1. Introduction

Recently chalcogenide glasses (ChGs) have attracted growing interests [1–14] in the fields of optical materials and nonlinear optics because of their unique properties such as wide infrared (IR) transparent window, low phonon energy, good photosensitivity, and high optical nonlinearity. Typically, mid-IR supercontinuum (SC) generation in nonlinear ChG waveguides is one of the hot topics [2,3,6,7,9,10,12]. The main research aim is to achieve bright broadband mid-IR light sources for a variety of significant applications such as optical metrology, spectroscopy and biomedical optics. It has been confirmed that pumping an optical waveguide with ultrashort pulses is an efficient approach to generate wide mid-IR SC [2,6,7,9]. Currently, mid-IR SC with spectral coverages of ~1.5-7 µm [15], ~2-14 µm [7] and ~2-16 µm [9] have been generated in sulfur, selenium and tellurium-based chalcogenide waveguides, respectively. These coverages have almost reached the transmission limits of corresponding waveguides. Furthermore, in order to achieve high-brightness mid-IR SC for practical applications, the nonlinear chalcogenide waveguide has to be pumped by a light source with a high average power, which could be a high-repetition-rate picosecond (ps)/femtosecond (fs) laser [16,17] or an energetic SC source [12]. To date, the highest-average-power SC generated in sulfur, selenium and tellurium-based chalcogenide waveguides are 1390 mW, 417 mW and ~1 mW [12,18], respectively. Experiments indicated that the output power of the SC generated in chalcogenide waveguides was mainly limited by laser damage thresholds (LDTs) of the waveguide materials. In an effort to reveal the damage mechanisms and provide guidances for improving the damage resistance of the materials, a few researchers have recently studied fs-laser induced damage of several ChGs. S. Messaddeq et al. [19] looked into the surface topography of the damaged area on Ge₂₅Ga₁As₉S₆₅ glass induced by 34 fs pulses (806 nm, 1 kHz). They observed periodical surface structures (or ripples) at the initial damage stage (pulse number ≤ 50), and the ripples started to disappear when the pulse number was more than 100 due to heat accumulation which led to glass softening. C. You et al. [20]. investigated mid-IR fs-laser-induced (3-5 µm, 150 fs, 1 kHz) damage in As₂S₃ and As₂Se₃ glasses, and concluded that the damage was initiated by the accumulation of conduction band electrons and subsequently driven by the thermal accumulation. Y. Zhang et al. [21] measured the damage thresholds of a few stoichiometric As₂S₃-GeS₂ glasses under the irradiation of 150 fs pulses at 3 µm (1 kHz), and found the glass containing more Ge showed better laser damage resistance. Yet, almost no literature has involved the laser damage of ChGs induced by high-repetition-rate fs-pulses. Besides, there is also a lack of comprehensive understanding on compositional dependence of fs-LDTs of ChGs.

In this work, we measured LDTs of a number of Ge-As-S glasses induced by fs-pulses with different repetition rates (RRs) from 1 kHz to 1 MHz. The associated damage mechanisms were discussed, and the compositional dependence of the LDT was systematically investigated. We chose Ge-As-S system for this investigation because it has a relatively large glass-forming region, and Ge-As-S glasses have relatively high LDTs [3], as well as superior thermal and mechanical properties, which makes them the potential materials for nonlinear optics.

2. Experiments

More than twenty glasses were selected for systematically studying the composition dependence of the LDT in Ge-As-S system. The glasses were intentionally selected to cover a large part of the glass forming region [22,23] and include the S-rich, stoichiometric, and S-deficient compositions. The degree of the composition Ge_xAs_yS_100-x-y departing from stoichiometry was quantified by dS = (100-x-y)-2x-1.5y = 100-3x-2.5y. Table 1 lists the compositions and corresponding dS. The Ge-As-S glasses were prepared by melting mixtures of high purity Ge (5N), As (6N) and S (6N) elements in vacuum silica tubes (<10⁻⁵ torr) in a rocking furnace. The experimental details can be found in ref [23].

