- Email: [email protected]
Gyrotropy of photoinduced light scattering in glassy As2S3
Physica B 245 (1998) 206—209
Gyrotropy of photoinduced light scattering in glassy As S 2 3 M. Klebanov, V. Lyubin* Department of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Received 7 July 1997; received in revised form 16 October 1997
Abstract Results of the investigation of photoinduced light-scattering gyrotropy and its reorientation in bulk As S glass are 2 3 presented together with the data on photoinduced transmittance gyrotropy. Close correlation between scattering and transmittance gyrotropy permitted to conclude that generation of gyrotropically scattering centers is the basis of the whole group of gyrotropy phenomena induced by sub-band-gap light in bulk chalcogenide glasses. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 78.66.Jg; 42.70.Gi; 61.43.Fs; 31.70.Hg Keywords: Photoinduced phenomena; Photoinduced gyrotropy; Chalcogenide glasses; As S 2 3
1. Introduction It was shown earlier that photoinduced linear dichroism of transmittance and photoinduced circular dichroism of transmittance, referred to as photoinduced optical anisotropy and gyrotropy respectively, can arise in bulk chalcogenide glasses (ChG) when they are irradiated by linearly polarized or circularly polarized sub-band-gap light [1—3]. The phenomenon of photoinduced light scattering was also discovered in bulk ChG [3]. Several serious investigations both experimentally and theoretically of photoinduced anisotropy in ChG were accomplished later [4—11], but the phenomena of photoinduced gyrotropy and photoinduced light scattering continue to be poorly studied. Light scat* Corresponding author: Tel.: #972 7 646 1249; fax: 972 7 647 2903; e-mail: [email protected].
tering was shown to be anisotropic when it is induced by linearly polarized light and a very close correlation between transmittance and scattering anisotropies was also demonstrated [12]. These data permitted to conclude that the generation of scattering centers is the process that determines the whole group of sub-band-gap light-induced anisotropy effects in bulk chalcogenide glasses. The photoinduced optical activity and ellipticity were also shown to be characteristic of the scattered light [12], and these observations permitted to assume that the scattering centers created by inducing light in ChG are not only anisotropic but also gyrotropic. In this paper we present the results of the detailed investigation of photoinduced light-scattering gyrotropy and its reorientation in bulk As S glass 2 3 and discuss the mechanism of photoinduced gyrotropy phenomena in ChG.
0921-4526/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 8 8 4 - 3
M. Klebanov, V. Lyubin / Physica B 245 (1998) 206—209
207
2. Experimental We investigated the As S glassy samples that 2 3 are 2—5 mm thick and have carefully polished parallel facets. The experiments were carried out applying the same experimental installation as was used previously for the study of the photoinduced light-scattering anisotropy [12]. The installation was the modified variant of the set-up discussed in Ref. [3]. He—Ne-laser radiation (hv"1.96 eV, ¼" 10 mW) that is the sub-band-gap radiation for As S glass (E "2.3 eV) was used in all experi2 3 ' ments carried out at room temperature. Electrooptical modulator of this installation allowed to produce left- or right-hand circularly polarized light for both gyrotropy excitation and gyrotropy measurement. Maximum laser light intensity (&2.7 W/cm2) and 80 times weakened light intensity were used for excitation and measurements respectively. The installation allowed to measure simultaneously the laser radiation transmitted through the ChG sample and radiation scattered by the sample to various angles up to 230 mrad. Investigation of angular distribution of the light transmitted through As S sample permitted to consider the light at 2 3 the angles up to 5 mrad as a directly transmitted light and the light at larger angles as a scattered one [12]. We irradiated the sample by strong circularly polarized or non-polarized light (exciting laser beam) and measured the intensity of scattered and transmitted weak circularly polarized probing laser beam.
3. Results and discussion Fig. 1 demonstrates the kinetics of scattered light intensity changes induced by strong non-polarized light and also by strong right-hand and left-hand circularly polarized light (E -radiation and E -radi3 ation). It is seen from the figure that non-polarized radiation induced additional isotropic light-scattering (scattered right-hand and left-hand circularly polarized probing light intesities I and I are 43 4approximately equal). The subsequent strong circularly polarized radiation led to different values of
Fig. 1. The kinetics of scattered circularly polarized probing light intensity changes induced by strong non-polarized and circularly polarized light.
