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Abstract
In this work, we characterized the optical transmittance, optical band gap, nonlinear refraction and nonlinear absorption of a series of GeS2-Sb2S3-CsCl chalcogenide glasses, and monitored their compositional dependencies. We found that the number of lone-pair electrons and the bandgap energy are two dominate factors that determined these linear and third-order nonlinear optical properties. Besides, evaluation of figure of merit verified that the GeS2-Sb2S3-CsCl glasses are suitable for optical limiting devices.
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1. Introduction
Chalcogenide glasses (ChGs) have been regarded as an important platform for both nonlinear and infrared photonicsv [1–4] since they have remarkably high third-order optical nonlinearity (TONL, χ(3) at least 100 times to SiO2) with ultra-fast response (< 100 fs) as well as ultra-wide optical transmission range (0.5∼25 µm) and low temperature coefficient of refractive index. Besides, they are amorphous materials that can be easily molded, suitable for fabrication of optical devices in targeting shapes and sizes [5–8]. In recent years, with the vigorous development of integrated photonics theory and technology, the combination of chalcogenide glass materials and integrated photonics forms a unique research field, called “chalcogenide photonics" [9] which has become the hotspots in the current photonics research realm.
In many ChG systems, the Ge-Sb-S ternary system had been intensively studied for its large glass formation and high glass forming ability [10–12], while incorporation of cesium halide to the Ge-Sb-S ChGs had also attracted attentions for its capacity in modification of various properties of the ChGs, especially their transparent region and crystallization behavior. Lin et al. [13] recently discovered that Ge-Sb-S ChGs are perfect residence for various kinds of cesium halide nano-crystals. Other researchers [14–17] had found that cesium chloride (CsCl) nano-crystals or merely CsCl-doping are able to enhance the mechanical strength of Ge-Sb-S ChGs. Nevertheless, for the TONL properties of CsCl doped Ge-Sb-S ChGs, it remains not fully understood since no relevant study can be found to the best of our knowledge. Fedus et al. [18] had shown that CsI incorporation had significant impact on TONL properties of Ge-Sb-S ChGs, and the GeS2-Sb2S3-CsI ChGs had been considered as a candidate for these all-optical switching applications.
Our recent study [19] had discovered the precipitation of CsCl nano-crystals in the classic 62.5GeS2-12.5Sb2S3-25CsCl glass [20–22] as a composite has superior TONL performance. As a continuity, the aim of this work is to give a detailed investigation with respect to the composition-dependent TONL properties of GeS2-Sb2S3-CsCl (GSC) pure ChGs, in order to optimize the host composition for nano-crystallization. Meanwhile, the general optical attributes of the GSC ChGs including infrared transmission, bandgap energy, and their relationship with the TONL properties have also been investigated.
2. Experimental
The GSC ChGs were prepared by a traditional melt-quench process. High purity germanium (5N), antimony (4N), sulfur (5N) and cesium chloride (4N) were used as the original materials, and mixed in a quartz tube under vacuum. The sealed quartz tube was placed in a swing furnace at temperature of 850 °C and melted for 10 hours. Then, the molten was quenched in water and annealed at 20 °C below the glass transition temperature from 200 to 240 °C depending on composition for 24 hours to eliminate inner stress. After, the glass rods were cut into pieces with a thickness of 1 ± 0.1 mm and polished until both sides were mirror-like for the subsequent optical testing.
The optical transmission spectra of GSC ChGs in the wavelength range of 0.4∼2.5 µm were measured by a VIS-NIR spectrophotometer (Perkin-Elmer, Lamda950, USA), and the range of 2.5∼15 µm by a Fourier transform infrared spectrometer (FTIR, Nicolet381, USA).
The TONL parameters including nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of the GSC ChGs were measured by a self-built automatic Z-scan system. A Ti: sapphire laser system (Coherent, Mira 900-D, USA) with operating wavelength of 800 nm was used as the excitation source. The laser with repetition rate of 76 MHz and pulse duration of 130 fs was focused on the sample through a CaF2 lens with a focal length of 7.5 cm and the laser pulse energy was 0.76 nJ. The laser beam waist radius (ω0) was 16.1 ± 2.4 µm, corresponding to the laser power intensity (I0) of 1.39 ± 0.23 GW/cm2 at the focal point, which has an error of 15%. In addition, the thermoelectric power detector (Laser Probe, Rkp-575, USA) has an error of 5% due to the thermal effects. The scattering effect of the laser light passing through the lens and the samples causes an error of about 1.3%. Ultimately, there is a total error of 21% for the calculation of the n2 and β values.
