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Abstract
We report a systematical investigation on the mid-infrared nonlinear performances of Ge-Sb-S glasses. Laser damage threshold (Ith) of Ge-Sb-S glasses was measured under femtosecond pulsed laser incidence ranging between 1.55-3.6 μm. It is found that the Ith has the maximum value at stoichiometric composition. Moreover, the relationship between the refractive index refractive (n0) and nonlinear refractive indices (n2) was obtained, following the semi-empirical Miller’s rule. The n2 shows a nonlinear decay with the increase of wavelength. The multi-photon (up to 7-photon) absorption coefficients of Ge-Sb-S glasses were characterized. The composition Ge25Sb10S65 with high Ith was selected as the core of the designed fiber. A compatible composition Ge25Sb8S67 was chosen as the cladding glass. A 10 μm-diameter-core fiber was made via rod-in-tube method. By pumping a 10-cm-long fiber at 4.8 μm with 170 fs (100 kHz) pulses, we achieved a supercontinuum covering the 3–8 μm spectral range. It indicates that Ge-Sb-S glass family is a type of environment-friendly host materials for mid-infrared nonlinear applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In order to fabricate mid-infrared photonic devices, such as fiber sensors, lasers, supercontinuum (SC) generators and amplifiers, materials with good optical properties in the range from 2 μm up to 10 μm or more are generally needed [1,2]. One of the most promising materials in this spectral region is chalcogenide glass, which is an amorphous material based on chalcogen elements including S, Se, and Te [3,4]. Chalcogenide glasses have attracted wide attention due to the excellent optical properties, including broadband infrared transmittance, low vibrational phonon energy, high linear and nonlinear refractive index, and good chemical stability [5,6]. Therefore, it is regarded as one of the ideal host materials for mid-infrared optical devices. For instance, infrared lenses based on Ge-As-Se and Ge-Sb-Se chalcogenide glasses have been widely employed in night-vision systems [7]. Chalcogenide waveguides have been used as nonlinear media to obtain mid-infrared SC light sources [8,9], Raman and Brillouin lasers [10,11].
In chalcogenide glasses, sulfide glasses generally show better laser damage resistance compared with selenide or telluride counterparts, due to the stronger bonding strength. Hence, sulfide glasses are considered as better optical materials for high laser power applications in the mid-infrared [8,12–15]. Amongst chalcogenide glasses, Ge-Sb-S glass has large glass forming area, good mechanical property, high glass transition temperature (Tg) and high laser damage threshold (Ith) [3,15]. Meanwhile, it is environment-friendly because it is arsenic-free. Therefore, it is considered as a promising optical material [16–18]. In recent years, Ge-Sb-S glass and film have been studied, including the research on its application and structural characteristics. For example, Zhang et al. prepared AgI-containing Ge(Ga)–Sb–S glasses and investigated their thermal performances and conductive properties [17]. Du et al. reported photonic device fabrication in Ge23Sb7S70 chalcogenide glass, and they obtained a low loss of 0.5 dB/cm and a high Q-factor of 1.2 × 106 [18]. Mid-infrared laser damage threshold and nonlinearity of chalcogenide glass are of great significance to the design of military and civilian optical devices. The transmitted power in chalcogenide optical waveguides is mainly limited by the Ith of the material. Nonlinear refractive indices (n2) of the glass is closely associated with the nonlinear optical processes, such as advanced light sources, correlated photon pair generation for quantum communication, and soliton formation for data transmission. It has been proved that chalcogenide glasses exhibit better nonlinearity performance at longer wavelengths because of the low attenuation of multiphoton absorption (MPA) [19,20]. Therefore, the quantitative determination of nonlinear performances, including Ith, n2, and MPA coefficients of chalcogenide glasses in mid-infrared region is of great significance for designing the mid-infrared waveguide devices. Sohn et al. reported the MPA (up to 11-photon absorption at 5.1 μm) of Ge23Sb7S70 chalcogenide glass within the measurement spectral range of 1.1-5.5 µm [19]. You et al. reported the measurements of mid-infrared femtosecond laser-induced damage in two commonly used chalcogenide glasses, As2S3 and As2Se3 at 3 and 4 μm [21]. Qiao et al. observed the third-order nonlinearities of Ge-Sn-Se glasses at 3.0 and 3.5 μm [22]. Lin et al. investigated mid-infrared optical properties of Sn-Sb-Se glass system at 4 µm by Z-scan method [23].