Fig. 2. The kinetics of scattering gyrotropy growth and reorientation caused by circularly polarized inducing light.
scattered probing light intensities I and I , it 43 4means, to the appearance of scattering gyrotropy. This gyrotropy g can be written as G " 4 4 2(I !I )/(I #I ) and its kinetics is shown in 43 4- 43 4Fig. 2. As it is seen, the scattering gyrotropy can be reoriented when polarization state of inducing light is changed to the orthogonal one. It was mentioned above that the transmitted light intensity changes were investigated simultaneously with the scattered light study. Kinetics of transmitted right-hand and left-hand circularly polarized probing light intensities I and I changes 53 5caused by non-polarized, right-hand and left-hand
208
M. Klebanov, V. Lyubin / Physica B 245 (1998) 206—209
circularly polarized inducing light is demonstrated in Fig. 3. We notice again that circularly polarized inducing light results in different values of probing light intensities I and I . Kinetics of transmission 53 5gyrotropy G "2(I !I )/(I #I ) is shown in 5 53 5- 53 5Fig. 4. Comparison of data of Figs. 2 and 4 allows to make a conclusion that scattering gyrotropy and transmittance gyrotropy change in opposite directions: increase of one of them is accompanied by a decrease of the other one and vice versa. This correlation allows us to assume that, in analogy with the case of photoinduced anisotropy [12], the
Fig. 3. The kinetics of transmitted circularly polarized probing light intensity changes induced by strong non-polarized and circularly polarized light.
Fig. 4. The kinetics of transmission gyrotropy growth and reorientation caused by circularly polarized inducing light.
creation of gyrotropically scattering centers is the basis of all phenomena of photoinduced gyrotropy observed previously in bulk ChG, namely, photoinduced transmission gyrotropy, photoinduced optical activity and photoinduced ellipticity [3]. We have to remember that gyrotropy in our case is induced by the sub-band-gap light that is unable to excite lone-pair electrons of the ChG. Such subband-gap excitation could not result in breaking of the interatomic covalent bonds and in initiation of the photostructural transformations as it proceeds under the action of the above-band-gap light [13]. At the same time, the sub-band-gap light can produce some changes in the system of weaker bonds of ChG (Van der Waals bonds or three-center bonds [14]), leading to creation of scattering centers. The structure of such photoinduced centers may be different and they will scatter the probing light isotropically, anisotropically or gyrotropically depending on the polarization state of the inducing radiation. This isotropic, anisotropic or gyrotropic light-scattering, in its turn, is accompanied by observation of isotropic, anisotropic or gyrotropic light-transmission because the transmitted light intensity is always the difference between the incident light intensity and the sum of reflected, absorbed and scattered light intensities. Hisakuni and Tanaka, basing on the results of their experiments [15], assumed that optical phenomena induced in As S bulk samples by the 2 3 He—Ne laser light are due to formation of birefringent self-focusing structure and not due to photoinduced light scattering. In experiments described in Ref. [15], the authors focused the He—Ne laser beam with the outward lens, and in this case the non-orthogonal fall of light to the studied sample actually can complicate the picture of light scattering, supposing even very weak photoinduced selffocusing of the light by the chalcogenide sample. In opposite, in experiments discussed in the present paper and in Ref. [12], we used a non-focused laser beam which falls orthogonally to the surface of the studied sample. In this case, a possible selffocusing effect is very faint and must not distort the picture of light scattering. As to the existence of photo-induced light scattering, we cannot doubt in this effect due to several typical signs of this phenomenon and first of all due to observation of
M. Klebanov, V. Lyubin / Physica B 245 (1998) 206—209
photoinduced light scattering at large angles including 90° scattering that was demonstrated already in Ref. [3].
209
Acknowledgements This research was supported by a grant from the Israel Science Foundation.
4. Conclusion References The results of this research demonstrate that excitation of bulk As S glass by the circularly 2 3 polarized sub-band-gap light which was shown earlier to induce the gyrotropy of light transmission (circular dichroism), is characterized also by the appearance of the gyrotropic light scattering. Moreover, we made a conclusion that such excitation leads to creation of gyrotropically scattering centers that is the basis of all photoinduced gyrotropic phenomena in the studied chalcogenide glass: transmission gyrotropy, optical activity and ellipticity. We want to stress that all these conclusions relate only to the case of gyrotropy excitation by the sub-band-gap light in bulk ChG. The study of photoinduced gyrotropy and photoinduced light scattering in ChG is now in the initial phase. We can hope that further detailed study of these phenomena will result in better understanding of their mechanism, in the same way as detailed investigation of photoinduced anisotropy already led to the development of interesting models of this process [4,6,8—11]. It is worth mentioning in conclusion that Elliott in his review article [16] already discussed the possibility to use the photoinduced gyrotropy in ChG as a probe of local chirality.
