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Measurement of impurity concentration in chalcogenide glasses using optical principle
Optik 125 (2014) 5794–5799
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Measurement of impurity concentration in chalcogenide glasses using optical principle G. Palai ∗ Gandhi Institute for Technological Advancement (GITA), Bhubaneswar, India
a r t i c l e
i n f o
Article history: Received 11 October 2013 Accepted 29 May 2014 Keywords: Chalcogenide glass Reflectance Absorption factor Transmitted intensity
a b s t r a c t Owing to impurity concentration, is important in chalcogenide glass to study various commercial applications, this paper presents a novel technique to measure the impurity concentration in chalcogenide glass at wavelength of 633 nm and 1500 nm using optical principle. Here both reflection and absorption losses are considered to estimate the same impurities. Reflectance is found using plane wave expansion method, where absorption factor is determined using Maxwell’s curl equations. Simulation result reveals that reflectance, absorption factor and transmitted intensity vary linearly with respect to different impurity concentrations. The excellent linear variation of transmitted intensity gives an accurate measurement of impurity concentration in chalcogenide at aforementioned wavelength. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Chalcogenide glasses are a recognized group in organic glassy materials which always contains one or more chalcogenide elements such as Ge, As, Sb, and Ga. These are low-phonon-energy materials and are generally transparent from the visible up to the infrared [1]. These glasses are optically highly nonlinear and could therefore be useful for all-optical switching (AOS) [2]. Chalcogenide glasses are sensitive to the absorption of electromagnetic radiation and show a variety of photoinduced effects as a result of illumination [3]. Various models have been put forward to explain these effects, which can be used to fabricate diffractive, waveguide and fiber structures [4]. Next-generation devices for telecommunication and related applications will rely on the development of materials which possess optimized physical properties that are compatible with packaging requirements for systems in planar or fiber form [5]. This allows suitable integration to existing fiberbased applications, and hence requires appropriate consideration as to material choice, stability, and long-term aging behavior [6]. Actually, the above applications of chalcogenide glasses depend on its intrinsic property. As far as intrinsic properties of chalcogenide glasses are concerned, the composition (impurity composition) in the same plays vital role for different applications.
∗ Tel.: +91 9439045946. E-mail address: [email protected] http://dx.doi.org/10.1016/j.ijleo.2014.07.004 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
Keeping in view of importance of impurity concentration, we propose a novel technique to measure the impurity concentration in chalcogenide grating structure using optical principle. This paper is organized as follows: Section 2 states the principle of measurement. Mathematical treatment is discussed in Section 3. Result and interpretation is presented in Section 4. Finally conclusions are drawn in Section 5. 2. Principle of measurement Chalcogenide glass structure is formed by the combination of two chalcogenide glass separated by air medium (which is called chalcogenide grating structure), such that different type’s chalcogenide glass ((100 − x)GeS2 –xGa2 S3 glasses, 10GeS2 –(30 − x)Ga2 S3–xCdS, (100 − x)GeS2 –x(0.5Ga2 S3 –0.5CdS), Ge10 As10 Se80−x Tex ) having different concentrations of impurities or compositions (x), which is shown in Fig. 1. Here, we measure the amount of impurity concentration in chalcogenide glass grating structure by the incident of light having wavelength 633 nm or 1500 nm. After incident, some amount of light will be reflected from this structure and some will be absorbed by it. Then rest amount of light will be transmitted through the chalcogenide grating structure. The arrow mark in Fig. 1, shows the direction of incident, reflected and transmitted light. In this case, the principle of measurement of concentration is based on the linear variations of reflectance, absorbance and transmittance with respect to impurity concentrations.
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Similarly, the final output intensity through chalcogenide grating structure will be IT3 = IT2 − IR3 IT3 = I0 (1 − R)4
Considering ˛1 , ˛2 be the absorption coefficient and t1 , t2 be the thickness of odd and even layers of the grating structure, the resultant transmitted intensity (IT3 ) chalcogenide grating structure is modified using Maxwell’s curl equation, which is given by [7]
Fig. 1. Schematic diagram of chalcogenide grating structure.
3. Mathematical treatment Though Fig. 1 shows the direction of incident, reflected and transmitted ray, Fig. 2 gives the complete informations the direction of same rays in each side of chalcogenide grating structure. In Fig. 2, it is seen that, ‘I0 ’ be the intensity of incident light, which falls on the left side of this structure. Assuming ‘R’ to be the reflectance from left side, the intensity of reflected light (IR ) is written as IR = RI0
(8)
IT4 = IT3 e−(˛1 t1 +˛2 t2 +˛1 t1 ) IT4 = I0 (1 − R)4 e−(2˛1 t1 +˛2 t2 )
(9)
Using plane wave expansion method, reflectance is found for each impurity concentration for different chalcogenide structures. Then transmitted intensity of light is obtained by substituting the values of R, ˛1 , ˛2 , t1 and t2 corresponding each impurity concentration.
(1)
So the amount of light transmitted (IT ) through the left side of the first layer is IT = I0 − IR ⇒ IT = I0 − RI0
(2)
⇒ IT = I0 (1 − R) Similarly, I0 (1 − R) amount of light incident at the other end of the first layer. Again the amount of light reflected (IR1 ) from this end is IR1 = R(1 − R)I0
(3)
So the amount of light transmitted through the first layer or entered to the second layer (air) is IT1 = I0 − IR1 IT1 = I0 (1 − R) − R(1 − R)I0 IT1 = I0 (1 − R)
(4)
2
Again ‘IT1 ’ be the incident light for end face of second layer and its corresponding amount of light reflected from this end is equal to IR2 = RIT1 IT1 = RI0 (1 − R)2
(5)
And the amount of light transmitted (IT2 ) through second layer (air) of the chalcogenide grating structure is written as IT2 = IT1 − IR2 IT1 − RT1 = I0 (1 − R)2 − R · I0 (1 − R)2 IT2 = I0 (1 − R)
(6)
3
This amount of intensity will be entered to the third layer and its corresponding amount of reflected intensity will be IR3 = RI0 (1 − R)3
Fig. 2. Direction of light ray in chalcogenide grating structure.