In this study, Ge-Sb-S chalcogenide glasses with different chemical ratio and network structure were prepared by melt-quenching technology. The mid-infrared nonlinear optical performances, including Ith, n2 and MPA coefficients, were systematically studied. A step-index fiber using the Ge25Sb10S65 core, which has a high Ith was fabricated. Broadband 3-8μm SC with ∼5 mW output power was generated in a 10-cm-long fiber, under the excitation of 4.8-μm ultrafast laser with 170 fs pulse width of and 100 kHz repetition rate.
2. Experiment
Two representative groups were designed to investigate the properties of Ge-Sb-S glasses. One group is GexSb10S(90-x), where x = 15, 20, 25, 28.5, 30, 32.5. The other group is Ge15SbxS(85-x), where x = 10, 25, 20, 22, 25, 28. These glasses cover sulfur-rich, stoichiometric, and sulfur-deficient compositions. Here, dS = (100-x-y)-2x-1.5y = 100-3x-2.5y is used to quantitatively indicate the deviation of the composition GexSbyS100-x-y away from the stoichiometry. Accordingly, the network structure of glass is characterized by average coordination number =4x+3y+2(1-x-y) = 2x + y+2.
The glasses were prepared by vacuum melt-quenching method. The experimental details can be found in our previous reports [8,24]. The fibers were prepared by a two-stage rod-in-tube method. The details and the used instruments can be found in Refs. [25–27].
A commercial differential scanning calorimeter (DSC, Q2000, TA Instruments) was employed to measure the glass Tg. The ramp rate was set as 10 ℃/min. The transmission spectra from 0.5 to 2.5 μm were measured by a spectrophotometer (Lambda 950, Perkin-Elmer), while the spectra in 2.5-25 μm were measured by a Fourier Transform Infrared Spectrophotometer (FTIR, Tensor 27, Bruker). An IR-VASE ellipsometer (J. A. Woollam) was used to measure the n0. Diffuse reflectance infrared spectroscopy method was used to determine the optical bandgaps (Eg) of the glasses, via the Lambda 950 spectrophotometer. Fine glass powders were used in the measurement. The spontaneous Raman scattering spectra of the glasses were collected by a home-made micro-Raman spectrometer under the excitation of a 785 nm laser diode.
Following US standard MIL-PRF-13830B, glass samples with two parallel polished faces were prepared. The polished surface was with the scratch-dig of 20/10, which is typically required for precision laser applications, such as laser damage and Z-Scan characterizations in the following. The experimental setup for the laser damage measurement has been introduced previously [3,8]. The Ith of the glass can be determined by [3,8,21]:
where Pcr, R, τ and d are the minimum average laser power which makes the observable modification on the sample, the repetition rate of the laser, the pulse width and the 1/e2 diameter of the focused beam spot. Light Conversion's ORPHEUS OPA (optical parametric amplifier), a commercial wavelength-tunable femtosecond laser was used as the source. The operation wavelengths were set at 1.55, 2.4, 3.0 and 3.6 μm, respectively. The pulse width and the repetition frequency were 170 fs and 100 kHz, respectively. The irradiation time was set as 60 s. The n2 and nonlinear absorption coefficients were obtained by the Z-Scan technique, whose measurement setup has been described elsewhere [8,28]. The OPA laser system was still chosen as a light source and the central wavelengths were set at 1.55, 2.4, 3.0 and 3.6 μm. The power was monitored by InGaAs detector and MCT (HgCdTe) detector according to the wavelength range. The SC generation scheme was similar to what was described in Ref. [8]. In short, the femtosecond pump was coupled into the fiber by an IR aspheric lenses (C036TME-E, Thorlabs). During the adjustment process, a camera (Win CamD-IR-BB, Dataray) monitored the coupling status in real time. Finally, the SC spectral data were collected by a liquid-nitrogen-cooled MCT photodetector through a monochromator (MS3504i, SOL instruments).3. Results and discussion
Figure 1 displays the transmission spectra of glasses with different compositions. It can be noticed that Ge-Sb-S glasses have good transparency in 1-10 μm. A few weak impurity absorption peaks exist at 4.1, 6.6, 7.5 and 9.1 μm, due to the impurity absorption of residual S-H, CS2, SO2 and Sb-O, respectively [29]. With the increase of sulfur content in the glass, either the short-wavelength edge (λS) or the long-wavelength edge (λL) shows blue-shifted.