[1] V.M. Lyubin, V.K. Tikhomirov, Sov. Phys. JETP Lett. 51 (1990) 587. [2] V.M. Lyubin, V.K. Tikhomirov, Sov. Phys. JETP Lett. 52 (1990) 79. [3] V.M. Lyubin, V.K. Tikhomirov, J. Non-Cryst. Solids. 135 (1991) 37. [4] H. Fritzsche, J. Non-Cryst. Solids 164—166 (1993) 1169. [5] V.K. Tikhomirov, S.R. Elliott, Phys. Rev. B 49 (1994) 17476. [6] H. Fritzsche, Phys. Rev. B 52 (1995) 15854. [7] K. Tanaka, M. Notani, H. Hisakuni, Solid State Commun. 95 (1995) 461. [8] V.K. Tikhomirov, S.R. Elliott, Phys. Rev. B 51 (1995) 5538. [9] K. Tanaka, K. Ishida, N. Yoshida, Phys. Rev. B 54 (1996) 9190. [10] V. Lyubin, M. Klebanov, Phys. Rev. B 53 (1996) 11924. [11] V.K. Tikhomirov, G.J. Adriaenssens, S.R. Elliott, Phys. Rev. B 55 (1997) 660. [12] V. Lyubin, M. Klebanov, S. Rosenwaks, V. Volterra, J. Non-Cryst. Solids. 164—166 (1993) 1165. [13] V. Lyubin, in: D. Adler, H. Fritzsche, S.R. Ovshinsky (Eds.), Physics of Disordered Materials, Plenum Press, New York, 1985, p. 673. [14] S.A. Dembovsky, Solid State Commun. 83 (1992) 761. [15] H. Hisakuni, K. Tanaka, Solid State Commun. 90 (1994) 483. [16] S.R. Elliott, Nature 354 (1991) 445.
Gyrotropy of photoinduced light scattering in glassy As S 2 3 M. Klebanov, V. Lyubin* Department of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Received 7 July 1997; received in revised form 16 October 1997
Abstract Results of the investigation of photoinduced light-scattering gyrotropy and its reorientation in bulk As S glass are 2 3 presented together with the data on photoinduced transmittance gyrotropy. Close correlation between scattering and transmittance gyrotropy permitted to conclude that generation of gyrotropically scattering centers is the basis of the whole group of gyrotropy phenomena induced by sub-band-gap light in bulk chalcogenide glasses. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 78.66.Jg; 42.70.Gi; 61.43.Fs; 31.70.Hg Keywords: Photoinduced phenomena; Photoinduced gyrotropy; Chalcogenide glasses; As S 2 3
1. Introduction It was shown earlier that photoinduced linear dichroism of transmittance and photoinduced circular dichroism of transmittance, referred to as photoinduced optical anisotropy and gyrotropy respectively, can arise in bulk chalcogenide glasses (ChG) when they are irradiated by linearly polarized or circularly polarized sub-band-gap light [1—3]. The phenomenon of photoinduced light scattering was also discovered in bulk ChG [3]. Several serious investigations both experimentally and theoretically of photoinduced anisotropy in ChG were accomplished later [4—11], but the phenomena of photoinduced gyrotropy and photoinduced light scattering continue to be poorly studied. Light scat* Corresponding author: Tel.: #972 7 646 1249; fax: 972 7 647 2903; e-mail: [email protected].
tering was shown to be anisotropic when it is induced by linearly polarized light and a very close correlation between transmittance and scattering anisotropies was also demonstrated [12]. These data permitted to conclude that the generation of scattering centers is the process that determines the whole group of sub-band-gap light-induced anisotropy effects in bulk chalcogenide glasses. The photoinduced optical activity and ellipticity were also shown to be characteristic of the scattered light [12], and these observations permitted to assume that the scattering centers created by inducing light in ChG are not only anisotropic but also gyrotropic. In this paper we present the results of the detailed investigation of photoinduced light-scattering gyrotropy and its reorientation in bulk As S glass 2 3 and discuss the mechanism of photoinduced gyrotropy phenomena in ChG.