3. Results and discussion
The full-band transmission spectra of GSC ChGs are given in Fig. 1(a), and the labels for the GSC ChGs given in the box at the top right-hand corner is the abbreviation of their compositions. For example, 15Ge80Sb5Cs represents the sample with molar composition of 15GeS2-80Sb2S3-5CsCl. It can be seen that GSC ChGs have a wide optical transparent range from near 0.5 µm to over 12 µm, and the overall transmittance stays above 60%. The two strong absorption peaks at 2.8 and 4 µm are due to H-O and S-H molecules groups, respectively, which was caused by the exposure of ChGs to air, while the relatively weak absorption peak at 6.3 µm is related to H-O-H in the raw materials. Both short-wavelength cut-off (SWC) and long-wavelength cut-off (LWC) show evident compositional dependency, while the later varied more strongly than the former. As monitored in inset of Fig. 1(a) sample 15Ge80Sb5Cs with highest Sb2S3/CsCl ratio possesses the longest LWC, while sample 60Ge15Sb25Cs with smallest Sb2S3/CsCl ratio has the shortest. GeS2 content has no significant impact on the LWC, but the large characteristic absorption of Ge-O bonds [23] can be observed at 13 µm which shortened the LWC. Figure 1(b) plots the relationship between the average transmittance from 1 to 2 µm of the GSC ChGs and their linear refractive indices (n0), which shows that the transmittance gradually decreases with n0 increase. This indicates that the optical transmittance of the GSC ChGs is affected by Fresnel reflection.
Fig. 1. (a) Full-band (from 0.4 to 15 µm) transmission spectra of the GSC ChGs; (b) the relationship between the average transmittance (from 1 to 2 µm) and linear refractive index (n0) of the GSC ChGs.
For the SWC, CsCl incorporation to ChGs had been shown to blue-shift its location [24,25] since electronegative chlorine could localize the lone-pair electrons (LPEs) which are the non-bonded electrons that do not participate in forming the covalent bonds in the outermost electron shell of chalcogen atoms. However, for the present GSC ChGs, the effect of CsCl on SWC location could be neutralized since GeS2 has four LPEs while Sb2S3 has the doubled LPE number due to the additional one LPE resident in Sb atom. Figure 2 gives the absorption spectra of the GSC ChGs divided into four groups by CsCl content, which illustrated that replacement of GeS2 with Sb2S3 can red-shift the SWC.
Fig. 2. Absorption spectra of the series of GSC ChGs with same CsCl content. (a) 5 mol%; (b) 10 mol%; (c) 15 mol%; (d) 25 mol%.
To be more specific, the SWC location can be quantified by bandgap energy (Eg as listed in Table 1) defined as the photon energy at wavelength (λ) having linear absorption coefficient (α) of 100 cm−1 [26,27], expressed as the following formula:
Fig. 3. Dependency of bandgap energy (Eg) on lone-pair electronic (LPE) number in 1 molar of each GSC glass sample. The solid line is linear fitting.
Table 1. Compositions, optical and TONL parameters of the GSC ChGs and some GeS2-Sb2S3 based ChGs in previous studies for references
The closed-aperture (CA) Z-scan traces of four glass samples, which represent the general n2 property of the GSC ChGs are shown in Fig. 4. The self-focusing behavior of the samples resulted in a valley-following-peak shape of the CA curves, which means that the GSC ChGs has positive n2 at the excitation wavelength of 800 nm. Since the depth between valley and peak (ΔTv-p) reflects the n2 value, it can be informed from Fig. 4(a) and (b) that replacement of CsCl with GeS2 could promote the n2 value, while comparison among Fig. 4(a), (c) and (d) informed that replacement of both CsCl and GeS2 with Sb2S3 could promote the n2 value as well. The maximum ΔTv-p presented in Fig. 4(d) further proved that the Sb2S3 content is the main compound that determines the n2 value of the GSC ChGs. By fitting the CA Z-scan traces via Kwak’s method [28] that is applicable for large nonlinear phase shifts (ΔTv-p > 1), and the concrete n2 value can be extracted using the express as follow:
Fig. 4. Closed-aperture Z-scan traces of the four representative GSC glass samples under laser intensity of 1.39 ± 0.23 GW/cm2. The solid curves are theoretical fitting according to Kwak’s method.