(7)
4. Results and discussion Using plane wave expansion method, simulation is carried out to obtain the reflectance from chalcogenide grating structure [8]. The reflectance from such structure depends on structure parameters such as refractive indices and thickness of odd and even layer, which is shown in Table 1 [9]. Table 1 gives information about the variation of refractive indices of different chacogenide materials with respect to impurity concentrations. Here t1 , t2 are the thickness of odd and even, respectively, which is considered, 20 nm and 10 nm. Using data from Table 1 and with the help of plane wave expansion method, simulation is made for reflectance with respect to impurity concentrations, for different glass materials having different impurities. The simulation result for (100 − x)GeS2 –xGa2 S3 glasses with respect to different impurities, which varies from 0 mole% to 30 mole% are shown in Fig. 3(a)–(d). The simulation for other materials (10GeS2 –(30 − x)Ga2 S3 –xCdS, (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], Ge10 As10 Se80−x Tex ) with respect to different impurity concentrations is done but not shown here. Fig. 3(a) represents the simulation result for reflectance of (100 − x)GeS2 –xGa2 S3 glass having impurity concentration ‘0’ mole%. In this graph, it is seen that wavelength (r.u.) is taken along y-axis, where wavelength in m is taken along x-axis. It is also seen from Fig. 3(a) that reflectance is 0.2105 at wavelength, 0.633 m. Similarly, Fig. 3(b) represents the simulation result for reflectance of (100 − x)GeS2 –xGa2 S3 glass having impurity concentration ‘10’ mole%. In this graph, it is seen that wavelength (r.u.) is taken along y-axis, where wavelength in m is taken along x-axis. It is also seen from Fig. 3(b) that reflectance is 0.2406 at wavelength, 0.633 m. Again, Fig. 3(c) represents the simulation result for reflectance of (100 − x)GeS2 –xGa2 S3 glass having impurity concentration ‘20’ mole%. In this graph, it is seen that wavelength (r.u.) is taken along y-axis, where wavelength in m is taken along x-axis. It is also seen from Fig. 3(c) that reflectance is 0.2802 at wavelength, 0.633 m. And, Fig. 3(d) represents the simulation result for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having impurity concentration ‘30’ mole%. In this graph, it is seen that wavelength (r.u.) is taken along y-axis, where wavelength in m is taken along x-axis. It is also seen from Fig. 3(d) that reflectance is 0.3 at wavelength of 0.633 m. Analyzing above paragraphs, a graph is plotted between reflectance from (100 − x)GeS2 –xGa2 S3 glasses along y-axis with
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Table 1 Input parameter of chalcogenide grating structure. Optical properties of GeS2 -based glasses
(100 − x)GeS2 –xGa2 S3 glasses
10GeS2 –(30 − x)Ga2 S3 –xCdS
(100 − x)GeS2 –x(0.5Ga2 S3 –0.5CdS)
Ge10 As10 Se80−x Tex
Thickness glass and air Impurity
Refractive index
0 10 20 30 0 10 15 20 10 20 30 40 0 10 15 20
2.01 2.064 2.21 2.25 2.24 2.2 2.16 2.14 2.12 2.15 2.17 2.19 2.52 2.82 2.9 2.95
respect to impurity (composition) in mole% along x-axis, which is shown in Fig. 4(a). From Fig. 4(a), it is seen that reflectance varies linearly with respect to impurity (composition) concentrations. It is also found that at wavelength 0.633 m, reflectance increases from 0.2105 to 0.3 as impurity increases from 0 mole% to 30 mole%. Apart from this, it is also seen that this linear variation is excellently fitted with linearship (R2 = 0.9861). This excellently linear variation leads to an accurate measurement of composition (x) in (100 − x)GeS2 –xGa2 S3 chalcogenide glass. Using same technique, the simulation is done to obtained the reflectance from 10GeS2 –(30 − x)Ga2 S3 –xCdS, (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], Ge10 As10 Se80−x Tex structure and the same is clearly indicated in Fig. 4(b)–(d). From Fig. 4(b), it is seen that reflectance from 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure varies linearly with respect to impurity (composition) concentrations. It is also found that at wavelength 0.633 m, reflectance decreases from 0.3 to 0.258 as impurity increases from 0 mole% to 20 mole%. Apart from this, it is also seen that this linear variation is excellently fitted with linearship (R2 = 0.9983). This excellent linear variation leads to an accurate measurement of composition in 10GeS2 –(30 − x)Ga2 S3 –xCdS chalcogenide glass. Similarly, from Fig. 4(c), it is seen that reflectance from (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure varies linearly
Wavelength in nm 633
Thickness glass (t1 ) = 20 nm thickness air (t2 ) = 10 nm
633
633
1500
with respect to impurity (composition) concentrations. It is also found that at wavelength 0.633 m, reflectance increases from 0.2498 to 0.2771 as impurity increases from 10 mole% to 40 mole%. Apart from this, it is also seen that this linear variation is excellently fitted with linearship (R2 = 0.9921). This excellent linear variation leads to an accurate measurement of composition in (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], chalcogenide glass. Again, from Fig. 4(d), it is seen that reflectance from Ge10 As10 Se80−x Tex , glass structure varies linearly with respect to impurity (composition) concentrations. It is also found that at wavelength 1.50 m, reflectance increases from 0.16 to 0.27 as impurity increases from 0 mole% to 20 mole%. Apart from this, it is also seen that this linear variation is excellently fitted with linearship (R2 = 0.9776). This excellent linear variation leads to accurate measurement of composition in Ge10 As10 Se80−x Tex , chalcogenide glass. Beside the reflection loss by chalcogenide grating structure, absorption loss by the same also plays important role to find out the resultant transmitted intensity through it. This absorption loss is determined by using Maxwell’s curl equations, which is given by AF = e−2˛1 t1 +˛2 t2 where ˛1 , ˛2 , are the absorption coefficient of glass and air layers, respectively. Similarly, t1 , t2 are the thickness of odd and even
