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Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film
Materials Chemistry and Physics xxx (2012) 1e5
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film A.R. Barik a, Mukund Bapna a, Ramakanta Naik a,1, Uday Deshpande b, T. Shripathi b, K.V. Adarsh a, * a b
Department of Physics, Indian Institute of Science Education and Research, Bhopal 462023, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India
h i g h l i g h t s < We present the intensity dependence of transient photodarkening and its kinetics. < Kinetics shows an exponential dependence on pump beam intensity. < Pump beam intensity can be used as an effective tool in controlling TPD.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2012 Received in revised form 10 September 2012 Accepted 25 November 2012
We report intensity-dependent transient photodarkening (TPD) in As50Se50 thin film when illuminated with sub-bandgap light of wavelength 671 nm. TPD is always accompanied by concurrent changes in the far infrared absorption spectrum and points out the structural units associated with such changes. Strikingly TPD and its kinetics show an exponential dependence on the pump beam intensity and we foresee that light intensity can be used as an effective tool to control such processes. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Amorphous materials Thin films Optical properties
1. Introduction In recent years, the demand for chalcogenide glasses (ChG) in light wave technology has increased substantially, because of their prominent photoinduced effects [1]. Among the many photoinduced effects shown by ChG, photodarkening (PD) is of particular importance and find applications in writing high bit rate holograms and optical wave guides [2e8]. Numerous researches have shown that, PD can be induced by continuous and pulsed (femto/nanosecond) lasers, however the mechanism by which PD occurs in both the cases are rather different [9,10]. In recent experiments, we have demonstrated the usefulness of PD as an effective tool to control the multiphoton absorption process [11]. Although, PD is desired for many applications, however it limits the application of ChG in IR
* Corresponding author. E-mail address: [email protected] (K.V. Adarsh). 1 Present address: Department of Physics, Utkal University, Vani Vihar, Bhubaneswar 751004, Odisha, India.
optics and also in ultrafast optical switching, where photo-stability is more important. It has been shown that the total change in PD consists of a transient and a metastable part [12,13], where the transient part decays once the illumination was switched off and leaving the metastable part, which could only be reversed by annealing near the glass transition temperature [14]. In all experiments, metastable PD (MPD) was studied extensively, however in contrast such information was lacking for TPD [15,16]. Naturally, it is quite important to understand TPD since its magnitude is larger than the thermal effects induced by light illumination and hence cause serious problems for device applications. Although, TPD is highly undesired for many applications, nevertheless the recent development shows that it can be effectively used for photonic applications like tunable optical limiters [11]. Further, TPD has an added advantage since the whole process is reversible in real time and thus eliminate the need of intermediate step, for example in MPD, we have to heat treat the samples near the glass transition temperature to reverse the effects. In order to utilize TPD within the framework of scientific applications, we have to increase the magnitude of TPD and at the same time without compromising the
0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
Please cite this article in press as: A.R. Barik, et al., Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
2
A.R. Barik et al. / Materials Chemistry and Physics xxx (2012) 1e5
response time (for onward and reversible time scales). In this context, controlling and enhancing TPD is therefore of crucial importance since ChG provide a versatile platform for many photonics applications. In this paper, we present the intensity dependence of TPD and kinetics in As50Se50 thin films, when illuminated with 671 nm light. Our experimental results clearly demonstrate the exponential growth of TPD followed by an exponential decay in kinetics, when the intensity of illuminating beam was increased from 0.07 to 1.84 W cm2. To understand the origin of TPD, we have performed a pump probe far infrared (FIR) absorption measurement and found that TPD is always accompanied by concurrent changes in the far infrared absorption spectrum. Moreover, our experimental results also point out the possible structural units associated with transient changes.