0921-4526/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 8 8 4 - 3
M. Klebanov, V. Lyubin / Physica B 245 (1998) 206—209
207
2. Experimental We investigated the As S glassy samples that 2 3 are 2—5 mm thick and have carefully polished parallel facets. The experiments were carried out applying the same experimental installation as was used previously for the study of the photoinduced light-scattering anisotropy [12]. The installation was the modified variant of the set-up discussed in Ref. [3]. He—Ne-laser radiation (hv"1.96 eV, ¼" 10 mW) that is the sub-band-gap radiation for As S glass (E "2.3 eV) was used in all experi2 3 ' ments carried out at room temperature. Electrooptical modulator of this installation allowed to produce left- or right-hand circularly polarized light for both gyrotropy excitation and gyrotropy measurement. Maximum laser light intensity (&2.7 W/cm2) and 80 times weakened light intensity were used for excitation and measurements respectively. The installation allowed to measure simultaneously the laser radiation transmitted through the ChG sample and radiation scattered by the sample to various angles up to 230 mrad. Investigation of angular distribution of the light transmitted through As S sample permitted to consider the light at 2 3 the angles up to 5 mrad as a directly transmitted light and the light at larger angles as a scattered one [12]. We irradiated the sample by strong circularly polarized or non-polarized light (exciting laser beam) and measured the intensity of scattered and transmitted weak circularly polarized probing laser beam.
3. Results and discussion Fig. 1 demonstrates the kinetics of scattered light intensity changes induced by strong non-polarized light and also by strong right-hand and left-hand circularly polarized light (E -radiation and E -radi3 ation). It is seen from the figure that non-polarized radiation induced additional isotropic light-scattering (scattered right-hand and left-hand circularly polarized probing light intesities I and I are 43 4approximately equal). The subsequent strong circularly polarized radiation led to different values of
Fig. 1. The kinetics of scattered circularly polarized probing light intensity changes induced by strong non-polarized and circularly polarized light.
Fig. 2. The kinetics of scattering gyrotropy growth and reorientation caused by circularly polarized inducing light.
scattered probing light intensities I and I , it 43 4means, to the appearance of scattering gyrotropy. This gyrotropy g can be written as G " 4 4 2(I !I )/(I #I ) and its kinetics is shown in 43 4- 43 4Fig. 2. As it is seen, the scattering gyrotropy can be reoriented when polarization state of inducing light is changed to the orthogonal one. It was mentioned above that the transmitted light intensity changes were investigated simultaneously with the scattered light study. Kinetics of transmitted right-hand and left-hand circularly polarized probing light intensities I and I changes 53 5caused by non-polarized, right-hand and left-hand
208
M. Klebanov, V. Lyubin / Physica B 245 (1998) 206—209
circularly polarized inducing light is demonstrated in Fig. 3. We notice again that circularly polarized inducing light results in different values of probing light intensities I and I . Kinetics of transmission 53 5gyrotropy G "2(I !I )/(I #I ) is shown in 5 53 5- 53 5Fig. 4. Comparison of data of Figs. 2 and 4 allows to make a conclusion that scattering gyrotropy and transmittance gyrotropy change in opposite directions: increase of one of them is accompanied by a decrease of the other one and vice versa. This correlation allows us to assume that, in analogy with the case of photoinduced anisotropy [12], the
Fig. 3. The kinetics of transmitted circularly polarized probing light intensity changes induced by strong non-polarized and circularly polarized light.
Fig. 4. The kinetics of transmission gyrotropy growth and reorientation caused by circularly polarized inducing light.