Table 2 Variation of absorption coefficient of different chlogenide glasses with their impurity concentartions. Absorption coefficient of GeS2 -based glasses
Air absorption coefficient (nm) Impurity
Absorption coefficient, nm−1 (×10−5 )
Wavelength in nm
(100 − x)GeS2 –xGa2 S3 glasses
0 10 20 30
55 49.5 44 38.5
633
10−12
10GeS2 –(30 − x)Ga2 S3 –xCdS
0 10 15 20
49.5 44.3 38.8 33.3
633
10−12
(100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS]
10 20 30 40
52.5 50 47.5 45
633
10−12
Ge10 As10 Se80−x Tex
0 10 15 20
49.8 × 10−4 44.3 × 10−4 38.8 × 10−4 33.3 × 10−4
1500
10−10
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Fig. 3. (a) Simulation results for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having doping concentration 0 mole%. (b) Simulation results for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having doping concentration 10 mole%. (c) Simulation results for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having doping concentration 20 mole%. (d) Simulation results for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having doping concentration 30 mole%.
layer, respectively. The absorption coefficient of different types of glass and air with respect to different impurity concentrations at wavelength 0.633 m and 1.50 m is found from literature, which is shown in Table 2 [10–12]. Using data from Tables 1 and 2, simulation is done for absorption factor and result for (100 − x)GeS2 –xGa2 S3 glasses, 10GeS2 –(30 − x)[Ga2 S3 –xCdS], (100 − x)GeS2 –x(0.5Ga2 S3 –0.5CdS), Ge10 As10 Se80−x Tex is shown in Fig. 5(a)–(d).
Fig. 4. (a) Variation of reflectance from (100 − x)GeS2 –xGa2 S3 glass structure with respect to impurity concentration (x), which varies from 0 mole % to 30 mole% at wavelength 0.633 m. (b) Variation of reflectance from 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure with respect to impurity concentration, which varies from 0 mole % to 20 mole% at wavelength, 0.633 m. (c) Variation of reflectance from (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength, 0.633 m. (d) Variation of reflectance from Ge10 As10 Se80−x Tex glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength 1.5 m.
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Fig. 6. Experimental set-up to measure the impurity in chalcogenide glass.
Fig. 5. (a) Variation of absorption factor for (100 − x)GeS2 –xGa2 S3 glass structure with respect to impurity concentration, which varies from 0 mole% to 30 mole% at wavelength 0.633 m. (b) Variation of absorption factor for 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength 0.633 m. (c) Variation of absorption factor for (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure with respect to impurity concentration, which varies from 10 mole% to 40 mole% at wavelength 0.633 m. (d) Variation of absorption factor from Ge10 As10 Se80−x Tex glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength 1.5 m.
In Fig. 5(a)–(d), absorption factor is taken along y-axis, where impurity concentration (composition) in mole% is taken along xaxis. From Fig. 5(a), it is seen that absorption factor for (100 − x)GeS2 –xGa2 S3 glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, absorption factor increases from 0.9782 to 0.98471 as impurity concentration increases from 0 mole% to 30 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9998). Similarly, from Fig. 5(b), it is seen that absorption factor for 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, absorption factor increases from 0.9802 to 0.9821 as impurity concentration increases from 0 mole% to 20 mole%. Apart from this, it is also found that this variation is an excellent fitted with linearship (R2 = 0.9683). And from Fig. 5(c), it is seen that absorption factor for (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, absorption factor increases from 0.9792 to 0.9887 as impurity concentration increases from 10 mole% to 40 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9994). And from Fig. 5(d), it is seen that absorption factor for Ge10 As10 Se80−x Tex , glass structure varies linearly with respect to impurity concentration at wavelength 1.50 m. It is found that, absorption factor decreases from 0.999 to 0.9979 as impurity concentration increases from 10 mole% to 40 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9908). After determining the reflectance and absorption factor, using Eq. (9), the transmitted intensity is found. Before going to discuss the transmitted intensity, let us focus on the experimental set-up by which one can find the amount of impurity in chalcogenide glass by knowing the values of transmitted intensity. The same experimental set-up is shown in Fig. 6. From Fig. 6, it is seen that light having wavelength 633 nm or 1500 nm incident on chalcogenide grating structure [(100 − x)GeS2 –xGa2 S3 , 10GeS2 –(30 − x)Ga2 S3 –xCdS, (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], Ge10 As10 Se80−x Tex ] having different impurity concentrations, then some amount of light will be reflected and some will be absorbed by it and rest amount of light will be transmitted through the chalcogenide grating structure and then reached at photodetector. And finally, it reaches at output end (power meter). Then the transmitted intensity is calculated with respect to different impurity concentrations and it is shown in Fig. 7(a)–(d). In this Fig. 7(a)–(d), transmitted intensity in eV is taken along yaxis, where impurity concentration (composition) in mole% is taken along x-axis. From Fig. 7(a), it is seen that transmitted intensity trough (100 − x)GeS2 –xGa2 S3 glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, transmitted intensity decreases from 0.7477 to 0.4632 as impurity concentration increases from 0 mole% to 30 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9808). This excellent linear
