1. Introduction

Chalcogenide glasses (ChG) exhibit numerous photoinduced effects with bandgap or sub-bandgap illumination. Among these, the most notable effects are photodarkening (PD) in As-based chalcogenides and photobleaching (PB) in Ge-based chalcogenides [1,2]. Besides the fundamental interest in PD/PB in ChG, these effects find useful technological applications in high bit rate waveguide writing, dense holographic recording, etc [3–6].

PD in As-based ChG has been investigated for many years and is believed to originate from photoinduced structural transformations. Interestingly, PD comprises of a transient and a metastable part [7, 8], such that the former decays quickly once the illumination is switched off, leaving behind the metastable part that can only be reversed by annealing near the glass transition temperature [2]. By contrast, light illumination of Ge-based ChG primarily results in PB, which appears to be irreversible [2]. However, recent studies on GeSe₂ thin films have shown that the irreversible PB is accompanied by a transient PD [9, 10]. Many models have been proposed to explain the observed PB in Ge-based ChG, but none of them appears to be applicable to all compositions [11, 12]. In general, it is believed that intrinsic structural changes and photo-oxidation are responsible for the observed PB in Ge-based ChG [10]. Naturally, PD in As based and PB in Ge based ChG glasses calls for experiments to establish the light-induced behaviour of a glassy system consisting of Ge, As and Se as major constituents and to determine how the associated As-Se and Ge-Se parts respond to light illumination jointly. Such information will provide new insight in understanding PD/PB in ChG. With this motivation, we have investigated photoinduced changes in the optical transmittance of Ge₁₉As₂₁Se₆₀ thin films. Our results clearly demonstrate an unusual coexistence of fast photodarkening (PD) and slow photobleaching (PB) in Ge₁₉As₂₁Se₆₀ thin films, when illuminated with a laser of wavelength 671 nm and 2W/cm² intensity.

2. Experimental details

Bulk Ge₁₉As₂₁Se₆₀ glass was prepared by melt-quench method using 99.999% pure As, Ge and Se powders. The cast sample was used as the source material for depositing thin films of average thickness ~1.0 μm by thermal evaporation in a vacuum of about 1 × 10⁻⁶ Torr. PD/PB in these films was studied by a pump-probe optical absorption method using the experimental set up described previously [13]. We have chosen the wavelength of the pump beam as λ = 671nm (from a diode pumped solid state laser, DPSSL) and kept its intensity at 2 W/cm². The probe beam was a low intensity white light in the wavelength regime of 450-1000 nm. During illumination, transmission of the sample was recorded using Ocean Optics high resolution composite grating spectrometer (HR 4000 CG), which has the capability to collect the entire optical pectrum in 2ms. In our experiments, full optical spectrum was collected in real time of 100 ms/spectrum

3. Results and discussions

To estimate the effect of pump beam illumination, first we recorded the transmission spectrum of as-prepared sample in dark condition and denoted it as T_i. Next, we turned on the pump beam and continuously recorded the transmission spectrum as a function of time (T_f) until the whole effect saturated, which took nearly an hour. Figure 1(a) and 1(b) shows the contour plot of T_f/T_i as a function of time for selected wavelengths close to the optical bandgap of the sample. It is clear from the fig. that PD begins almost instantaneously after turning on the pump beam and saturates within a few tens of seconds. To our great surprise, after complete saturation of PD, PB starts to develop and grows, showing a remarkable shift in the transmission. Finally, it also saturates at a value which is well above the initial value, producing overall bleached film. To understand the role of intensity on the unusual coexistence of PD and PB, we have performed the same experiment at a moderately low intensity of 0.2 W/cm². Basically, the whole effect is reproduced even at such low intensities. These experimental results clearly demonstrate the unusual coexistence of PD and PB in Ge₁₉As₂₁Se₆₀ thin films.

