 2012
Published: May 16, 2012

Citation
Pritam Khan, A. R. Barik, E. M. Vinod, K. S. Sangunni, H. Jain, and K. V. Adarsh, "Coexistence of fast photodarkening and slow photobleaching in Ge19As21Se60 thin films," Opt. Express 20, 12416-12421 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-11-12416


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References

  1. B. J. Eggleton, B. L. Davies, and K. Richardson, “Chalcogenide photonics,” Nat. Photonics5, 141–148 (2011).
  2. K. Shimakawa, A. Kolobov, and S. R. Elliott, “Photoinduced effects and metastability in amorphous semiconductors and insulators,” Adv. Phys.44(6), 475–588 (1995). [CrossRef]
  3. K. E. Asatryan, S. Frédérick, T. Galstian, and R. Vallée, “Recording of polarization holograms in photodarkened amorphous chalcogenide films,” Appl. Phys. Lett.84(10), 1626–1628 (2004). [CrossRef]
  4. M. L. Trunov, P. M. Lytvyn, S. N. Yannopoulos, I. A. Szabo, and S. Kokenyesi, “Photoinduced mass-transport based holographic recording of surface relief gratings in amorphous selenium films,” Appl. Phys. Lett.99(5), 051906 (2011). [CrossRef]
  5. M. Hughes, W. Yang, and D. Hewak, “Fabrication and characterization of femtosecond laser written waveguides in chalcogenide glass,” Appl. Phys. Lett.90(13), 131113 (2007). [CrossRef]
  6. O. M. Efimov, L. B. Glebov, K. A. Richardson, E. Van Stryland, T. Cardinal, S. H. Park, M. Couzi, and J. L. Brunéel, “Waveguide writing in chalcogenide glasses by a train of femtosecond laser pulses,” Opt. Mater.17(3), 379–386 (2001). [CrossRef]
  7. A. Ganjoo, K. Shimakawa, K. Kitano, and E. A. Davis, “Transient photodarkening in amorphous chalcogenides,” J. Non-Cryst. Solids299-302, 917–923 (2002). [CrossRef]
  8. K. Antoine, H. Jain, M. Vlcek, S. D. Senanayake, and D. A. Drabold, “Chemical origin of polarization-dependent photoinduced changes in an As36Se64glass film via in situ synchrotron x-ray photoelectron spectroscopy,” Phys. Rev. B79(5), 054204 (2009). [CrossRef]
  9. E. Flaxer, M. Klebanov, D. Abrahamoff, S. Noah, and V. Lyubin, “Photodarkening of As50Se50 glassy films under μs light pulses,” Opt. Mater.31(4), 688–690 (2009). [CrossRef]
  10. Q. Yan, H. Jain, J. Ren, D. Zhao, and G. Chen, “Effect of photo-oxidation on photobleaching of GeSe2 and Ge2Se3 films,” J. Phys. Chem. C115(43), 21390–21395 (2011). [CrossRef]
  11. L. Tichý, H. Ticha, P. Nagels, and E. Sleeckx, “A review of the specific role of oxygen in irreversible photo- and thermally induced changes of the optical properties of thin film amorphous chalcogenides,” Opt. Mater.4(6), 771–779 (1995). [CrossRef]
  12. C. A. Spence and S. R. Elliott, “Light-induced oxidation and band-edge shifts in thermally evaporated films of germanium chalcogenide glasses,” Phys. Rev. B Condens. Matter39(8), 5452–5463 (1989). [CrossRef] [PubMed]
  13. R. Naik, K. V. Adarsh, R. Ganesan, and K. S. Sangunni, “Giant photoinduced optical bleaching at room and liquid helium temperatures in Sb/As2S3 multilayered films,” Opt. Commun.284(22), 5319–5322 (2011). [CrossRef]
  14. A. R. Barik, K. V. Adarsh, R. Naik, C. S. S. Sandeep, R. Philip, D. Zhao, and H. Jain, “Photoinduced transparency of effective three-photon absorption coefficient for femtosecond laser pulses in Ge16As29Se55 thin films,” Appl. Phys. Lett.98(20), 201111 (2011). [CrossRef]
  15. A. R. Barik, K. V. Adarsh, R. Naik, R. Ganesan, G. Yang, D. Zhao, H. Jain, and K. Shimakawa, “Role of rigidity and temperature in the kinetics of photodarkening in Ge(x)As(45-x)Se55 thin films,” Opt. Express19(14), 13158–13163 (2011). [CrossRef] [PubMed]
  16. G. Yang, H. Jain, A. Ganjoo, D. Zhao, Y. Xu, H. Zeng, and G. Chen, “A photo-stable chalcogenide glass,” Opt. Express16(14), 10565–10571 (2008). [CrossRef] [PubMed]

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