2. Experimental details Amorphous As50Se50 thin film of w1.0 mm thickness is deposited on a microscope glass substrate by conventional thermal evaporation method in a vacuum of 5 106 torr. TPD in these films are studied by a pump probe optical absorption method by using the experimental setup described previously [17]. In our experiments, we have used a 671 nm (diode pumped solid state laser) light as pump beam and its intensity can be easily varied from 0.07 to nearly 1.8 W cm2 by adjusting the diode current. The probe beam was a low intensity white light in the wavelength range of 350e 1000 nm. Probe beam transmission through the sample was monitored using Ocean Optics HR 400CG Spectrometer, which is capable of measuring the entire spectrum in 50 ms interval. In our experiments, pump beam was expanded considerably to encompass an area that was larger than the probe beam, so that a region of uniform light intensity was examined. The spectra were collected from the time when the pump beam was switched on and continued until the PD was saturated. TPD (pump beam on and off) measurements were carried out only after complete saturation of PD. Transient changes in the FIR absorption was also measured by a similar pump probe setup, where the probe beam wavelength was in the range of 100e400 cm1. In this regard As50Se50 films were also deposited on Si substrate, since the microscope glass is
not transparent in the far infrared region, where the vibrational bands of AsSe occur. 3. Results and discussions At first, we have investigated TPD in As50Se50 thin film at constant pump beam intensity of 1.84 W cm2. Before discussing the results of TPD, it is important to show the time evolution of PD (Fig. 1a) with pump beam illumination. We see from Fig. 1a that PD appears to begin instantaneously, grows gradually, and saturates within a few minutes. The total increase in absorption coefficient (Da) during pump beam illumination consists of both transient and metastable PD. As a result, to study the transient effects, consequent on and off cycle of the pump beam was repeated many times and finally for each illumination we could only see the TPD. Fig. 1b shows the time evolution of TPD and it can be seen that transient effects decay instantaneously and saturate within a few minutes. Following our experimental results, we found that TPD constitutes nearly 5% of the total changes induced during illumination. After demonstrating the large TPD, in an attempt to explain the observed effects, we considered that photoinduced structural transformation leads to two metastable states X and Y, of which X is at slightly higher energy than the Y state. During pump beam illumination, a percolative growth of photon-assisted site switched chalcogenide clusters of high energy X state is formed. However, on turning off the pump beam, the system relaxes to slightly lower energy Y state which is energetically more favored. Naturally, site switching from X to Y state produce TPD and the effects can be easily reversed by turning on the pump beam. Finally, the net PD can be attributed to the transformations from ground state to Y state, which can be reversed only by annealing the sample near the glass transition temperature. In the foregoing, we tried to observe experimentally the transient structural units associated with TPD using pump probe FIR absorption spectrum. In chalcogenide glasses, the constituent atoms are heavy and hence the fundamental phonon vibrations are of low energy. FIR data of asprepared As50Se50 samples on Si substrate show a large number of features in the range of 220e260 cm1 (Fig. 2). However, in describing the data, we concentrated more on the main FIR features at 228 and 249 cm1 correspond to AsSe3 pyramids and Se8 rings
Fig. 1. (a) Temporal evolution of the change in absorption coefficient (Da) of the sample at different times during illumination of the pump beam having an intensity of 1.8 W cm2. Da is calculated as (1/d) ln (T0/T) where d, T and T0 are the thickness of the film, transmittance of the as prepared film and transmittance of the film during illumination, respectively. (b) Temporal evolution the difference in absorption coefficient of the sample at different times immediately after turning off the pump beam with an intensity of 1.8 W cm2. Transient effects saturates nearly after 100 s. Difference spectrum was calculated using the equation (ai af) where ai is the absorption coefficient of the sample after complete saturation of PD and af, absorption coefficient of the sample for a particular time t after turning off the pump beam.
Please cite this article in press as: A.R. Barik, et al., Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
A.R. Barik et al. / Materials Chemistry and Physics xxx (2012) 1e5
Fig. 2. FIR transmission spectra of As50Se50 thin films coated on Si substrate with and without pump beam illumination.