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variation leads to accurate measurement of composition in (100 − x)GeS2 –xGa2 S3 chalcogenide glass. Similarly, from Fig. 7(b), it is seen that transmitted intensity through 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, transmitted intensity increases from 0.4612 to 0.5801 as impurity concentration increases from 0 mole% to 20 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9911). This excellent linear variation leads to accurate measurement of composition in 10GeS2 –(30 − x)Ga2 S3 –xCdS chalcogenide glass. And from Fig. 7(c), it is seen that transmitted intensity through (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, transmitted intensity decreases from 0.614 to 0.525 as impurity concentration increases from 10 mole% to 40 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9901). This excellent linear variation leads to accurate measurement of composition in (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], chalcogenide glass. And from Fig. 7(d), it is seen that transmitted intensity through Ge10 As10 Se80−x Tex , glass structure varies linearly with respect to impurity concentration at wavelength 1.50 m. It is found that, transmitted intensity decreases from 0.41 to 0.23 as impurity concentration increases from 10 mole% to 40 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9711). This excellent linear variation leads to accurate measurement of composition in Ge10 As10 Se80−x Tex , chalcogenide glass. 5. Conclusions Using optical principle, the measurement of impurity concentration in chalcogenide glass is thoroughly discussed in this paper. Here both absorption and reflection loss is considered to find out the transmitted intensity from (100 − x)GeS2 –xGa2 S3 glasses, 10GeS2 –(30 − x)Ga2 S3 –xCdS, (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], Ge10 As10 Se80−x Tex . It is found that reflectance as well as absorption factor varies linearly with respect to impurity concentration. Apart from that, it is also seen that transmitted intensity varies excellently linear with same composition, which leads to an accurate measurement of impurity concentration in chalcogenide glass. References
Fig. 7. (a) Variation of transmitted intensity for (100 − x)GeS2 –xGa2 S3 glass structure with respect to impurity concentration, which varies from 0 mole% to 30 mole% at wavelength 0.633 m. (b) Variation of transmitted intensity for 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength 0.633 m. (c) Variation of transmitted intensity for (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure with respect to impurity concentration, which varies from 10 mole% to 40 mole% at wavelength 0.633 m. (d) Variation of transmitted intensity for Ge10 As10 Se80−x Tex glass structure with respect to impurity concentration (composition), which varies from 0 mole% to 20 mole% at wavelength 1.5 m.
[1] A.B. Seddon, Chalcogenide glasses: a review of their preparation, properties and applications, J. Non-cryst. Solids 184 (1995) 44–50. [2] A. Zakery, S.R. Elliott, Optical Nonlinearities in Chalcogenide Glasses and their Applications Springer Series in Optical Science, Springer-Verlag, Berlin, Heidelberg, 2007. [3] M.C. Rao, K. Ravindranadh, A. Christy Ferdinand, M.S. Shekhawat, Physical properties and applications of chalcogenide glasses – a brief study, IJAPBC 2 (2013). [4] C. Tsay, F. Toor, C.F. Gmachl, B.C. Arnold, Chalcogenide glass waveguides integrated with quantum cascade lasers, Opt. Lett. 35 (2010) 3324–3326. [5] R. Corby, N. Ponnampalam, M. Pai, H. Nguyen, High index contrast waveguides in chalcogenide glass and polymer, Opt. Lett. 11 (2005) 539–546. [6] J. Hu, Planar Chalcogenide Glass Materials and Devices, Institute of Technology, Massachusetts, 2009 (DSpace@MIT). [7] M. Vijaya, G. Rangarajan, Materials Science, Tata McGraw-Hill Education, New Delhi, 2003. [8] G. Palai, Realization of Braggs grating for SOI application using PWE method, Trends Opto-Electro Opt. Commun. 3 (2013) 30–35. [9] A. Zakery, S.R. Elliot, Optical Nonlinearities in Chalcogenide Glasses and their Applications, Springer, Heidelberg, 1999, pp. 92. [10] S. Kang, M. Kwak, B. Park, S. Kim, H. Ryu, D. Chung, S. Jeong, D. Kang, S. Choi, M. Paek, E. Cha, K. Kang, Optical and dielectric properties of chalcogenide glasses at terahertz frequencies, ETRI J. 31 (2009) 667–674. [11] M. Zalkovskij, C. Bisgaard, A. Novitsky, R. Malureanu, D. Savastru, A. Popescu, P. Jepsen, A. Lavrinenko, Ultrabroadband terahertz spectroscopy of chalcogenide glasses, Appl. Phys. Lett. 100 (2012) 031901–31905. [12] A. Singh, A comparative study on optical properties of Se–Zn–In and Se–Zn–Te–In chalcogenide glasses, Optik 124 (2013) 2187–2190.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Measurement of impurity concentration in chalcogenide glasses using optical principle G. Palai ∗ Gandhi Institute for Technological Advancement (GITA), Bhubaneswar, India
a r t i c l e
i n f o
Article history: Received 11 October 2013 Accepted 29 May 2014 Keywords: Chalcogenide glass Reflectance Absorption factor Transmitted intensity