Fig. 5. (a) Variation of nonlinear refractive index (n2) vs lone-pair electron (LPE) number and (b) bandgap energy (Eg).
Fig. 6. (a) Linear refractive index (n0) vs nonlinear refractive index (n2); (b) nonlinear absorption coefficient (β) vs bandgap energy (Eg).
For β property of the GSC ChGs in Fig. 7, the valley in central of the open-aperture (OA) Z-scan traces illustrate presence of reversed saturated absorption (RSA), namely positive sign of β that can be assigned to two-photon absorption (TPA) [29] because the normalized photon energy (hv/Eg, the photon energy hv at 800 nm is 1.55 eV) of the GSC ChGs are between 0.5 and 1. Further, depth of the valley (ΔTv) which reflected the β value follows the similar trend with ΔTv-p, indicating that the LPE number indirectly effected the β property of the GSC ChGs. The concrete β value can be extracted via fits of the OA Z-scan traces using the well-established fitting formula for TPA:
The calculation results in Table1 reveal that the increase rate from the minimum β to the maximum is about 12 times. By plotting β versus Eg as given in Fig. 6(b), it can be found that the trend of β variation is analogous to that of n2, both are inversely proportional to the Eg.
Fig. 7. Open-aperture Z-scan traces of the four representative GSC glass samples under laser intensity of 1.39 ± 0.23 GW/cm2. The solid curves are theoretical fitting according to the TPA formula.
The experimental results from the above Z-scan measurements demonstrated that the n2 value augment with LPE number is faster than the β value, which indicated that the trade-off between the n2 and β of the GSC ChGs had been amplified. Since the hv/Eg value also increased with LPE number, such behavior is in general consistent with the spectral variations of n2 and β for indirect-gap crystalline semiconductors as shown in Fig. 1 of Ref. [30]. Therefore, the suitability of GSC ChGs for TONL-based photonic devices can be evaluated through a figure of merit (FOM), using the following expression [31]:
where λ is excitation wavelength of 0.8 µm in this study. For n2-based devices, such as all-optical switching and wavelength conversion, optical materials with FOM above 1 are required, while β-based devices, such as optical limiter need materials with the opposite FOM [32,33]. The experimental data of the current GSC ChGs are given in Table 1, their FOM value are all below 1 owing to the strong TPA at the excitation wavelength. The TONL parameters of GeS2-Sb2S3 binary ChGs as well as other GeS2-Sb2S3 based ternary ChGs reported in previous studies [34–36] are also given in Table 1 for comparison, and it turns out that the β value of the present GSC ChGs are remarkably larger than their counterparts, indicating the greater potential of the GSC ChGs for optical limiter devices.Accordingly, the optical limiting behavior of the sample having the maximum β (25Ge60Sb15Cs) is illustrated in Fig. 8. It shows the theoretical output laser intensity (Iout) as a function of input laser power (Iin) considering without (straight line) and with TPA at 0.8 µm (β = 4.247×10−10 m/W) by the following express:
Fig. 8. Optical limiting behavior for the sample 25Ge60Sb15Cs with Leff of 1.18 mm. The circles with 21% error bars are the experimental data, the dash line is what expected in the case without TPA and the solid line is the theoretical curve
4. Conclusions
In this paper, the linear and third-order nonlinear optical properties of chalcogenide glasses within a GeS2-Sb2S3-CsCl pseudo-ternary system were investigated. The optical transmission cut-offs at both short- and long-wavelength regions exhibit a red-shifting trend with the Sb2S3 content, while a blue-shifting trend with the CsCl content. Z-scan measurements showed that the GeS2-Sb2S3-CsCl glasses have positive nonlinear refractive index and two-photon absorption (TPA) at excitation wavelength of 0.8 µm, and both the nonlinear optical parameters exhibit positive dependency on number of lone-pair electrons within the glasses and negative dependency on the bandgap energy. Evaluation of figure of merit indicated that the GeS2-Sb2S3-CsCl glasses meet the requirement for TPA-based photonic devices, and the capacity for optical limiting has been verified.
Funding
National Natural Science Foundation of China (62075108, 62075107, 61935006); K. C. Wong Magna Fund in Ningbo University.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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