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Abstract
We have prepared GexGa8S92-x glasses with x=20, 26.67 and 36, and investigated the photoinduced effects under illumination at sub-bandgap wavelength with different power densities. It was found that, Ge20 and Ge36 undergo photodarkening (PD) and photobleaching (PB), respectively, and the change of transmission ratio with and without illumination increases with increasing illumination power density as well as prolonging illumination time. On the other hand, Ge26.67 is almost optical stable in any cases. This potentially offers a chance to reduce additional optical loss induced by PD and achieve net optical gain in the erbium doped chalcogenide planar waveguide amplifier using chemically stoichiometric Ge26.67 glass.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Chalcogenide glasses are useful in various photonic devices in the infrared because of their high linear and nonlinear refractive index, broad transmission range, ultrafast nonlinear response and low phonon energy [1–4]. Especially, low phonon energy makes rare earth ions-doped chalcogenide glasses promising to emit or amplify the light for various active applications in the infrared, since the radiative emission quantum efficiency of rare-earth ions dissolved in the glass matrix can be significantly enhanced [5,6]. For this reasons, rare earth ion-doped chalcogenide glasses have been extensively investigated as a solution available to develop new lasers working at the mid-infrared [3–7].
However, dissolving high content rare earth ions into chalcogenide is challenging since the rare earth ion clusters can be easily formed in the glasses and thus photoluminescence can be readily quenched. For example, typical As2S3 glasses only can host 0.1%mol Er [4,5]. One of the solutions is to add metal Ga into glass matrix, since Ga can be well dispersed in the glasses and the formation of the chemical bonds between Ga and rare earth ions can effectively reduce the number of the rare earth ion clusters [8,9]. Therefore Ga-contained chalcogenide glasses are considered as an effective medium to host rare earth ions.
Nevertheless, in the last few decades, only few successful reports on active applications from rare earth ion-doped glasses can be found in the literatures, and the lasing wavelength is mostly limited to the near infrared [10–14], but recently extended to 5-6 µm [15–16]. While lots of effects have been paid to purify Ga-contained glasses in order to decrease the defects in the glasses and thus reduce the radiation recombination, we realize that, the nature of weak chemical bonds in chalcogenide glasses compared with those in SiO2 glasses leads to strong photo-induced effects, and their impact on active applications like optical amplifier and lasing has seldom been investigated. As mentioned above, while Ga-contained chalcogenide glasses are considered as an effective medium to host rare earth ions, we found that, optical loss can be significantly increased at a short time scale (up to tens of seconds) when Er3+doped chalcogenide waveguides were exposed to laser, and could be recovered with prolonged illumination [17]. This is clearly due to photo-induced effects in the chalcogenide glasses.
The most observable photo-induced effects in chalcogenide glasses are photodarkening (PD) and photobleaching (PB). PB represents the shift of optical transmission edge to higher energy (blue shift), and a corresponding decrease in absorption coefficient with the increase in optical bandgap Eg. PD is opposite to PB with a red shift of the optical transmission edge upon illumination. [4,18]. While their origin is still arguable, alternative explanations are from photo-induced structural change from homopolar to heteropolar bonds triggered by external energy input or photo-oxidation [19,20]. Many studies also show that chemical composition plays a decisive role in the photo-induced effects. For example, Calvez et.al. showed that PD and photo- expansa broad band” ion tend to vanish in GexAsxSe1-2x glasses [21]. Yang et al. [22], Su et al. [23]. and Nemec et al. [24]. have independently demonstrated a photo-stable phase in Ge-As-Se films with very distinct compositions. All these indicate that, the photosensitivity can be tuned by varying the composition in Ge-As-Se ternary system [25]. However, there are no reports on Ge-Ga-S system which is very useful as the host material for rare earth ion doping. Therefore, the results in Ge-As-Se glasses inspire us to investigate the dependence of photo-induced effects on the compositions in Ge-Ga-S system.
In the present paper, we prepared Ge-Ga-S glasses with various compositions that were S-poor, chemically stoichiometric and S-rich, and investigated the transmittance with and without illumination using sub-bandgap wavelength at different power densities to understand the photo-induced effects in Ge-Ga-S glasses. The time-varying exponential function was used to model the dynamic process of photo-induced effects. The results demonstrated the possibility of tuning photo-induced effects via glass compositions, and potential decrease of optical losses in the materials via suppression of photo-induced effects could open up the avenue of the use of Ge-Ga-S glasses in various active applications.