a b s t r a c t Owing to impurity concentration, is important in chalcogenide glass to study various commercial applications, this paper presents a novel technique to measure the impurity concentration in chalcogenide glass at wavelength of 633 nm and 1500 nm using optical principle. Here both reflection and absorption losses are considered to estimate the same impurities. Reflectance is found using plane wave expansion method, where absorption factor is determined using Maxwell’s curl equations. Simulation result reveals that reflectance, absorption factor and transmitted intensity vary linearly with respect to different impurity concentrations. The excellent linear variation of transmitted intensity gives an accurate measurement of impurity concentration in chalcogenide at aforementioned wavelength. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Chalcogenide glasses are a recognized group in organic glassy materials which always contains one or more chalcogenide elements such as Ge, As, Sb, and Ga. These are low-phonon-energy materials and are generally transparent from the visible up to the infrared [1]. These glasses are optically highly nonlinear and could therefore be useful for all-optical switching (AOS) [2]. Chalcogenide glasses are sensitive to the absorption of electromagnetic radiation and show a variety of photoinduced effects as a result of illumination [3]. Various models have been put forward to explain these effects, which can be used to fabricate diffractive, waveguide and fiber structures [4]. Next-generation devices for telecommunication and related applications will rely on the development of materials which possess optimized physical properties that are compatible with packaging requirements for systems in planar or fiber form [5]. This allows suitable integration to existing fiberbased applications, and hence requires appropriate consideration as to material choice, stability, and long-term aging behavior [6]. Actually, the above applications of chalcogenide glasses depend on its intrinsic property. As far as intrinsic properties of chalcogenide glasses are concerned, the composition (impurity composition) in the same plays vital role for different applications.
∗ Tel.: +91 9439045946. E-mail address: [email protected] http://dx.doi.org/10.1016/j.ijleo.2014.07.004 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
Keeping in view of importance of impurity concentration, we propose a novel technique to measure the impurity concentration in chalcogenide grating structure using optical principle. This paper is organized as follows: Section 2 states the principle of measurement. Mathematical treatment is discussed in Section 3. Result and interpretation is presented in Section 4. Finally conclusions are drawn in Section 5. 2. Principle of measurement Chalcogenide glass structure is formed by the combination of two chalcogenide glass separated by air medium (which is called chalcogenide grating structure), such that different type’s chalcogenide glass ((100 − x)GeS2 –xGa2 S3 glasses, 10GeS2 –(30 − x)Ga2 S3–xCdS, (100 − x)GeS2 –x(0.5Ga2 S3 –0.5CdS), Ge10 As10 Se80−x Tex ) having different concentrations of impurities or compositions (x), which is shown in Fig. 1. Here, we measure the amount of impurity concentration in chalcogenide glass grating structure by the incident of light having wavelength 633 nm or 1500 nm. After incident, some amount of light will be reflected from this structure and some will be absorbed by it. Then rest amount of light will be transmitted through the chalcogenide grating structure. The arrow mark in Fig. 1, shows the direction of incident, reflected and transmitted light. In this case, the principle of measurement of concentration is based on the linear variations of reflectance, absorbance and transmittance with respect to impurity concentrations.
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Similarly, the final output intensity through chalcogenide grating structure will be IT3 = IT2 − IR3 IT3 = I0 (1 − R)4
Considering ˛1 , ˛2 be the absorption coefficient and t1 , t2 be the thickness of odd and even layers of the grating structure, the resultant transmitted intensity (IT3 ) chalcogenide grating structure is modified using Maxwell’s curl equation, which is given by [7]
Fig. 1. Schematic diagram of chalcogenide grating structure.
3. Mathematical treatment Though Fig. 1 shows the direction of incident, reflected and transmitted ray, Fig. 2 gives the complete informations the direction of same rays in each side of chalcogenide grating structure. In Fig. 2, it is seen that, ‘I0 ’ be the intensity of incident light, which falls on the left side of this structure. Assuming ‘R’ to be the reflectance from left side, the intensity of reflected light (IR ) is written as IR = RI0
(8)
IT4 = IT3 e−(˛1 t1 +˛2 t2 +˛1 t1 ) IT4 = I0 (1 − R)4 e−(2˛1 t1 +˛2 t2 )
(9)
Using plane wave expansion method, reflectance is found for each impurity concentration for different chalcogenide structures. Then transmitted intensity of light is obtained by substituting the values of R, ˛1 , ˛2 , t1 and t2 corresponding each impurity concentration.
(1)
So the amount of light transmitted (IT ) through the left side of the first layer is IT = I0 − IR ⇒ IT = I0 − RI0
(2)
⇒ IT = I0 (1 − R) Similarly, I0 (1 − R) amount of light incident at the other end of the first layer. Again the amount of light reflected (IR1 ) from this end is IR1 = R(1 − R)I0
(3)
So the amount of light transmitted through the first layer or entered to the second layer (air) is IT1 = I0 − IR1 IT1 = I0 (1 − R) − R(1 − R)I0 IT1 = I0 (1 − R)
(4)
2
Again ‘IT1 ’ be the incident light for end face of second layer and its corresponding amount of light reflected from this end is equal to IR2 = RIT1 IT1 = RI0 (1 − R)2
(5)
And the amount of light transmitted (IT2 ) through second layer (air) of the chalcogenide grating structure is written as IT2 = IT1 − IR2 IT1 − RT1 = I0 (1 − R)2 − R · I0 (1 − R)2 IT2 = I0 (1 − R)
(6)
3
This amount of intensity will be entered to the third layer and its corresponding amount of reflected intensity will be IR3 = RI0 (1 − R)3
Fig. 2. Direction of light ray in chalcogenide grating structure.