Table 1 lists the Ith of Ge-Sb-S glasses under pulsed laser irradiation at different wavelengths. From Fig. 2, it can be noticed that the maximum value of Ith appears at stoichiometric composition (dS=0), indicating that the stoichiometric glass possesses the best resistance to laser damage. It is worth noting that Ith of the stoichiometric composition(s) (dS=0) of Ge-Sb-S glasses is approximately doubly enhanced compared to that of As40S60 glass under the same conditions. In addition, the S-rich (dS>0) ones show higher Ith than the S-deficient (dS<0) glasses. Raman spectra are used to determine the glass structure, to look into the association between the glass structure and the laser damage mechanism. Figure 3 plots the Raman spectra of the selected two glass groups. Table 2 gives the assigned structural vibrations in the Raman spectra. It is seen that the symmetric stretching vibration of [GeS4] tetrahedra located at 340 cm-1 and [SbS3] pyramids near 314 cm-1 are essentially dominant in Ge-Sb-S glass network [30–33]. For the dS=0 glasses (Ge25Sb10S65 and Ge15Sb22S63), only [GeS4] tetrahedra (at 340 cm-1, 415 cm-1) and [SbS3] pyramids (at 290 cm-1) are seen on the Raman spectra. Therefore, the stoichiometric glass is composed of Ge-S and Sb-S heteropolar bonds [30,31], while virtually no homopolar bonds, such as Ge-Ge, Sb-Sb and S-S, were observed [30,31,34]. For the dS>0 glasses, except for vibration peaks caused by heteropolar bonds in the above [GeS4] tetrahedral structure and [SbS3] triangular cone structure, some peaks were also found at 150 cm-1, 215 cm-1 and 457 cm-1, which are assigned to either S-S bridge bonds in the network structure or S-S bond in the S chains. With the increase of S content, the peak intensities of these S-S homopolar bonds increase gradually. Similarly, for the dS<0 glasses, in addition to the vibration peak caused by the heteropolar bond of Ge-S and Sb-S, there are vibration peaks at 160 cm-1, 210 cm-1 and 254 cm-1, which are attributed to the vibration of Sb-Sb and Ge-Ge homopolar bonds. Previous studies indicated that the heteropolar bonds have a higher bond strength than the homopolar bond [3,15]. Similarly, since the S-S bond has higher bond strength than the Ge-Ge and Sb-Sb bonds, the S-rich glasses exhibit higher Ith than the S-deficient ones.
Table 1. Measured Tg and Ith for Ge-Sb-S glasses
Table 2. Peak assignments of Raman shifts in Ge-Sb-S glasses
Figure 4 shows the dispersion curves of Ge-Sb-S glasses. It can be seen that n0 of the glasses increases with the decrease of S content. Table 3 summarizes n0 and n2 of various Ge-Sb-S glasses at the wavelengths of 1.55, 2.4, 3.0 and 3.6 μm. For comparison, n0 and n2 of some chalcogenide glasses reported in literatures were also included in the table. It is seen that n0 increases with the decrease of dS and the increase of . This is because the coordination number of S in the studied glass network is two, smaller than those of Ge (4) and Sb (3). According to the calculation formula of , the increase of <r > actually means the decrease of S content in the glass.