2. Experiments
Ge-Ga-S glasses were prepared by the melt-quenching method. High pure Ge, Ga, S elements (5 N) were weighted and introduced into cleaned quartz glass ampule before the ampule was evacuated to ∼ 10−3 Pa and sealed. The ampoules were then placed in a rocking furnace, the mixture was melted at 950 °C for 12 hours, and then quenched in cold water. Then, the glass rod was annealed at 300 °C for 4 hours, and then cut into glass disks ($\emptyset $10 mm ${\times} $ 1 mm). The glass disks were further mechanically polished into a thickness of tens of micrometers.
X-ray diffraction measurements showed that, all the XRD patterns of the glasses exhibit broad bands and no any sharp peaks can be detected, confirming amorphous nature of the glasses. The chemical compositions of the glasses were determined by an energy dispersive X-ray spectrometer installed in a Tescan VEGA3 scanning electron microscope, and the difference from their respective nominal composition is less than 0.3%. Some parameters of the physical properties of the glasses can be found in Ref.(26).
We used the pump-probe method to study the photo-induced effects in these samples. The schematic diagram of the experimental set-up is shown in Fig. 1. In short, we choose high-power collimated light-emitting diodes (LED) with a central wavelength of 830 nm (1.49 eV) and a beam diameter of 5 mm. The illumination power is adjusted by a filter in front of the sample. The detection beam is low intensity white light with a diameter of 2 mm and a wavelength range of 550–1000 nm. High resolution optical absorption spectrometer (Ocean Optics HR2000) was used to record the transmittance changes before and after the pump beam irradiation at 10 ms intervals, and all the measurements were performed in air.
Fig. 1. Schematic diagram of the experimental set-up. The frame indicates the distribution of the illumination (red) and probe light (white) beam on the surface of the sample where the probe light beam is overlapped with the illumination beam, but the former one is less than the latter one in the size of the beam as stated in the text.
3. Results and discussion
Regarding photo-induced effects, the difference in as-prepared, annealed films and bulk glasses in terms of quasi-stability and structural disorder have been emphasized [4,18]. The annealed films and bulks have the lowest free energy and smallest disorder, and photo-excitation always produces more unstable and more disordered states, therefore photo-induced effects in the annealed films and bulks are reversible. On the other hand, the as-prepared films are likely to be energetically higher or lower than the illuminated state, therefore, illumination effects become irreversible [4]. Irreversible or Reversible change are defined as whether photo-induced effects can be erased by thermal annealing at some temperatures. In the present paper, thin bulk glasses were used to investigate photo-induced effects in Ge-Ga-S system.
On the other hand, if Ga content is fixed at 8%, the chemically stoichiometric composition is Ge26.67Ga8S65.33, considering that Ge, Ga and S is four-, three- and two-coordinated, respectively, in the glasses [26]. In the present experiments, we measured photo-induced effects in the glasses with various compositions and we found three different behaviors in the glasses corresponding to three different compositional range, e.g., S-rich, chemically stoichiometric and S-poor, respectively. Therefore we only report the typical results here. The LED with four different power density of 10, 122, 244, 400 mW / cm2 were used to illuminate the samples with an illumination time up to 10000 s. Below are the details of the experimental results.
The transmission spectra of as-prepared and illuminated samples were measured and the results were shown in Figs. 2(a), 1(b) and 1(c) for Ge20 (Ge20Ga8S72), Ge26.67 (Ge26.67Ga8S65.33) and Ge36 (Ge36Ga8S56), respectively. It is clear seen that, the transmission edge of the illuminated sample has a significant red shift (moving to longer wavelength) compared with the as-prepared sample in Fig. 1(a) for Ge20, while the dash shifted (moves to shorter wavelength) for Ge36 in Fig. 1(c), and it seems no change upon illumination for chemically stoichiometric Ge26.67 sample as shown in Fig. (b).
Fig. 2. Transmission spectra of as-prepared (red, solid) and illuminated (10000 s, dash) Ge-Ga-S samples (a) Ge20 (Ge20Ga8S72), (b)Ge26.67 (Ge26.67Ga8S65.33), and (c) Ge36 (Ge36Ga8S56), respectively.
We used the Tauc plots to determine the optical band gap of these samples in the as-prepared and illuminated states under different power densities of 10, 122, 244, and 400 mW/cm2, respectively. After the samples were illuminated under a power density of 400 mW/cm2, they were annealed at a temperature that was 20°C lower than their respective glass transition temperature (following the data in Ref.(26)) for 2h, and their optical band gaps were also measured using the same method. All the results are listed in Table 1. From the change of the optical band gap, we can conclude that for Ge20 sample, the energy of the optical band gap decreases from 2.45 eV to 2.4 eV, but this is reversible to 2.45 eV with further annealing, and thus this is a PD process. However, Ge26.67 and Ge36 show different behaviors. There is no visible change in the optical band gap of Ge26.67 before and after illumination, while the optical bandgap energy of Ge36 increases from 1.73 eV to 1.76 eV, and this further increases to 1.81 eV upon annealing, indicating an irreversible PB process.