(7)
4. Results and discussion Using plane wave expansion method, simulation is carried out to obtain the reflectance from chalcogenide grating structure [8]. The reflectance from such structure depends on structure parameters such as refractive indices and thickness of odd and even layer, which is shown in Table 1 [9]. Table 1 gives information about the variation of refractive indices of different chacogenide materials with respect to impurity concentrations. Here t1 , t2 are the thickness of odd and even, respectively, which is considered, 20 nm and 10 nm. Using data from Table 1 and with the help of plane wave expansion method, simulation is made for reflectance with respect to impurity concentrations, for different glass materials having different impurities. The simulation result for (100 − x)GeS2 –xGa2 S3 glasses with respect to different impurities, which varies from 0 mole% to 30 mole% are shown in Fig. 3(a)–(d). The simulation for other materials (10GeS2 –(30 − x)Ga2 S3 –xCdS, (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], Ge10 As10 Se80−x Tex ) with respect to different impurity concentrations is done but not shown here. Fig. 3(a) represents the simulation result for reflectance of (100 − x)GeS2 –xGa2 S3 glass having impurity concentration ‘0’ mole%. In this graph, it is seen that wavelength (r.u.) is taken along y-axis, where wavelength in m is taken along x-axis. It is also seen from Fig. 3(a) that reflectance is 0.2105 at wavelength, 0.633 m. Similarly, Fig. 3(b) represents the simulation result for reflectance of (100 − x)GeS2 –xGa2 S3 glass having impurity concentration ‘10’ mole%. In this graph, it is seen that wavelength (r.u.) is taken along y-axis, where wavelength in m is taken along x-axis. It is also seen from Fig. 3(b) that reflectance is 0.2406 at wavelength, 0.633 m. Again, Fig. 3(c) represents the simulation result for reflectance of (100 − x)GeS2 –xGa2 S3 glass having impurity concentration ‘20’ mole%. In this graph, it is seen that wavelength (r.u.) is taken along y-axis, where wavelength in m is taken along x-axis. It is also seen from Fig. 3(c) that reflectance is 0.2802 at wavelength, 0.633 m. And, Fig. 3(d) represents the simulation result for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having impurity concentration ‘30’ mole%. In this graph, it is seen that wavelength (r.u.) is taken along y-axis, where wavelength in m is taken along x-axis. It is also seen from Fig. 3(d) that reflectance is 0.3 at wavelength of 0.633 m. Analyzing above paragraphs, a graph is plotted between reflectance from (100 − x)GeS2 –xGa2 S3 glasses along y-axis with
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Table 1 Input parameter of chalcogenide grating structure. Optical properties of GeS2 -based glasses
(100 − x)GeS2 –xGa2 S3 glasses
10GeS2 –(30 − x)Ga2 S3 –xCdS
(100 − x)GeS2 –x(0.5Ga2 S3 –0.5CdS)
Ge10 As10 Se80−x Tex
Thickness glass and air Impurity
Refractive index
0 10 20 30 0 10 15 20 10 20 30 40 0 10 15 20
2.01 2.064 2.21 2.25 2.24 2.2 2.16 2.14 2.12 2.15 2.17 2.19 2.52 2.82 2.9 2.95
respect to impurity (composition) in mole% along x-axis, which is shown in Fig. 4(a). From Fig. 4(a), it is seen that reflectance varies linearly with respect to impurity (composition) concentrations. It is also found that at wavelength 0.633 m, reflectance increases from 0.2105 to 0.3 as impurity increases from 0 mole% to 30 mole%. Apart from this, it is also seen that this linear variation is excellently fitted with linearship (R2 = 0.9861). This excellently linear variation leads to an accurate measurement of composition (x) in (100 − x)GeS2 –xGa2 S3 chalcogenide glass. Using same technique, the simulation is done to obtained the reflectance from 10GeS2 –(30 − x)Ga2 S3 –xCdS, (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], Ge10 As10 Se80−x Tex structure and the same is clearly indicated in Fig. 4(b)–(d). From Fig. 4(b), it is seen that reflectance from 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure varies linearly with respect to impurity (composition) concentrations. It is also found that at wavelength 0.633 m, reflectance decreases from 0.3 to 0.258 as impurity increases from 0 mole% to 20 mole%. Apart from this, it is also seen that this linear variation is excellently fitted with linearship (R2 = 0.9983). This excellent linear variation leads to an accurate measurement of composition in 10GeS2 –(30 − x)Ga2 S3 –xCdS chalcogenide glass. Similarly, from Fig. 4(c), it is seen that reflectance from (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure varies linearly
Wavelength in nm 633
Thickness glass (t1 ) = 20 nm thickness air (t2 ) = 10 nm
633
633
1500
with respect to impurity (composition) concentrations. It is also found that at wavelength 0.633 m, reflectance increases from 0.2498 to 0.2771 as impurity increases from 10 mole% to 40 mole%. Apart from this, it is also seen that this linear variation is excellently fitted with linearship (R2 = 0.9921). This excellent linear variation leads to an accurate measurement of composition in (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], chalcogenide glass. Again, from Fig. 4(d), it is seen that reflectance from Ge10 As10 Se80−x Tex , glass structure varies linearly with respect to impurity (composition) concentrations. It is also found that at wavelength 1.50 m, reflectance increases from 0.16 to 0.27 as impurity increases from 0 mole% to 20 mole%. Apart from this, it is also seen that this linear variation is excellently fitted with linearship (R2 = 0.9776). This excellent linear variation leads to accurate measurement of composition in Ge10 As10 Se80−x Tex , chalcogenide glass. Beside the reflection loss by chalcogenide grating structure, absorption loss by the same also plays important role to find out the resultant transmitted intensity through it. This absorption loss is determined by using Maxwell’s curl equations, which is given by AF = e−2˛1 t1 +˛2 t2 where ˛1 , ˛2 , are the absorption coefficient of glass and air layers, respectively. Similarly, t1 , t2 are the thickness of odd and even