bond stretching respectively [18,19]. The comparison of the time evolution of FIR transmission spectra of 228 and 249 cm1 at on and off states of the pump beam helps us in understanding qualitatively how differently coordinated Se atoms are affected by exposure to laser light (Fig. 3). It is clearly evident from the figure that absorption at 228 cm1 wavelength increases with pump beam illumination and shows transient characteristics while turning off the pump beam (Fig. 3a). However, on the other hand, at 249 cm1, absorption decreases with illumination and does not show any appreciable transient characteristics (Fig. 3b). The observed absorption increase at 228 cm1 and decrease at 249 cm1 during illumination can be understood by considering the compositional heterogeneities created during thermal evaporation of the film on the glass substrate. During deposition, various non-stochiometric atomic fragments of As, As4, Se2, Se8, As4Se5, As4Se4, As2, etc. are formed from the vapor phase [14]. When such a film is illuminated with 671 nm CW light, a considerable fraction of metastable homopolar bonds present in the atomic fragments are broken and subsequently converted into energetically favored AseSe heteropolar bonds. Consequently, light-induced creation of AseSe bonds with illumination results in the increase of FIR absorption at 228 cm1 and decrease at 249 cm1. Our results thus clearly point that chemical ordering is taking place in As50Se50 with illumination. Quite surprisingly on turning off the pump beam, FIR absorption at 228 cm1 decreases considerably and is likely, however that some of the light produced AseSe bonds are decreased while turning off the pump beam. Further, transient characteristics in AseSe FIR absorption are concurrent with the
3
observed effects in the optical absorption. Nevertheless the role of AseSe on TPD can be estimated from FIR data, but at this stage it is too difficult to attribute the origin of TPD only to these bonds, because the whole picture is complex possibly with several interconnected effects. Having demonstrated strong TPD in As50Se50 thin films, as our next step we have examined the intensity dependence of TPD. To avoid any ambiguity related to the magnitude of TPD, we confirmed that the transmission spectra of the samples were same before illumination and secondly, we turned off the pump beam only after complete saturation of PD. In this context, we have selected one wavelength at 630 nm close to the optical bandgap of the sample. Fig. 4a shows intensity dependence of TPD at 630 nm for various pump beam intensities. It can be seen from the figure that TPD shows a dramatic increase with intensity of the pump beam. Further, for all selected intensities of the pump beam, TPD appears to begin instantaneously and it seems there exist no induction period. To get more detailed information on the kinetics and magnitude of TPD, let us assume that during illumination there exist NX photodarkened sites in X state. When we turn off the pump beam NX sites are converted into NY sites and the rate of growth of NY sites can be written as:
dNY ¼ KP ðNX NY Þ KR NY dt
(1)
where KP is the rate at which NX sites are converted into NY sites and KR is the rate at which NY sites converted back into NX sites. By assuming a time-dispersive reactions, i.e. KP ¼ Atb1 and KR ¼ Btb1, the changes in absorption coefficient (Da) can be written as:
Da ¼ C 1 exp
b t
s
þ Das
(2)
where Das, C, t, b and s are the persistent value of Da (MPD), C is temperature-dependent quantity and is equal to maximum change in TPD, illumination time, dispersion parameter (0 < b < 1) and the effective time constant, respectively. As our next step, we have calculated the values of C (which is a measure for TPD) and effective time constant by numerically fitting the experimental data using Eq. (2). As can be seen in Fig. 4b, which depicts the saturated change in TPD for the probe wavelength at 630 nm, shows an exponential increase on the pump beam intensity. From our experimental results, it can be concluded that pump beam intensity has a remarkable role in determining the magnitude of TPD. After demonstrating the large intensity dependence of TPD, it is of great interest to look at their kinetics. In this context, we have plotted the
Fig. 3. Variation of transmittance with time for the wave numbers (a) 228 cm1 (b) 249 cm1, during on and off cycle of the pump beam.
Please cite this article in press as: A.R. Barik, et al., Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
4
A.R. Barik et al. / Materials Chemistry and Physics xxx (2012) 1e5
Fig. 4. (a) Normalized absorbance with time for the wavelength at 630 nm, during on and off cycle at different pump beam intensities. (b) Variation of C (TPD) with pump beam intensity, where C is calculated by numerically fitting the experimental data with Eq. (2). (c) Variation of s with pump beam intensity. For measuring s, we irradiated the sample with 1.8 W cm2 until complete saturation of PD and then turned off the pump beam. Changes in absorption coefficient values are measured simultaneously and s values are calculated by numerically fitting the experimental data with Eq. (2). (d) Variation of s1 with pump beam intensity. For measuring s1, we switched on the pump beam after complete saturation of TPD and measured the change in absorption coefficient. s1 values are calculated by fitting the experimental data thus obtained with Eq. (3).
values of s at 630 nm for various pump beam intensities and are shown in Fig. 4c. Our experimental results clearly demonstrate that s decays exponentially as we increase the intensity of the pump beam. At low intensities, s values are higher, decreases drastically with increase in intensity and eventually saturates when the intensity crosses a threshold (1.5 W cm2). After showing the strong intensity-dependent kinetics of TPD, it is of great importance to see the reversibility of the effect when we turn on the pump beam. Fig. 4a clearly displays that the TPD is indeed fully reversible, and that the response time for the shift is of the order of a few seconds. In order to measure the time at which the effect occurs, we plotted the change in absorption coefficient at 630 nm as a function of time. The change in absorption coefficient follows a stretched exponential behavior
Da ¼ Das 1 exp
b t
s1
(3)
where Das is the saturated value of TPD. Naturally, s1 decays exponentially with the intensity and is shown in Fig. 4d.