Table 1. The optical bandgap energy of the as-prepared, illuminated (with different power densities) and annealed samples extracted from the Tauc plots.
It is well known that, the determination of the bandgap from the Tauc plots could have an error bar of 0.01 eV. The present results regarding the change of the optical bandgap is larger than the error bar, and thus confirming the validity of the present approach. The change of the optical bandgap upon illumination in Table 1 is also in agreement with Fig. 2, where Figs. 2(a) and (c) exhibit observable shift of the transmission edge.
We excluded the possible thermal effect during illumination. Schardt et al. [27] reported that under the irradiation of 800nm laser with a power from 50 to 400 mW/cm2, the temperature of the irradiated point was only about 5K. Our previous results using infrared thermal image also showed that the temperature rise of the film irradiated by the 655 nm laser with power of 400 mW/cm2 is less than 3 K [28]. Such a smaller temperature raise has negligible effect on photo-induced effects in the samples.
We recorded the transmission spectra of the samples in the dark, and expressed it as Ti. Then, we turned on the pump beam and recorded the transmission spectrum as (Tf). Finally, Tf/Ti at a certain wavelength was used to investigate the kinetic process of photo-induced effects in samples. In our case, we took a wavelength where the sample has a transmittance of 30% in the dark in the as-prepared state. Figure 3 show three distinct behaviors. For Ge20, Tf / Ti decreases down to 0.85 with prolonged illumination time up to 10000s. For the Ge26.67, Tf/ Ti ratio is almost unchanged at 1. For the Ge36, when the pump laser is turned on, the real-time transmission decreases initially up to 200 s, and then increases gradually with the prolonged illumination. We can conclude that the photobleaching occurs in Ge36 during this process.
Fig. 3. Time evolution of Tf/Ti for Ge20, Ge26.67, and Ge36 samples illuminated by an 830 nm LED with a power intensity of 244 mW/cm2.
The whole dynamic process can be regarded as the linear sum of the contributions from both PD and PB processes. In order to simulate the reaction kinetics of photo-induced effects under four different illuminated power densities, we use the combination of stretching exponential functions to describe photo-induced effects [29] with the quality of the fit as R-square=0.97463:
Figures 4(a) and (b) show Tf/Ti vs. illumination time for Ge20 and Ge36 samples with different illumination power densities, respectively. Each curve in Fig. 3 is fitted by Eq. (1) and all the fitting parameters are listed in Table 2. For Ge20 sample, Tf/Ti exhibits a tendency to decrease with prolonged illumination, corresponding to a PD process. Such a decrease can be accelerated with the increase of illumination power density as shown in Fig. 3(a). Such an acceleration process is reflected by both decreasing τd and ΔTsd in Table 2. When the illumination power density increases from 10 to 400 mW/cm2, ${\tau _d}$ decreases from 10640 s to 2700 s, but ΔTsd decreases from 0.88 to 0.61. On the other hand, Tf/Ti in Ge36 shows an initial decrease and then converts to a gradual increase overtaking its initial value of 1, Such mixed PD and PB behavior is similar to that has been found in Ge19As21Se60 [29]. In Table 2, both τd and τb in Ge36 gradually increases with decreasing power density, more than one order of magnitude difference in τd and τb indicates that, PD is a fast response to the illumination while PB is relatively slow.
Fig. 4. Time evolution of Tf/Ti at 10, 122 and 400 mW/cm2 for Ge20 (a), and Ge36 (b), respectively. The black lines are the experimental results, while the red lines are the fitting based on the Eq. (1).
Table 2. The fitting parameters of photo-induced effects in Ge20 and Ge36 samples at different power densities
Figures 5(a) and (b) show the response of Ge20 and Ge36 samples to switching-on and -off of the illumination, respectively. Switching-off the illumination causes the increase of Tf /Ti. The initial value of Tf/Ti can be restored by turning on the light again. In all cases, the difference of Tf /Ti between switching-on and -off increases with increasing illumination power. The behavior is similar to that in [25,29,30], where the change of Tf /Ti during on-off cycles was ascribed to the mixture of the transient photodarkening (during the pump period) and metastable PD for Ge20 or PB for Ge36 (during the rest period). Once the pump beam is switched off, the transmission increases and saturates quickly. When the illumination is switched on, transmission decreases and reaches the values before the illumination is switched off. In subsequent “on/off” series, transient photodarkening is followed by build-up of metastable PD for Ge20 or PB for Ge36 glass, leading to enhanced amplitude of PD for Ge20 in Fig. 5(a) or PB for Ge36 in Fig. 5(b) during each next resting cycle. This especially becomes obvious with increasing illumination power density or prolonged illumination duration.
