Chemical stoichiometric Ge-As-S glasses were prepared, and their thermal properties, refractive index (n), optical bandgap, Raman gain, and femtosecond laser damage were examined. Results revealed that the n and density (ρ) of the glasses decreased as Ge concentration increased, whereas the bandgap and glass transition temperature (Tg) increased. The Raman gain coefficients (gR) of the samples were calculated on the basis of spontaneous Raman scattering spectra. gR decreased from 2.79 × 10−11 m/W for As2S3 to 1.06 × 10−11 m/W for GeS2 as Ge concentration increased. However, the smallest gR was 100 times higher than that of fused silica (0.89 × 10−13 m/W). When these glasses were irradiated by a laser with a pulse width of 150 fs and a power of 33 mW at 3 μm, the damaged area and depth decreased and the damage threshold increased gradually as Ge concentration increased. Raman spectra and composition analysis indicated that surface oxidation probably occurred and sulfur gasified at a high laser power. Although the gR decreased as Ge was added, the laser damage threshold of Ge-As-S glasses was higher than that of the As2S3 glass. Thus, these glasses are potential materials for Raman gain media.
© 2017 Optical Society of America
Stimulated Raman scattering (SRS) is an important third-order nonlinear effect in liquids, gases and solids [1, 2]. SRS can be used as lasers and amplifiers at wavelengths inaccessible by using traditional rare-earth-doped materials. Raman lasers and amplifiers based on silica glasses are commercially available in the wavelength region near 1.5 μm for telecommunication applications [3, 4]. However, the wavelengths of Raman lasers and amplifiers based on silica materials are limited to less than 2 μm because of their multi-phonon absorption edge . Although fluoride glass exhibits low loss in the mid-infrared region up to 6 μm, Raman gain coefficient (gR = 3.52 × 10−14 m/W) is very low, and this requires tens of meters of long fibers to develop Raman lasers . Compared with silica and fluoride glass fibers, chalcogenide glass fiber has a lower phonon energy (~300–450 cm−1) and a wider transparency window in the mid-IR . For example, arsenic sulfide- and selenide-based fibers are transparent up to 7 and 10 μm, respectively . Chalcogenide glasses exhibit large linear and nonlinear refractive indices and high gR (~300 times than that of silica fibers). Chalcogenide glass fibers, including As-Se and As-S, are characterized by a much narrower Raman line width (~60 cm−1) and smaller Raman shifts . With these characteristics, these glass fibers are promising materials for Raman fiber lasers.
Numerous applications require lasers with long wavelengths, and the shift of laser wavelengths based on Raman effect is an efficient method to obtained laser wavelengths longer than 2 μm. For example, Kulkarni et al. reported a maximum of three cascaded Raman Stokes orders in a 12 m long sulfide fiber . However, a low laser damage threshold of sulfide fibers of approximately 1 GW/cm2 limits the fourth or more cascaded Raman orders . White et al. reported fourth-order cascaded SRS in large-core (∼65 μm diameter) As2S3 and As2Se3 fibers with 1.55 and 1.9 μm nanosecond pump pulses, respectively . The respective optical damage thresholds of As2S3 and As2Se3 fibers are approximately 2.9 and 2.8 GW/cm2 for 2 ns pulses at a wavelength of 1.9 μm. The enhanced infrared Raman gain media require materials with high laser damage thresholds. For covalent chalcogenide glasses, the addition of four-fold coordinated Ge to binary As-S glasses can improve the connectivity of a glassy network, help increase Tg and create a glass system with an exceptionally wide glass-forming range. Thus, many physical properties, such as linear and nonlinear refractive index, bandgap, and Tg, have been tuned on the basis of composition. Laser damage threshold also possibly increases as Ge is added to As-S glasses. However, this phenomenon has yet to be reported.
Among chalcogenide glasses, Ge-As-S glasses possess a large glass-forming region, good mechanical, thermal, optical properties, and tunable photosensitivity; as such, these glasses are considered good candidates of infrared nonlinear optical materials [13, 14]. In this work, physical properties, such as refractive index (n), density (ρ), bandgap, and Tg of chemically stoichiometric Ge-As-S glasses, were investigated. The evolution of the gR and optical damage threshold of the Ge-As-S glasses were also systematically investigated. Structural analyses before and after laser damage were conducted through Raman spectroscopy. The corresponding compositions were determined through SEM-EDS.