absorption during illumination can be understood by considering the compositional heterogeneities created during thermal evaporation of the film and consequently, the transient effects are likely due to some of the light-induced bonds are converted back to its initial configuration [14]. Following our experimental observation, we assume that photoinduced structural transformation leads to two metastable states X and Y of which transitions from X to Y state is responsible for TPD and from ground to Y state accounts for MPD. Further, our experimental results clearly demonstrate that TPD grows exponentially with increase in intensity and saturates above 1.5 W cm2. However on the other hand, s decays exponentially as we increase the intensity. All these results point out that the intensity has a predominant role in determining TPD and can be used as an effective tool to control such processes.
Acknowledgement The authors thank DST (Project no: SR/S2/LOP-003/2010) for financial support and UGC-DAE consortium for scientific research for FIR measurements.
4. Conclusion References In conclusion, we have shown that TPD changes dramatically with pump beam intensity. The comparison of the time evolution of FIR transmission spectrum of 228 and 249 cm1 at on and off states of the pump beam qualitatively shows how differently coordinated Se atoms are affected by exposure to laser light. Interestingly, absorption at 228 cm1 wavelength increases with pump beam illumination; nevertheless it exhibits transient characteristics when the pump beam was switched off. The observed increase in
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Please cite this article in press as: A.R. Barik, et al., Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
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[12] Ashtosh Ganjoo, K. Shimakawa, K. Kitano, E.A. Davis, J. Non-Cryst. Solids 299e 302 (2002) 917. [13] A. Ganjoo, H. Jain, Phys. Rev. B 74 (2006) 024201. [14] K. Shimakawa, A. Kolobov, S.R. Elliot, Adv. Phys. 44 (1995) 475e588. [15] K. Tanaka, Thin Solid Films 157 (1988) 35. [16] A.R. Barik, K.V. Adarsh, R. Naik, R. Ganesan, G. Yang, D. Zhao, H. Jain, K. Shimakawa, Opt. Express 19 (2011) 13158. [17] R. Naik, R. Ganesan, K.V. Adarsh, K.S. Sangunni, V. Takats, S. Kokenyesi, J. NonCryst. Solids 355 (2009) 1943. [18] V. Kovanda, M. Vleck, H. Jain, J. Non-Cryst. Solids 326e327 (2003) 88. [19] G. Lucovsky, A. Mooradian, W. Taylor, G.B. Wright, R.C. Keezer, Solid State Commun. 5 (1967) 113.
Please cite this article in press as: A.R. Barik, et al., Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film A.R. Barik a, Mukund Bapna a, Ramakanta Naik a,1, Uday Deshpande b, T. Shripathi b, K.V. Adarsh a, * a b
Department of Physics, Indian Institute of Science Education and Research, Bhopal 462023, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India
h i g h l i g h t s < We present the intensity dependence of transient photodarkening and its kinetics. < Kinetics shows an exponential dependence on pump beam intensity. < Pump beam intensity can be used as an effective tool in controlling TPD.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2012 Received in revised form 10 September 2012 Accepted 25 November 2012
We report intensity-dependent transient photodarkening (TPD) in As50Se50 thin film when illuminated with sub-bandgap light of wavelength 671 nm. TPD is always accompanied by concurrent changes in the far infrared absorption spectrum and points out the structural units associated with such changes. Strikingly TPD and its kinetics show an exponential dependence on the pump beam intensity and we foresee that light intensity can be used as an effective tool to control such processes. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Amorphous materials Thin films Optical properties
1. Introduction In recent years, the demand for chalcogenide glasses (ChG) in light wave technology has increased substantially, because of their prominent photoinduced effects [1]. Among the many photoinduced effects shown by ChG, photodarkening (PD) is of particular importance and find applications in writing high bit rate holograms and optical wave guides [2e8]. Numerous researches have shown that, PD can be induced by continuous and pulsed (femto/nanosecond) lasers, however the mechanism by which PD occurs in both the cases are rather different [9,10]. In recent experiments, we have demonstrated the usefulness of PD as an effective tool to control the multiphoton absorption process [11]. Although, PD is desired for many applications, however it limits the application of ChG in IR
* Corresponding author. E-mail address: [email protected] (K.V. Adarsh). 1 Present address: Department of Physics, Utkal University, Vani Vihar, Bhubaneswar 751004, Odisha, India.