 

Fig. 1 Contour plot of T_f/T_i as a function of time at selected wavelengths close to the optical bandgap of Ge₁₉As₂₁Se₆₀ thin film when irradiated with a 671nm laser of intensity: (a) 2 W/cm² and (b) 0.2 W/cm². The color bars indicate the value of T_f/T_i, where T_f and T_i are transmission spectra of the sample at a particular time after the laser was turned on and for as-prepared sample in dark condition, respectively. At indicated wavelengths, transmission decreases initially, followed by a sharp increase in transmission. Time evolution of T_f/T_i at λ = 590nm for the pump beam intensity (c) 2W/cm² and (d) 0.2W/cm². Here the black and red lines represent the experimental data and theoretical fit (using Eq. (5), respectively. Figures clearly demonstrate the coexistence of PD and PB with the former being a significantly faster process.

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To explain the unusual coexistence of PB and PD within the same sample, we assume that the compositional units exist (As-As, Ge-Ge etc) in the film, which were created from the vapour containing various atomic fragments [14,15]. When such a film is illuminated with λ = 671nm light, the structural units comprising of As-Se and Ge-Se respond rather independently and a considerable fraction of metastable homopolar bonds present in the atomic fragments are broken and subsequently converted into energetically favored heteropolar bonds. The reduction in As homopolar bonds presumably present in As-rich clusters results in PD [16], i.e.

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

Thus there exist two parallel mechanisms of PD and PB in these samples with light illumination. Between them PD is the faster process which saturates rapidly. It is then followed by the slower PB, which requires prolonged illumination to saturate. As a result, we observed an unusual coexistence of PD and PB. To model their reaction kinetics, we used a combination of stretched exponential functions that describe PD and PB separately. Rate equation for PD can be written as:

The experimental data fit very well to the stretched exponential functional forms as described in Eq. (5)—see Fig. 1(c) and 1(d). Fitting parameters calculated from theoretical fit are listed in Table 1 . Note that the effective reaction time for PD is relatively short, a few seconds for the laser intensity of 2 W/cm². By contrast, PB is a slower process compared to PD, with much longer reaction times. Interestingly, as we decrease the intensity by a factor of ten, the magnitude of PD and PB remain about the same, but the kinetics change remarkably. In our experiments, we observe a ten times decrease in kinetics, when the pump beam intensity is reduced from 2 to 0.2 W/cm². Thus the intensity of the pump beam predominantly determines the kinetics of PD and PB.

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

View Table

To observe the reversibility of combined transient effects in Ge₁₉As₂₁Se₆₀ film we turned off and on the pump beam after the complete saturation of PB. On turning the pump beam off, the transmission increased further and saturated quickly (Fig. 2 ). When the illumination was switched on subsequently, transmission decreased and reached the values before the illumination was switched off. Subsequent on-off cycles of the pump beam showed reversible transient photodarkening (TPD) every time. The TPD component in Ge₁₉As₂₁Se₆₀ is probably of the same origin (in bond switching and atom movement) that was observed in As chalcogenide films and more recently in Ge₂Se₃ and GeSe₂ [9, 10].

 

Fig. 2 The observation of reversible, transient photodarkening (TPD). When the pump beam was turned off, the relative transparency of the film (T_f/T_i) increases and saturates quickly. However, when the pump beam is turned on, it decreases and quickly returns to the pre-switching value.

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We have measured further the reversible TPD, while the film undergoes PD in the initial few seconds. Such study will help to determine whether it is a process with time lag similar to PB or instantaneous like that of PD. Recall that in our experiments we observed an initial fast PD which is followed by a slow PB. In that context, we illuminated the sample for a few seconds and turned off the pump beam. Interestingly, after turning off the pump beam, transparency of the film increases, which grows gradually and saturates quickly (Fig. 3 ), however it never reaches the initial value prior to illumination indicating the contributions from PD and reversible TPD. As our next step, we turned on the pump beam again and saw the transmission of the film quickly revert back to previous transmission value.

 

Fig. 3 TPD measurements in Ge₁₉As₂₁Se₆₀ thin film for λ = 671 nm, when it undergoes initial PD with pump beam illumination. The dotted red and blue lines indicate the time at which the pump beam is turned on and off respectively. With pump beam illumination transmission decreases quickly. However, when the pump beam is turned off, transmission increases, but never recovers completely showing the metastable and transient component of PD. For consecutive on and off cycle for short time, the TPD is fully reversible. However, when the sample is illuminated for a long time, the sample begins to show the effect of PB.

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

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