Table 1. Laser damage thresholds (LDTs) of Ge-As-S glasses under the irradiation of 216-fs pulses (1030 nm) at different repetition rates

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The obtained glass ingots were cut into ~2 mm thick discs and polished according to 20/10 level (US standard MIL-PRF-13830B) for measurements. Before laser damage measurements, each polished disc was inspected using a home-made perspective imaging detection system to ensure that no macroscopic defects (e.g. stripes, bubbles, inclusions, etc.) appear inside the glass. The disc was also examined by a Perkin-Elmer Lambda 950 spectrophotometer to exclude microscopic scattering defects. The experimental setup for the laser damage measurement is illustrated in Fig. 1. The irradiation source was a Light Conversion Pharos femtosecond laser with the pulse width of 216-fs at 1030 nm, and the RR could be tuned from 1 MHz to 1 kHz via changing the pulse picker. A beam splitter was used to divide the laser beam into two, one was focused onto the surface of the samples by a K9 lens with the focal length of 100 mm, and the other one was sent into a detector as a reference beam. The irradiation power on the sample was adjusted with an attenuator. The irradiation duration of the test was 60 s. An optical microscope was used to monitor the surface change of the sample during the test. The minimum average laser power which makes the observable modification on the sample is defined as the critical power (P_cr), and the LDT of the glass can be determined by [20,24]

 

Fig. 1 Experimental set-up for the laser damage threshold measurements.

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3. Results and discussion

Table 1 summarizes the measured LDTs of studied Ge-As-S glass samples when they were irradiated by the 216-fs pulses with different RRs at 1030 nm. Figure 2(a) shows the typical dependence of the LDT as a function of the RR. The LDT decreases sharply with the RR increasing from 1 kHz to 10 kHz, and is getting much gentler with the RR further increasing from 10 kHz to 1 MHz, which could be associated with different optical damage mechanisms of the glasses with fs pulse irradiation at different RRs. It has been demonstrated [20,25] that both avalanche ionization and thermal accumulation may contribute to the optical damage of transparent materials when they are exposed to ultra-short pulses. The damage is generally dominated by avalanche ionization (which is generally motivated by multiphoton ionization) when the RR of the pulses is sufficiently low, while thermal accumulation is getting more prominent and materials melting could happen when they are irradiated by high-repetition-rate pulses [26]. As to the optical damage of Ge-As-S glasses in this research, we believe that avalanche ionization causes the damage when the RR operates at 1 kHz, and the contribution of thermal accumulation is not noticeable; while the thermal accumulation appears as the RR exceeds 10 kHz and becomes a major damage factor when the RR is over 100 kHz, which could explain the decrease trend of the LDT with the increasing RR.

 

Fig. 2 (a) Variation of the laser damage threshold (LDT) of Ge_17.5As₁₅S_67.5 glass when irradiated by fs pulses at different repetition rates and composition dependence of the LDT in Ge₁₅As_xS_85-x (b) and Ge_xAs₁₀S_90-x (c) glasses.

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Figures 2(b) and 2(c) show the composition dependence of the LDT in Ge-As-S glasses. The LDT shows a maximum value at dS = 0, which suggests that the stoichiometric composition has the best resistance to laser irradiation. It is also clear that the S-rich glasses generally possess higher LDTs than the S-deficient ones. Previous studies [27] on the structure of Ge-As-S glasses indicated that quantities of Ge-Ge/As-As homopolar bonds (or wrong bonds) presented in the glass networks of S-deficient compositions (dS<0), and a large number of S-S homopolar bonds appeared in S-rich ones (dS>0), while substantially only heteropolar bonds (e.g. Ge-S and As-S) formed in stoichiometric compositions (dS = 0). Considering the higher bond strengths of the heteropolar bonds (Ge-S 551 kJ/mol, As-S 478 kJ/mol) than those of the homopolar bonds (S-S 425 kJ/mol, As-As 382 kJ/mol, Ge-Ge 274 kJ/mol) [28], the stoichiometric compositions are supposed to have higher average bond energies than the S-deficient/rich ones, which suggests that the high LDTs of the stoichiometric compositions could be attributed to the high average bond energies. Similarly, the higher bond strength of S-S than Ge-Ge/As-S could account for the higher LDTs of the S-rich compositions compared with the S-deficient ones.