optics and also in ultrafast optical switching, where photo-stability is more important. It has been shown that the total change in PD consists of a transient and a metastable part [12,13], where the transient part decays once the illumination was switched off and leaving the metastable part, which could only be reversed by annealing near the glass transition temperature [14]. In all experiments, metastable PD (MPD) was studied extensively, however in contrast such information was lacking for TPD [15,16]. Naturally, it is quite important to understand TPD since its magnitude is larger than the thermal effects induced by light illumination and hence cause serious problems for device applications. Although, TPD is highly undesired for many applications, nevertheless the recent development shows that it can be effectively used for photonic applications like tunable optical limiters [11]. Further, TPD has an added advantage since the whole process is reversible in real time and thus eliminate the need of intermediate step, for example in MPD, we have to heat treat the samples near the glass transition temperature to reverse the effects. In order to utilize TPD within the framework of scientific applications, we have to increase the magnitude of TPD and at the same time without compromising the
0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
Please cite this article in press as: A.R. Barik, et al., Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
2
A.R. Barik et al. / Materials Chemistry and Physics xxx (2012) 1e5
response time (for onward and reversible time scales). In this context, controlling and enhancing TPD is therefore of crucial importance since ChG provide a versatile platform for many photonics applications. In this paper, we present the intensity dependence of TPD and kinetics in As50Se50 thin films, when illuminated with 671 nm light. Our experimental results clearly demonstrate the exponential growth of TPD followed by an exponential decay in kinetics, when the intensity of illuminating beam was increased from 0.07 to 1.84 W cm2. To understand the origin of TPD, we have performed a pump probe far infrared (FIR) absorption measurement and found that TPD is always accompanied by concurrent changes in the far infrared absorption spectrum. Moreover, our experimental results also point out the possible structural units associated with transient changes.
2. Experimental details Amorphous As50Se50 thin film of w1.0 mm thickness is deposited on a microscope glass substrate by conventional thermal evaporation method in a vacuum of 5 106 torr. TPD in these films are studied by a pump probe optical absorption method by using the experimental setup described previously [17]. In our experiments, we have used a 671 nm (diode pumped solid state laser) light as pump beam and its intensity can be easily varied from 0.07 to nearly 1.8 W cm2 by adjusting the diode current. The probe beam was a low intensity white light in the wavelength range of 350e 1000 nm. Probe beam transmission through the sample was monitored using Ocean Optics HR 400CG Spectrometer, which is capable of measuring the entire spectrum in 50 ms interval. In our experiments, pump beam was expanded considerably to encompass an area that was larger than the probe beam, so that a region of uniform light intensity was examined. The spectra were collected from the time when the pump beam was switched on and continued until the PD was saturated. TPD (pump beam on and off) measurements were carried out only after complete saturation of PD. Transient changes in the FIR absorption was also measured by a similar pump probe setup, where the probe beam wavelength was in the range of 100e400 cm1. In this regard As50Se50 films were also deposited on Si substrate, since the microscope glass is
not transparent in the far infrared region, where the vibrational bands of AsSe occur. 3. Results and discussions At first, we have investigated TPD in As50Se50 thin film at constant pump beam intensity of 1.84 W cm2. Before discussing the results of TPD, it is important to show the time evolution of PD (Fig. 1a) with pump beam illumination. We see from Fig. 1a that PD appears to begin instantaneously, grows gradually, and saturates within a few minutes. The total increase in absorption coefficient (Da) during pump beam illumination consists of both transient and metastable PD. As a result, to study the transient effects, consequent on and off cycle of the pump beam was repeated many times and finally for each illumination we could only see the TPD. Fig. 1b shows the time evolution of TPD and it can be seen that transient effects decay instantaneously and saturate within a few minutes. Following our experimental results, we found that TPD constitutes nearly 5% of the total changes induced during illumination. After demonstrating the large TPD, in an attempt to explain the observed effects, we considered that photoinduced structural transformation leads to two metastable states X and Y, of which X is at slightly higher energy than the Y state. During pump beam illumination, a percolative growth of photon-assisted site switched chalcogenide clusters of high energy X state is formed. However, on turning off the pump beam, the system relaxes to slightly lower energy Y state which is energetically more favored. Naturally, site switching from X to Y state produce TPD and the effects can be easily reversed by turning on the pump beam. Finally, the net PD can be attributed to the transformations from ground state to Y state, which can be reversed only by annealing the sample near the glass transition temperature. In the foregoing, we tried to observe experimentally the transient structural units associated with TPD using pump probe FIR absorption spectrum. In chalcogenide glasses, the constituent atoms are heavy and hence the fundamental phonon vibrations are of low energy. FIR data of asprepared As50Se50 samples on Si substrate show a large number of features in the range of 220e260 cm1 (Fig. 2). However, in describing the data, we concentrated more on the main FIR features at 228 and 249 cm1 correspond to AsSe3 pyramids and Se8 rings