creation of gyrotropically scattering centers is the basis of all phenomena of photoinduced gyrotropy observed previously in bulk ChG, namely, photoinduced transmission gyrotropy, photoinduced optical activity and photoinduced ellipticity [3]. We have to remember that gyrotropy in our case is induced by the sub-band-gap light that is unable to excite lone-pair electrons of the ChG. Such subband-gap excitation could not result in breaking of the interatomic covalent bonds and in initiation of the photostructural transformations as it proceeds under the action of the above-band-gap light [13]. At the same time, the sub-band-gap light can produce some changes in the system of weaker bonds of ChG (Van der Waals bonds or three-center bonds [14]), leading to creation of scattering centers. The structure of such photoinduced centers may be different and they will scatter the probing light isotropically, anisotropically or gyrotropically depending on the polarization state of the inducing radiation. This isotropic, anisotropic or gyrotropic light-scattering, in its turn, is accompanied by observation of isotropic, anisotropic or gyrotropic light-transmission because the transmitted light intensity is always the difference between the incident light intensity and the sum of reflected, absorbed and scattered light intensities. Hisakuni and Tanaka, basing on the results of their experiments [15], assumed that optical phenomena induced in As S bulk samples by the 2 3 He—Ne laser light are due to formation of birefringent self-focusing structure and not due to photoinduced light scattering. In experiments described in Ref. [15], the authors focused the He—Ne laser beam with the outward lens, and in this case the non-orthogonal fall of light to the studied sample actually can complicate the picture of light scattering, supposing even very weak photoinduced selffocusing of the light by the chalcogenide sample. In opposite, in experiments discussed in the present paper and in Ref. [12], we used a non-focused laser beam which falls orthogonally to the surface of the studied sample. In this case, a possible selffocusing effect is very faint and must not distort the picture of light scattering. As to the existence of photo-induced light scattering, we cannot doubt in this effect due to several typical signs of this phenomenon and first of all due to observation of
M. Klebanov, V. Lyubin / Physica B 245 (1998) 206—209
photoinduced light scattering at large angles including 90° scattering that was demonstrated already in Ref. [3].
209
Acknowledgements This research was supported by a grant from the Israel Science Foundation.
4. Conclusion References The results of this research demonstrate that excitation of bulk As S glass by the circularly 2 3 polarized sub-band-gap light which was shown earlier to induce the gyrotropy of light transmission (circular dichroism), is characterized also by the appearance of the gyrotropic light scattering. Moreover, we made a conclusion that such excitation leads to creation of gyrotropically scattering centers that is the basis of all photoinduced gyrotropic phenomena in the studied chalcogenide glass: transmission gyrotropy, optical activity and ellipticity. We want to stress that all these conclusions relate only to the case of gyrotropy excitation by the sub-band-gap light in bulk ChG. The study of photoinduced gyrotropy and photoinduced light scattering in ChG is now in the initial phase. We can hope that further detailed study of these phenomena will result in better understanding of their mechanism, in the same way as detailed investigation of photoinduced anisotropy already led to the development of interesting models of this process [4,6,8—11]. It is worth mentioning in conclusion that Elliott in his review article [16] already discussed the possibility to use the photoinduced gyrotropy in ChG as a probe of local chirality.
[1] V.M. Lyubin, V.K. Tikhomirov, Sov. Phys. JETP Lett. 51 (1990) 587. [2] V.M. Lyubin, V.K. Tikhomirov, Sov. Phys. JETP Lett. 52 (1990) 79. [3] V.M. Lyubin, V.K. Tikhomirov, J. Non-Cryst. Solids. 135 (1991) 37. [4] H. Fritzsche, J. Non-Cryst. Solids 164—166 (1993) 1169. [5] V.K. Tikhomirov, S.R. Elliott, Phys. Rev. B 49 (1994) 17476. [6] H. Fritzsche, Phys. Rev. B 52 (1995) 15854. [7] K. Tanaka, M. Notani, H. Hisakuni, Solid State Commun. 95 (1995) 461. [8] V.K. Tikhomirov, S.R. Elliott, Phys. Rev. B 51 (1995) 5538. [9] K. Tanaka, K. Ishida, N. Yoshida, Phys. Rev. B 54 (1996) 9190. [10] V. Lyubin, M. Klebanov, Phys. Rev. B 53 (1996) 11924. [11] V.K. Tikhomirov, G.J. Adriaenssens, S.R. Elliott, Phys. Rev. B 55 (1997) 660. [12] V. Lyubin, M. Klebanov, S. Rosenwaks, V. Volterra, J. Non-Cryst. Solids. 164—166 (1993) 1165. [13] V. Lyubin, in: D. Adler, H. Fritzsche, S.R. Ovshinsky (Eds.), Physics of Disordered Materials, Plenum Press, New York, 1985, p. 673. [14] S.A. Dembovsky, Solid State Commun. 83 (1992) 761. [15] H. Hisakuni, K. Tanaka, Solid State Commun. 90 (1994) 483. [16] S.R. Elliott, Nature 354 (1991) 445.