Table 3. Measured n0 and n2 for Ge-Sb-S glasses
The n2 of the glasses were obtained at 1.55, 2.4, 3.0 and 3.6 μm, according to the Z-scan measurement under the closed-aperture conditions. Figure 5 shows Z-scan data of Ge25Sb10S65 glass. The data were fitted to calculate n2, according to the nonlinear transmission model provided in previous studies [19,23,35]. The obtained results can be found in Table 3.
Fig. 5. Z-scan curves obtained in closed aperture measurements at 1.55, 2.4, 3 and 3.6 μm for Ge25Sb10S65 glass.
The experimental results show that a glass with a higher n0 has a higher n2. Such a trend is consistent with Miller's rule, which is usually represented by [8,36]:
Fig. 6. (a) Correlations between n2 and n0 in Ge-Sb-S glasses. (b) Measured n2 of Ge25Sb10S65 glass from 1.55 µm to 3.6 µm.
The open z-scan measurements were carried out at 1.55, 2.4, 3.0 and 3.6 μm, respectively. In order to judge the higher-order MPA effects, we further measured the Eg (see the results shown in Table 4). For the Nth order MPA, the contributed number of photons can be determined by N ≥ Eg/hν, where hν is the photon energy. Due to the large bandgap of Ge-Sb-S glass, 7PA can occur in the mid-infrared regime. The open z-scan measurements of Ge25Sb10S65 at different wavelengths are shown in Fig. 7. These data were further fitted according to the nonlinear transmission model [19,35]. The obtained nonlinear absorption coefficients are listed in Table 4. For comparison, the nonlinear absorption coefficients of some chalcogenide glasses reported in literatures were also included in the table. Figure 8 shows the correlation between the nonlinear absorption coefficient and the normalized photon energy (hν/Eg) in the Ge-Sb-S glasses. In the case of the same order of MPA, nonlinear absorption coefficient increases with hν/Eg. With the increase of hν/Eg, the order of MPA decreases and the absorption value increases dramatically.
Fig. 7. Z-scan curves obtained in open aperture measurements at 1.55, 2.4, 3 and 3.6 μm for Ge25Sb10S65 glass.
Fig. 8. The correlation between nonlinear absorption coefficient and the normalized photon energy (hν/Eg) in the Ge-Sb-S glasses.
Table 4. Measured nonlinear absorption coefficients for Ge-Sb-S glassesa
Judging from the measured nonlinear performances, including Ith, n2, and MPA coefficients, the composition Ge25Sb10S65, which showed the best laser damage resistance, was chosen as the core of the designed nonlinear fiber. The composition Ge25sbS67 with a lower n0 [ Fig. 9(a)] was used as the cladding of the fiber. The glass transition temperatures of the core and cladding glasses were measured to be 339 oC and 327 oC, respectively, indicating good thermal match between the two glasses for fiber drawing. The dispersion curves Fig. 9(a) are fitted by Sellmeier formula as the following:
Fig. 9. (a) Dispersion curves of linear refractive indexes of Ge-Sb-S glasses. (b) Calculated dispersion parameters of Ge25Sb10S65 glass and Ge25Sb10S65 / Ge25Sb8S67 fiber with 10 μm-core diameter.
SC generation is a nonlinear phenomenon with strong spectral broadening of pulsed light. It involves multiple complex nonlinear processes, including stimulated Raman scattering, soliton fission, four-wave mixing, self-phase modulation, cross phase modulation, and so on. Chalcogenide fiber is the ideal nonlinear medium for generating broadband mid-infrared SC, due to its excellent mid-infrared nonlinear performances. In order to obtain broadband high-brightness SC, it is an effective way to pump fiber waveguides in anomalous group velocity dispersion (GVD) regions near zero dispersion wavelength (ZDW) [8,27]. The calculated ZDWs of the core material and the 10 μm-core Ge25Sb10S65/ Ge25Sb8S67 fiber were 4.57 μm and 4.35 μm, respectively [see Fig. 9(b)]. Therefore, such a fiber ensures single-mode operation when using anomalous dispersion pumping scheme.