We notice that, while the degree of photo-induced effects is sensitive to light intensity as the data presented above, photon energy of the illumination light more or less than optical bandgap of the glasses could also have different effect on the photo-induced effects [4,18]. Nevertheless, the illumination energy of 1.49 eV (830 nm) used in the paper is close to or less than the band gap energy of the samples in any states (e.g., as-prepared, illuminated, annealed states) as shown in Table 1, and this makes it possible to compare all the PIEs induced by the sub-bandgap light in different samples.
Regarding the origin of the photo-induced effects, it has been suggested that the darkening effect is caused by the photoinduced broadening at the top of the valence band, increasing Ge content at the expense of S results in a sulfur deficient network, promoting a conversion of isolated GeS4 to corner- or edge-sharing GeS4 tetrahedra units and GexS(3-x)Ge–GeS(3-x)Gex units, with a corresponding reduction in S–S bonds. The presence of Ge–Ge bonds, which have lower bond energy and higher bond susceptibility than those of S–S bonds, is responsible for the decrease in optical band gap and enhancement of refractive index [25,30,31]. However, we do not observe any structural changes from Raman scattering spectra of the as-prepared and illuminated samples, probably due to the thick bulk samples used in the experiments.
While the origin of the photo-induced effects in chalcogenide glasses is still arguable, our on-going interest is to develop stable host glasses for rare earth ion doping, in which PD should be minimized otherwise the additional optical loss can be introduced. Andriesh et.al. investigated the optical loss and optical-induced absorption in chalcogenide fiber [32]. When extrinsic impurities like hydrides (OH, H2) oxides and hydro-carbonates impurities, which are one of the important factors to induce optical loss in the materials, can be further reduced with the development of the technologies in the research laboratory, intrinsic photo-induced absorption effect can cause the decrease of the intensity of the probing light at the output of the fibre when illuminating the lateral surface of the fiber with bandgap light. Photo-induced increase in optical loss has also been discovered in waveguide of Ge-Ga-Se [17]. This is a big concern for the active applications like amplifier and lasing, since the signal enhancement from Er3+ should exceed the loss of the host glass before the net gain is achieved [14]. We note that, in previous studies, the composition used for chalcogenide waveguide amplifiers are mostly Ge-poor, where the strong PD leads to additional loss and thus reduce the net optical gain in the waveguide.
The present results present clear evidence that, chemically stoichiometric Ge26.67Ga8S65.33 samples exhibit the minimum photo-induced effect in the continuous illumination process, where Tf /Ti has no observable change with the increase of laser power density. Therefore, it could be the best choice of glass composition for waveguide manufacturing in applications of optical amplifiers. This is understandable based on the conversion of heteropolar to homoploar bonds induced by PD [4,18]. The formation of the hompoloar bonds increases the number of the wrong bonds in the glasses and thus increases the optical loss. Therefore, a significant approach could be reducing the intrinsic losses caused by PD via compositional design, which is obviously viable from the present results. Further investigation on the correlation between PB and optical loss may offer an interesting opportunity to see whether strong PB effect could further reduce the optical losses and thus increase the gain of the waveguide using Ge-rich Ge-Ga-S glasses.
4. Conclusion
We have prepared Ge-Ga-S glasses with different compositions, and studied the photo-induced effects in the as-prepared, illuminated and annealed states of the samples. We found that, while PD and PB appear in the Ge-poor and Ge-rich samples, the chemically stoichiometric Ge26.67 sample show no visible photo-induced effects. The dependence Tf / Ti on illumination time and power density indicates that, PD and PB increases with increasing illumination time and power density in Ge20 and Ge36 samples, respectively. While PD behavior certainly has negative effect on the optical loss of the glass, the present results demonstrate that, the chemically stoichiometric Ge26.67 sample could be the best choice as host glasses for rare earth ion doping in waveguide-based amplifier since additional optical loss induced by PD can be suppressed. Nevertheless, it is also possible to use PB effects decreasing optical loss of the materials if further investigation can confirm the positive correlation between the PB and optical loss.
Funding
Key Technologies Research and Development Program (2020YFB1805900); National Natural Science Foundation of China (61775109); 3315 Innovation Team in Ningbo City.
Acknowledgements
This work is supported by the National Key R&D Program of China (2020YFB1805900); Natural Science Foundation of China (Grant No. 61775109); 3315 Innovation Team in Ningbo City, Zhejiang Province, China.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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