2.1 Sample Preparation
xGeS2-(100-x)As2S3 (x = 0, 20, 40, 60, 80, 100) glasses labeled S1 to S6 were prepared with the standard melt-quenching method. High purity Ge, As, and S were used as starting materials. These elements (10 g) were weighted in a glove box and loaded into a low -OH silica tube with an inner diameter of 9 mm. The filled tube was evacuated to 10−7 Torr at 90 °C for 2 h, flame sealed, and heated at 900 °C for at least 16 h in a rocking furnace. In the end, the tube containing the melt was quenched in water, and the formed glass was annealed below its Tg for 3 h. Glass samples were cut into disks (~1 mm) and polished to high optical quality for further testing.
Glass transition temperatures were measured using a differential scanning calorimeter (Q2000, TA) under the protection of N2 gas. The samples (5–10 mg) were gradually heated to 500 °C at a rate of 10 K/min. The n at 1.7–17 μm was determined with an infrared variable angle spectroscopic ellipsometer (IR-VASE Mark II, J. A. Wollam). The optical absorption spectrum was obtained with a spectrophotometer (Lambda 950 UV/VIS/NIR, PerkinElmer) at a range from 400 nm to 2500 nm. The ρ of the glasses was measured in pure water with Archimedes method with a precision of 0.001 g/cm3. Raman spectra were obtained by utilizing a confocal micro Raman spectrometer (InVia, Renishaw) with a 785 nm excitation laser at a range of 100–700 cm−1 with a 0.5 cm−1 resolution. The incoming polarized laser beam was focused on the polished surface of the sample by a 50 × microscope objective with an exposure time of 5 s at 0.05 mW. The spectral peak values were considered to calculate the gR. For comparison, the Raman spectrum of the fused silica was also obtained under the same conditions, but exposure time was set to 300 s. Laser damage experiments were performed using a Ti: sapphire femtosecond laser (Mira900D + , Coherent) and an optical parametric amplifier (Legend Elite + OperA Solo, Coherent) delivering 150 fs and 1 kHz pulses at 3 μm. A 33 mW laser was injected into the glass surface for 60 s to create a damaged hole. Laser shining was repeated thrice to measure the average size of the laser damage. The laser damage size and depth were determined by using a super-long depth-of-view optical microscope (VHX-1000E, Keyence) in a 3-D mode. The morphological characteristics and composition of the sample before and after laser damage were acquired by utilizing a scanning electron microscope (SEM, VEGA3 SB-EasyProbe, Tescan) equipped with an energy dispersive spectrometer (EDS, Quantax, Bruker). Elemental mapping images were taken by a SEM (Nova NanoSEM 450, FEI) equipped with an EDS (EDAX Inc.). All of the measurements were conducted at room temperature.
3. Results and discussion
3.1 Refractive index
Figure 1(a) shows the n of the glasses. Among the studied glasses, GeS2 yielded the lowest n and As2S3 exhibited the highest n at the same wavelength. As the Ge concentration increased, the n decreased. It is relative to the ρ of the glass sample and the ionic polarization (p) of constitute elements, but the relationship between them is complicated . The n increased as ρ increased [Fig. 1(b)]. ρ was also associated with the relative concentration of S. The glass with high S concentration was characterized with low ρ because of the small atomic weight of S. The electron cloud around As likely appears distorted and tends to show a high n because of the large radiation radius of As under the action of an external electric field. p of Ge4+ (0.143 Å3) is lower than that of As3+ (0.496 Å3) . When Ge gradually replaced As, the degree of ionic polarization was reduced in the glass. Therefore, a large polarizability corresponded to a high n in the Ge-As-S glasses.
3.2 Optical bandgap
Figure 2 shows the absorption spectra of the Ge-As-S samples. As the Ge concentration increased, a wider range of transmission could be reached, and the absorption edge experienced a blue shift. The absorption spectra also indicated that the bandgap (Eg) became enlarged as the Ge concentration increased. Bandgap wavelength was set at a wavelength at which the linear absorption coefficient was α = 10 cm−1 (Table 1) . The bandgap of glass depends on the average electron affinity of anions, average bond energy of glass and average polarization energy of ions; among these factors, last two are mainly considered . The bond energies of Ge-S, As-S, S-S, Ge-Ge and As-As are 265, 260, 280, 185 and 200 kJ/mol, respectively . For glasses with a stoichiometric composition, the effect of Ge-S is relatively large, that is, a high Ge concentration corresponds to a large bandgap. Furthermore, As3+ is more easily polarized than Ge4+; thus, the glass with high As concentration yields low Eg. In comparison with the n in Fig. 1(a), the glass with a low Eg exhibited a high n .