Fig. 1. (a) Temporal evolution of the change in absorption coefficient (Da) of the sample at different times during illumination of the pump beam having an intensity of 1.8 W cm2. Da is calculated as (1/d) ln (T0/T) where d, T and T0 are the thickness of the film, transmittance of the as prepared film and transmittance of the film during illumination, respectively. (b) Temporal evolution the difference in absorption coefficient of the sample at different times immediately after turning off the pump beam with an intensity of 1.8 W cm2. Transient effects saturates nearly after 100 s. Difference spectrum was calculated using the equation (ai af) where ai is the absorption coefficient of the sample after complete saturation of PD and af, absorption coefficient of the sample for a particular time t after turning off the pump beam.
Please cite this article in press as: A.R. Barik, et al., Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
A.R. Barik et al. / Materials Chemistry and Physics xxx (2012) 1e5
Fig. 2. FIR transmission spectra of As50Se50 thin films coated on Si substrate with and without pump beam illumination.
bond stretching respectively [18,19]. The comparison of the time evolution of FIR transmission spectra of 228 and 249 cm1 at on and off states of the pump beam helps us in understanding qualitatively how differently coordinated Se atoms are affected by exposure to laser light (Fig. 3). It is clearly evident from the figure that absorption at 228 cm1 wavelength increases with pump beam illumination and shows transient characteristics while turning off the pump beam (Fig. 3a). However, on the other hand, at 249 cm1, absorption decreases with illumination and does not show any appreciable transient characteristics (Fig. 3b). The observed absorption increase at 228 cm1 and decrease at 249 cm1 during illumination can be understood by considering the compositional heterogeneities created during thermal evaporation of the film on the glass substrate. During deposition, various non-stochiometric atomic fragments of As, As4, Se2, Se8, As4Se5, As4Se4, As2, etc. are formed from the vapor phase [14]. When such a film is illuminated with 671 nm CW light, a considerable fraction of metastable homopolar bonds present in the atomic fragments are broken and subsequently converted into energetically favored AseSe heteropolar bonds. Consequently, light-induced creation of AseSe bonds with illumination results in the increase of FIR absorption at 228 cm1 and decrease at 249 cm1. Our results thus clearly point that chemical ordering is taking place in As50Se50 with illumination. Quite surprisingly on turning off the pump beam, FIR absorption at 228 cm1 decreases considerably and is likely, however that some of the light produced AseSe bonds are decreased while turning off the pump beam. Further, transient characteristics in AseSe FIR absorption are concurrent with the
3
observed effects in the optical absorption. Nevertheless the role of AseSe on TPD can be estimated from FIR data, but at this stage it is too difficult to attribute the origin of TPD only to these bonds, because the whole picture is complex possibly with several interconnected effects. Having demonstrated strong TPD in As50Se50 thin films, as our next step we have examined the intensity dependence of TPD. To avoid any ambiguity related to the magnitude of TPD, we confirmed that the transmission spectra of the samples were same before illumination and secondly, we turned off the pump beam only after complete saturation of PD. In this context, we have selected one wavelength at 630 nm close to the optical bandgap of the sample. Fig. 4a shows intensity dependence of TPD at 630 nm for various pump beam intensities. It can be seen from the figure that TPD shows a dramatic increase with intensity of the pump beam. Further, for all selected intensities of the pump beam, TPD appears to begin instantaneously and it seems there exist no induction period. To get more detailed information on the kinetics and magnitude of TPD, let us assume that during illumination there exist NX photodarkened sites in X state. When we turn off the pump beam NX sites are converted into NY sites and the rate of growth of NY sites can be written as:
dNY ¼ KP ðNX NY Þ KR NY dt
(1)
where KP is the rate at which NX sites are converted into NY sites and KR is the rate at which NY sites converted back into NX sites. By assuming a time-dispersive reactions, i.e. KP ¼ Atb1 and KR ¼ Btb1, the changes in absorption coefficient (Da) can be written as:
Da ¼ C 1 exp
b t
s
þ Das
(2)
where Das, C, t, b and s are the persistent value of Da (MPD), C is temperature-dependent quantity and is equal to maximum change in TPD, illumination time, dispersion parameter (0 < b < 1) and the effective time constant, respectively. As our next step, we have calculated the values of C (which is a measure for TPD) and effective time constant by numerically fitting the experimental data using Eq. (2). As can be seen in Fig. 4b, which depicts the saturated change in TPD for the probe wavelength at 630 nm, shows an exponential increase on the pump beam intensity. From our experimental results, it can be concluded that pump beam intensity has a remarkable role in determining the magnitude of TPD. After demonstrating the large intensity dependence of TPD, it is of great interest to look at their kinetics. In this context, we have plotted the
Fig. 3. Variation of transmittance with time for the wave numbers (a) 228 cm1 (b) 249 cm1, during on and off cycle of the pump beam.