A Ge25Sb10S65 / Ge25Sb8S67 fiber was made by the multi-step rod-in-tube method. The fiber core diameter was 10 μm. The inset of Fig. 10 shows the cross sectional view of the fiber. A multimode fiber with a ∼70 μm large core of was also made in the same draw, in order to measure the fiber attenuation. Figure 10 shows the measured loss spectrum, by the cutback method. The background loss of the fiber was measured to be ∼2 dB/m. The highly absorptive bands at 2.9 μm, 3.1 μm, 4.1 μm and 4.8 μm are due to the residual O-H, S-H, S-H, and Ge-H impurities, respectively.
Fig. 10. Attenuation of fabricated Ge25Sb10S65 / Ge20Sb8S67 fiber with a core diameter of 70 μm. The inset provides the cross-sectional view of the 10 µm-core fiber.
Broadband mid-infrared SC was generated in the 10 μm-core Ge25Sb10S65 / Ge25Sb8S67 fiber with a length of 10 cm. The pump at 4.8 µm was with a pulse width of 170 fs and a repetition rate of 100 kHz. Figure 11 shows the evolution of SC with the incident pump power. It can be seen that with the increase of the average pump power, the pulse spectrum shows rapidly broadening. It is seen that the SC spans from 3 to 8 μm under the maximum average pump power of 40 mW. The coupling efficiency was estimated to be 50%, hence the light coupled into the fiber was ∼20 mW, corresponding to a peak power of 1176 kW. The average power of the SC output was measured to be ∼5 mW.
Fig. 11. Measured SC generated in the Ge25Sb10S65 / Ge25Sb8S67 fiber with a diameter of 10 µm when pumped at 4.8 µm (170 fs, 100 kHz).
For femtosecond pulses, the generation of SC is mainly affected by soliton generation, Raman soliton self-frequency shift, and dispersive waves [8,14,26]. Essentially, the initial generated Raman solitons shows self-frequency shifting to the long wavelength side. On the opposite direction, dispersive waves, which was phase-matched with the long wavelength solitons, forms in the short wavelength side. When the pump power increases, the pairs of Raman soliton and dispersive wave extend towards the long wave and short wave directions, respectively. Ultimately, broadband, flat mid-infrared SC is generated.
It should be noted that broadband mid-infrared SC can be generated in the short-length Ge-Sb-S fiber under only tens of milliwatts pump power, because of the high nonlinearity and decent mid-infrared loss of Ge-Sb-S glass fiber. It is therefore shown that Ge-Sb-S glass is indeed a promising mid-infrared fiber host material.
4. Conclusion
In summary, Ge-Sb-S glasses with a variety of different chemical stoichiometry and glass network were prepared by melt-quenching method. The transmission property and refractive index of glass were characterized and analyzed. A systematic investigation on the mid-infrared nonlinear performances of the glasses, including laser damage thresholds Ith, nonlinear refractive index n2, and the MPA coefficients, were carried out. The Ith reaches the maximum value at stoichiometric composition, while n2 increases with n0 following the semi-empirical Miller’s rule. With the increase of hν/Eg, the order of MPA decreases and the absorption value increases dramatically. For the design and fabrication of the nonlinear fiber, the composition Ge25Sb10S65 was selected as the core, while a thermally-compatible composition Ge25Sb8S67 was chosen as the cladding. A 10 µm-core, and 10 cm-long fiber was used for the supercontinuum generation. A 4.8-μm pump was employed with 170 fs pulse-width and 100 kHz repetition rate. Finally 3-8 μm SC with an output average power of 5 mW was obtained. It indicates that Ge-Sb-S glass is a promising environment-friendly fiber material for mid-infrared nonlinear applications, such as efficient broadband mid-infrared SC generation.
Funding
National Natural Science Foundation of China (61575086, 61805109); Natural Science Foundation of Jiangsu Province (BK20170229); Natural Science Research of Jiangsu Higher Education Institutions of China (18KJB180004); Priority Academic Program Development of Jiangsu Higher Education Institutions; Jiangsu Collaborative Innovation Centre of Advanced Laser Technology and Emerging Industry.
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
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