3.3 Raman gain
The gR of the glasses was calculated by examining the spontaneous Raman scattering spectra after some corrections were introduced and by comparing the obtained spectra with that of the fused silica glass, which is a well-characterized standard. The spectra were excited by a 785 nm laser with vertical polarization under a laser power of 0.05 mW. The excited photon energy is lower than the bandgap energy of the glasses; thus, the photo-darkening effect can be avoided with a low excited laser power . Figure 3(a) illustrates the measured Raman spectra of the Ge-As-S glasses (main panel) and the SiO2 sample (inset). The broad vibrational bands at 280–450 cm−1 belong to the characteristic vibration of [GeS4] tetrahedra and [AsS3] triangular pyramids. A detailed analysis of the Raman spectra is presented in the latter part.
Samples must be obtained under the same conditions to calculate gR accurately. However, exposure time was prolonged to obtain a sufficient Raman scattering signal of SiO2. As such, the measured Raman spectra of SiO2 was divided by 60 (ratio of the exposure time = 300 s/5 s) to normalize the measurement time. The peak Raman intensity of fused silica (approximately at 440 cm−1), which was designated as PSiO2, was used to normalize the Raman spectra of the samples. The gR is expressed as follows :23], is 0.89 × 10−13 m/W at 1064 nm. Therefore, gR of SiO2 at 785 nm could be calculated as 1.2 × 10−13 m/W. Icorr is the corrected Raman intensity expressed as follows:22, 24]:
The refractive indexes of the glasses at 785 nm were extrapolated (Table 1). The n and gR decreased as the Ge concentration increased. Figure 3(b) shows the gR spectra of the samples in comparison with the spectra of silica glass.
3.4 Femtosecond laser damage
With narrow pulse widths and high peak powers, femtosecond lasers can instantaneously induce material gasification as a result of the accumulation of numerous electrons at the conduction band and the absence of heat accumulation . The main causes of femtosecond laser damage on optical materials are avalanche ionization, multi-photon ionization and impurity defect absorption of laser-induced damage. Among these causes, avalanche ionization is the most dominant and is attributed to multi-photon ionization and surface impurities or defects, but these factors are difficult to distinguish . After an ultra-short pulse laser interacts with optical materials, the effects of conduction electrons because of various mechanisms are rapidly increased in a short time. Damage occurs when the conduction band electron density exceeds the critical density . In our study, all of the glasses were irradiated by 150 fs multi-pulses at 3 μm for 60 s with a repetition rate of 1 kHz and an average power of 33 mW. The morphological characteristics of the six damaged glasses are shown in Figs. 4(a)–4(f).
The images in the first column from the top to the bottom correspond to the damaged surfaces of S1 to S6. Black oval pits with surrounding halo formed on the surface of the glasses in all cases, but the area of the damage decreased as the Ge concentration increased. The second column in Fig. 4 is the corresponding 3-D profile of the damaged area. The depth of each pit decreased from 73 μm for sample S1 to 43 μm for sample S6. The images in the third column were obtained with SEM. A damaged morphology could be observed in the internal part of the pit. It consisted of many gully sections, which are similar to a crater. The formation of pits is due to the formation and explosion of plasma during laser radiation . Many particles were also observed around the holes possibly because of the condensed sulfur after sulfur underwent gasification induced by laser irradiation.30]. Under small pulse interval and high intensity irradiation, the energy is deposited in a small volume around the focus point because of multi-photon absorption when a femtosecond pulse is tightly focused in the sample. The photo-generated hot electron plasma rapidly transfers its energy to the lattice, and consequently increase the temperature and pressure . This phenomenon could induce heat accumulation and thus cause sulfur evaporation.