Table 2 Variation of absorption coefficient of different chlogenide glasses with their impurity concentartions. Absorption coefficient of GeS2 -based glasses
Air absorption coefficient (nm) Impurity
Absorption coefficient, nm−1 (×10−5 )
Wavelength in nm
(100 − x)GeS2 –xGa2 S3 glasses
0 10 20 30
55 49.5 44 38.5
633
10−12
10GeS2 –(30 − x)Ga2 S3 –xCdS
0 10 15 20
49.5 44.3 38.8 33.3
633
10−12
(100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS]
10 20 30 40
52.5 50 47.5 45
633
10−12
Ge10 As10 Se80−x Tex
0 10 15 20
49.8 × 10−4 44.3 × 10−4 38.8 × 10−4 33.3 × 10−4
1500
10−10
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Fig. 3. (a) Simulation results for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having doping concentration 0 mole%. (b) Simulation results for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having doping concentration 10 mole%. (c) Simulation results for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having doping concentration 20 mole%. (d) Simulation results for reflectance of (100 − x)GeS2 –xGa2 S3 glasses having doping concentration 30 mole%.
layer, respectively. The absorption coefficient of different types of glass and air with respect to different impurity concentrations at wavelength 0.633 m and 1.50 m is found from literature, which is shown in Table 2 [10–12]. Using data from Tables 1 and 2, simulation is done for absorption factor and result for (100 − x)GeS2 –xGa2 S3 glasses, 10GeS2 –(30 − x)[Ga2 S3 –xCdS], (100 − x)GeS2 –x(0.5Ga2 S3 –0.5CdS), Ge10 As10 Se80−x Tex is shown in Fig. 5(a)–(d).
Fig. 4. (a) Variation of reflectance from (100 − x)GeS2 –xGa2 S3 glass structure with respect to impurity concentration (x), which varies from 0 mole % to 30 mole% at wavelength 0.633 m. (b) Variation of reflectance from 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure with respect to impurity concentration, which varies from 0 mole % to 20 mole% at wavelength, 0.633 m. (c) Variation of reflectance from (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength, 0.633 m. (d) Variation of reflectance from Ge10 As10 Se80−x Tex glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength 1.5 m.
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Fig. 6. Experimental set-up to measure the impurity in chalcogenide glass.
Fig. 5. (a) Variation of absorption factor for (100 − x)GeS2 –xGa2 S3 glass structure with respect to impurity concentration, which varies from 0 mole% to 30 mole% at wavelength 0.633 m. (b) Variation of absorption factor for 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength 0.633 m. (c) Variation of absorption factor for (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure with respect to impurity concentration, which varies from 10 mole% to 40 mole% at wavelength 0.633 m. (d) Variation of absorption factor from Ge10 As10 Se80−x Tex glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength 1.5 m.
In Fig. 5(a)–(d), absorption factor is taken along y-axis, where impurity concentration (composition) in mole% is taken along xaxis. From Fig. 5(a), it is seen that absorption factor for (100 − x)GeS2 –xGa2 S3 glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, absorption factor increases from 0.9782 to 0.98471 as impurity concentration increases from 0 mole% to 30 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9998). Similarly, from Fig. 5(b), it is seen that absorption factor for 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, absorption factor increases from 0.9802 to 0.9821 as impurity concentration increases from 0 mole% to 20 mole%. Apart from this, it is also found that this variation is an excellent fitted with linearship (R2 = 0.9683). And from Fig. 5(c), it is seen that absorption factor for (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, absorption factor increases from 0.9792 to 0.9887 as impurity concentration increases from 10 mole% to 40 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9994). And from Fig. 5(d), it is seen that absorption factor for Ge10 As10 Se80−x Tex , glass structure varies linearly with respect to impurity concentration at wavelength 1.50 m. It is found that, absorption factor decreases from 0.999 to 0.9979 as impurity concentration increases from 10 mole% to 40 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9908). After determining the reflectance and absorption factor, using Eq. (9), the transmitted intensity is found. Before going to discuss the transmitted intensity, let us focus on the experimental set-up by which one can find the amount of impurity in chalcogenide glass by knowing the values of transmitted intensity. The same experimental set-up is shown in Fig. 6. From Fig. 6, it is seen that light having wavelength 633 nm or 1500 nm incident on chalcogenide grating structure [(100 − x)GeS2 –xGa2 S3 , 10GeS2 –(30 − x)Ga2 S3 –xCdS, (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], Ge10 As10 Se80−x Tex ] having different impurity concentrations, then some amount of light will be reflected and some will be absorbed by it and rest amount of light will be transmitted through the chalcogenide grating structure and then reached at photodetector. And finally, it reaches at output end (power meter). Then the transmitted intensity is calculated with respect to different impurity concentrations and it is shown in Fig. 7(a)–(d). In this Fig. 7(a)–(d), transmitted intensity in eV is taken along yaxis, where impurity concentration (composition) in mole% is taken along x-axis. From Fig. 7(a), it is seen that transmitted intensity trough (100 − x)GeS2 –xGa2 S3 glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, transmitted intensity decreases from 0.7477 to 0.4632 as impurity concentration increases from 0 mole% to 30 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9808). This excellent linear