Please cite this article in press as: A.R. Barik, et al., Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.11.077
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A.R. Barik et al. / Materials Chemistry and Physics xxx (2012) 1e5
Fig. 4. (a) Normalized absorbance with time for the wavelength at 630 nm, during on and off cycle at different pump beam intensities. (b) Variation of C (TPD) with pump beam intensity, where C is calculated by numerically fitting the experimental data with Eq. (2). (c) Variation of s with pump beam intensity. For measuring s, we irradiated the sample with 1.8 W cm2 until complete saturation of PD and then turned off the pump beam. Changes in absorption coefficient values are measured simultaneously and s values are calculated by numerically fitting the experimental data with Eq. (2). (d) Variation of s1 with pump beam intensity. For measuring s1, we switched on the pump beam after complete saturation of TPD and measured the change in absorption coefficient. s1 values are calculated by fitting the experimental data thus obtained with Eq. (3).
values of s at 630 nm for various pump beam intensities and are shown in Fig. 4c. Our experimental results clearly demonstrate that s decays exponentially as we increase the intensity of the pump beam. At low intensities, s values are higher, decreases drastically with increase in intensity and eventually saturates when the intensity crosses a threshold (1.5 W cm2). After showing the strong intensity-dependent kinetics of TPD, it is of great importance to see the reversibility of the effect when we turn on the pump beam. Fig. 4a clearly displays that the TPD is indeed fully reversible, and that the response time for the shift is of the order of a few seconds. In order to measure the time at which the effect occurs, we plotted the change in absorption coefficient at 630 nm as a function of time. The change in absorption coefficient follows a stretched exponential behavior
Da ¼ Das 1 exp
b t
s1
(3)
where Das is the saturated value of TPD. Naturally, s1 decays exponentially with the intensity and is shown in Fig. 4d.
absorption during illumination can be understood by considering the compositional heterogeneities created during thermal evaporation of the film and consequently, the transient effects are likely due to some of the light-induced bonds are converted back to its initial configuration [14]. Following our experimental observation, we assume that photoinduced structural transformation leads to two metastable states X and Y of which transitions from X to Y state is responsible for TPD and from ground to Y state accounts for MPD. Further, our experimental results clearly demonstrate that TPD grows exponentially with increase in intensity and saturates above 1.5 W cm2. However on the other hand, s decays exponentially as we increase the intensity. All these results point out that the intensity has a predominant role in determining TPD and can be used as an effective tool to control such processes.
Acknowledgement The authors thank DST (Project no: SR/S2/LOP-003/2010) for financial support and UGC-DAE consortium for scientific research for FIR measurements.
4. Conclusion References In conclusion, we have shown that TPD changes dramatically with pump beam intensity. The comparison of the time evolution of FIR transmission spectrum of 228 and 249 cm1 at on and off states of the pump beam qualitatively shows how differently coordinated Se atoms are affected by exposure to laser light. Interestingly, absorption at 228 cm1 wavelength increases with pump beam illumination; nevertheless it exhibits transient characteristics when the pump beam was switched off. The observed increase in
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Please cite this article in press as: A.R. Barik, et al., Intensity-dependent transient photodarkening in visible and far infrared absorption spectra of As50Se50 thin film, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.11.077