The Raman spectra excited at 785 nm are illustrated in Fig. 5(a) to elucidate the structural changes before and after laser damage occurred. We assigned the Raman bands in accordance with previously reported studies [32–36]. In summary, the region 260 cm−1 to 430 cm−1 of sample S1 can be respectively ascribed to ν1 and ν3 vibrational modes of the C3ν symmetry AsS3 pyramidal unit at 342 and 310 cm−1, and to ν3’ vibrational mode of the C2v symmetry As-S-As water-like unit at 392 cm−1 [Fig. 5(b)]. For the spectrum of S6 (g-GeS2), the bands at 340 and 430 cm−1 were assigned to the core-sharing (CS) and F2 modes of GeS4/2 tetrahedra [Fig. 5(d)], respectively. The bands at 370 and 390 cm−1 were attributed to the edge-sharing (ES) of GeS4/2 tetrahedra [Fig. 5(d)]. As the Ge concentration increased, the shape of the main bands changed from a typical As2S3 glass to GeS2 glass from S2 to S5. Two weak vibration bands at 205 and 236 cm−1 for S3, S4, and S5 before irradiation were assigned to the vibrations of As-rich (As4S4) molecular units. After laser irradiation was induced, four vibration bands appeared at 186, 222, 236, and 273 cm−1 for S1, and these bands were well matched to the positions of the lower frequency bands in the spectrum of crystalline As2S2 . This finding indicated that the structure changed from As2S3 (orpiment) to As2S2 (realgar). As the arsenic concentration decreased, 186, 222, and 273 cm−1 gradually disappeared. The assignments of broad vibration bands from 280 cm−1 to 460 cm−1 were very complicated. After deconvolution, a shoulder peak formed at approximately 366 cm−1, which can be attributed to the ν2 modes of As-O bands  [Fig. 5(c)]. For S6, the vibration band at 420 cm−1 became noticeable [Fig. 5(e)], and this band is attributed to the symmetric stretching mode of Ge-O-Ge bridging oxygens . Hence, surface oxidation was probably observed during femtosecond laser irradiation on the sample surface. The homopolar bands moved from 236 cm−1 (As-As in As4S4 units) to 257 cm−1 (Ge-Ge in S3Ge-GeS3) as the Ge concentration increased and became stronger after laser irradiation. Conversely, the As-S bonds were likely broken to form As-As homopolar bonds in realgar because of the large increase in the low-frequency region (~236 cm−1) in the samples containing As.
The glass compositions before and after damage were determined through EDS. The statistical analysis of elemental variation in Fig. 6 indicated that S was significantly reduced after damage. Therefore, S gasification is a major process at high laser powers. In Fig. 6, 20 mol%–30 mol% of O was also identified in the damaged area. This observation suggested that surface oxidation might occur in the laser damaged region because glass samples were irradiated under the atmosphere.
SEM-EDS mapping helped clarify elemental distribution. Figure 7 illustrates the EDS of the damaged area of S4 (40As2S3-60GeS2) and the elemental mapping of Ge, As, S, and O. The mapping of Ge, As, and O was brightened after damage [upper right region of Figs. 7(c), 7(e), and 7(f)]. By contrast, the mapping of S darkened [Fig. 7(d)]. Hence, the composition of S4 was modified with a high laser power.
In summary, the physical properties of stoichiometric Ge-As-S glasses were investigated in this work. The n and ρ decreased as the Ge concentration increased. By contrast, the bandgap and Tg of Ge-As-S glasses increased. The gR of the samples decreased from 2.79 × 10−11 m/W for As2S3 to 1.06 × 10−11 m/W for GeS2 as the Ge concentration increased. However, the smallest gR was 100 times higher than that of fused silica (0.89 × 10−13 m/W). The area and depth of damage gradually decreased when the glasses were irradiated by laser with a pulse width of 150 fs and a power of 33 mW at 3 μm. The femtosecond laser damage threshold increased from 1057 GW/cm2 to 2647 GW/cm2 as the Ge concentration increased. Raman spectroscopy and composition analysis revealed that surface oxidation probably occurred, and sulfur gasified at high laser power. Although the gR decreased as Ge was added, the laser damage threshold of Ge-As-S glasses was higher than that of As2S3 glass. Therefore, Ge-As-S glasses are promising materials for Raman gain media.
National Natural Science Foundation of China (NSFC) (61435009, 61627815); National Key Research and Development Program of China (2016YFB0303803); K. C. Wong Magna Fund in Ningbo University.
The authors would like to express their gratitude to Prof. Rongping Wang of the Australian National University and Dr. Yuehao Wu of the Ningbo University for their helpful discussions and assistance in writing this manuscript in English.
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