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variation leads to accurate measurement of composition in (100 − x)GeS2 –xGa2 S3 chalcogenide glass. Similarly, from Fig. 7(b), it is seen that transmitted intensity through 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, transmitted intensity increases from 0.4612 to 0.5801 as impurity concentration increases from 0 mole% to 20 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9911). This excellent linear variation leads to accurate measurement of composition in 10GeS2 –(30 − x)Ga2 S3 –xCdS chalcogenide glass. And from Fig. 7(c), it is seen that transmitted intensity through (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure varies linearly with respect to impurity concentration at wavelength 0.633 m. It is found that, transmitted intensity decreases from 0.614 to 0.525 as impurity concentration increases from 10 mole% to 40 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9901). This excellent linear variation leads to accurate measurement of composition in (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], chalcogenide glass. And from Fig. 7(d), it is seen that transmitted intensity through Ge10 As10 Se80−x Tex , glass structure varies linearly with respect to impurity concentration at wavelength 1.50 m. It is found that, transmitted intensity decreases from 0.41 to 0.23 as impurity concentration increases from 10 mole% to 40 mole%. Apart from this, it is also found that this variation is an excellently fitted with linearship (R2 = 0.9711). This excellent linear variation leads to accurate measurement of composition in Ge10 As10 Se80−x Tex , chalcogenide glass. 5. Conclusions Using optical principle, the measurement of impurity concentration in chalcogenide glass is thoroughly discussed in this paper. Here both absorption and reflection loss is considered to find out the transmitted intensity from (100 − x)GeS2 –xGa2 S3 glasses, 10GeS2 –(30 − x)Ga2 S3 –xCdS, (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], Ge10 As10 Se80−x Tex . It is found that reflectance as well as absorption factor varies linearly with respect to impurity concentration. Apart from that, it is also seen that transmitted intensity varies excellently linear with same composition, which leads to an accurate measurement of impurity concentration in chalcogenide glass. References
Fig. 7. (a) Variation of transmitted intensity for (100 − x)GeS2 –xGa2 S3 glass structure with respect to impurity concentration, which varies from 0 mole% to 30 mole% at wavelength 0.633 m. (b) Variation of transmitted intensity for 10GeS2 –(30 − x)Ga2 S3 –xCdS glass structure with respect to impurity concentration, which varies from 0 mole% to 20 mole% at wavelength 0.633 m. (c) Variation of transmitted intensity for (100 − x)GeS2 –x[0.5Ga2 S3 –0.5CdS], glass structure with respect to impurity concentration, which varies from 10 mole% to 40 mole% at wavelength 0.633 m. (d) Variation of transmitted intensity for Ge10 As10 Se80−x Tex glass structure with respect to impurity concentration (composition), which varies from 0 mole% to 20 mole% at wavelength 1.5 m.
[1] A.B. Seddon, Chalcogenide glasses: a review of their preparation, properties and applications, J. Non-cryst. Solids 184 (1995) 44–50. [2] A. Zakery, S.R. Elliott, Optical Nonlinearities in Chalcogenide Glasses and their Applications Springer Series in Optical Science, Springer-Verlag, Berlin, Heidelberg, 2007. [3] M.C. Rao, K. Ravindranadh, A. Christy Ferdinand, M.S. Shekhawat, Physical properties and applications of chalcogenide glasses – a brief study, IJAPBC 2 (2013). [4] C. Tsay, F. Toor, C.F. Gmachl, B.C. Arnold, Chalcogenide glass waveguides integrated with quantum cascade lasers, Opt. Lett. 35 (2010) 3324–3326. [5] R. Corby, N. Ponnampalam, M. Pai, H. Nguyen, High index contrast waveguides in chalcogenide glass and polymer, Opt. Lett. 11 (2005) 539–546. [6] J. Hu, Planar Chalcogenide Glass Materials and Devices, Institute of Technology, Massachusetts, 2009 (DSpace@MIT). [7] M. Vijaya, G. Rangarajan, Materials Science, Tata McGraw-Hill Education, New Delhi, 2003. [8] G. Palai, Realization of Braggs grating for SOI application using PWE method, Trends Opto-Electro Opt. Commun. 3 (2013) 30–35. [9] A. Zakery, S.R. Elliot, Optical Nonlinearities in Chalcogenide Glasses and their Applications, Springer, Heidelberg, 1999, pp. 92. [10] S. Kang, M. Kwak, B. Park, S. Kim, H. Ryu, D. Chung, S. Jeong, D. Kang, S. Choi, M. Paek, E. Cha, K. Kang, Optical and dielectric properties of chalcogenide glasses at terahertz frequencies, ETRI J. 31 (2009) 667–674. [11] M. Zalkovskij, C. Bisgaard, A. Novitsky, R. Malureanu, D. Savastru, A. Popescu, P. Jepsen, A. Lavrinenko, Ultrabroadband terahertz spectroscopy of chalcogenide glasses, Appl. Phys. Lett. 100 (2012) 031901–31905. [12] A. Singh, A comparative study on optical properties of Se–Zn–In and Se–Zn–Te–In chalcogenide glasses, Optik 124 (2013) 2187–2190.