The surface morphology of the Ge_17.5As₁₅S_67.5 glass induced by the fs-pulses at different RRs was measured using SEM, as shown in Fig. 3. A crater full of periodically structured ripples forms on the surface of the irradiated area when the pulses operate at 1 kHz and 10 kHz. Similar periodic surface ripples were also observed in Ge₂₅Ga₁As₉S₆₅ and As₂S₃/As₂Se₃ ChGs irradiated by fs laser [19,20]. The ripple structure was considered to be a feature of the optical damage by avalanche ionization. When the pulses operate at 100 kHz, the ripple structure and smooth melting morphology appear simultaneously, implying that both avalanche ionization and thermal accumulation have considerable contributions to the damage. Furthermore, as the RR of the pulses is increased to 1 MHz, only smooth melting features are observed, indicating that thermal accumulation has a significant contribution to the damage. In addition, it is found that the diameter of the damaged area is smaller than that of the focused beam spot (d≈43µm) when the RR of the pulses is 1 kHz or 10 kHz. This is related to the Gaussian intensity distribution of the laser beam which could only make the intensity on the center area reach the LDT. In comparison, when the RR exceeds 100 kHz, the size of the damaged area gets larger than the focused beam spot; and a relatively deep damaged part appear in the center, indicating serious volatilization of materials could take place in the area, which is a typical feature of laser-induced melting [20,25] and implies significant thermal effects take place.

 

Fig. 3 The SEM images of the damaged areas on Ge_17.5As₁₅S_67.5 glass surface when irradiated by fs-pulses at different repetition rates

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To investigate composition changes on the damaged areas, elemental analyses were conducted using EDS. Table 2 lists the chemical compositions of the damaged and undamaged positions on the samples. When the irradiation pulses operate at 1 kHz, the sulfur concentration on the whole damaged area decreases by ~6 at.% and the comparable amount of oxygen appears in the damaged position, while the germanium and arsenic contents almost keep constant. When the RR is more than 10 kHz, the sulfur concentration has a decrease of ~8.7-10.4 at.%, ~8.2-10.1 at.% oxygen presents in the damaged area, and the arsenic content shows a decrease of ~0.5-2.3 at.% simultaneously, while the germanium content shows an increase of ~0.5-2.6 at.%. In the optical damage process of the Ge-As-S materials, multiphoton ionization would lead to the breaking of the chemical bonds, and the ionized elements might trap oxygen in the ambient environment [20,21], which may explain the detected oxygen in the damaged area. It has been observed [19] that the local temperature of irradiated areas on ChGs could increase markedly under high fluence of exposures, giving rise to softening or melting of the materials. We believe that the element losses of sulfur and arsenic are attributed to notable thermal accumulation on the exposed area. When the irradiation pulses operate at 1 kHz, the thermal accumulation might lead to a moderate temperature rise, which is adequate to cause the loss of high-volatile sulfur, but not enough to result in the loss of arsenic with moderate volatility. As the RR of the laser pulses is more than 10 kHz, the thermal accumulation is expected to increase significantly, and a higher temperature rise could occur, which is sufficient to cause volatilization of arsenic, and may explain the increased loss of sulfur and the reduced concentration of arsenic. The loss of arsenic would directly lead to the increase of the germanium content in the Ge-As-S glass, as seen in Table 2.

Table 2. Composition changes of the damaged area on Ge_17.5As₁₅S_67.5 glass surface when exposed to fs-pulses at different repetition rates

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4. Conclusion

When Ge-As-S glasses are exposed to 216-fs pulses with RRs of 1kHz-1MHz at 1030 nm, the LDT decreases with the increasing RR of the irradiation pulses. The stoichiometric compositions possess the best resistance to the optical damage, which could be related to the higher average bond energies of the stoichiometric glasses. When the irradiation pulses operate at 1kHz, the damage is mainly caused by avalanche ionization, and the thermal accumulation gets prominent when the RR is more than 10 kHz and even becomes a major damage factor when the RR exceeds 100 kHz. Significant decreases of sulfur and arsenic contents take place in the damaged area when the RR of the irradiation pulses is more than 10 kHz, further supporting the remarkable contribution of thermal accumulation in the optical damage.