1. Introduction

Over the past decades, the development of photonic devices based on optical nonlinearities has been considered as a possible way to solve the bottleneck effect caused by the speed restriction of optic–electronic signal conversion in optical communication [1]. Therefore, to find waveguide materials with high optical nonlinearity and ultra-fast response time is critical to fulfill an integrated all-optic communication network.

Chalcogenide glasses (ChGs) possess large third-order optical nonlinearity (TON, χ⁽³⁾~1000 times that of silica) and board infrared transmittance region. They are considered as a perfect material for the design and fabrication of infrared photonic devices [2–5], especially TON-based devices, such as wavelength conversion, all-optical switching, and supercontinuum generation. The TON properties of ChGs at near-infrared region (i.e., telecom wavelengths) have been intensively studied [6–9], and it has been proved that ChGs exhibit better TON performance at longer wavelengths than at shorter wavelengths because of the attenuation of multiple photon absorption (MPA) [10]. However, in longer wavelength regions, especially in the mid-infrared region, TON properties of ChGs are not well characterized. Our previous study [11] investigated the optical nonlinearities of Ge–Sb–Se ChGs at 2.5 μm, where the intensity of MPA was undetectable. For the mid-infrared atmosphere window at 3–5 μm [12], which has attracted considerable research attention for military and civilian applications, study on the TON properties of ChGs in such spectral region has not yet been reported to the best of our knowledge.

In this study, ChGs within Ge–Sn–Se ternary system that is completely environment-friendly and suitable for fiber drawing [13] is selected. TON properties of the Ge–Sn–Se glasses are investigated by Z-scan technique at mid-infrared window (3 and 3.5 μm). Moreover, the dependence of nonlinear properties on glass network structure and the testing wavelength is discussed.

2. Experimental

It is well known that ChG properties obey the constraint theory based on mean coordination number (MCN) [6, 14, 15]. To induce significant variation of the glass network of Ge–Sn–Se ChGs in this study, we deliberately altered the Sn and Se contents with different coordination numbers of 4 and 2 [16]. In accordance with the constraint theory, eight glasses with molar composition of Ge₁₅Sn_xSe_(85-x), where x = 0, 5, 10, 13, 15, 16.5, 17.5, and 18 (labeled as Snx), were synthesized in this work. Thus, MCN of the glass is equal to 2.3 + 0.02x. The Ge–Sn–Se ChGs were prepared from polycrystalline of germanium, tin, and selenium with 5N purity. The raw materials were carefully weighed and mixed in sealed quartz ampoules in a vacuum, and then melted at 950 °C for 12 h in a rocking furnace. The quartz ampoules filled with liquid glass melt were quenched in ice water to avoid crystallization. The ampoules were then annealed for 5 h and cooled down to room temperature to minimize inner stress. For the subsequent Z-scan measurements, the glass rods were cut and optically polished to a thickness of 1.0 mm, and then further polished to a thickness of 0.3 mm for the determination of optical band gap (E_opg) using Tauc’s method [17].

Absorption spectra of the ChGs were recorded using Perkin-Elmer-Lambda 950 UV-VIS-NIR spectrophotometer. Fourier transform infrared (FT-IR) spectra of the ChGs were measured by a Nicolet 380 FTIR spectrometer. Raman spectra of the samples were obtained through back (180°) scattering configuration with a Renishaw inVia laser confocal Raman spectrometer with an excitation wavelength of 488 nm and frequency resolution of ± 0.15cm⁻¹, in order to distinguish vibration energy of different bonds and structural units within the glass network.

Z-scan technique was used to study the TON properties of the ChGs, including nonlinear refraction and absorption behavior, at the mid-infrared wavelengths. By using an optical parametric amplifier system (Coherent, Mira 900, Legend Elite and OPerA Solo), a femtosecond laser with pulse width of 100 fs and repetition frequency of 1 kHz was generated. The testing wavelengths at 3.0 and 3.5 μm were selected by using a difference frequency generator (DFG). The femtosecond laser was focused on the glass samples using a CaF₂ lens. The average laser power incident on surface of glass samples was set at 3 ( ± 0.1) µW at both wavelengths, corresponding to laser density of 8.1 ( ± 0.3) and 4.4 ( ± 0.2) GW/cm² at the lens focus. The laser power was recorded by a highly sensitive pyroelectric power probe (Laser Probe, RkP-575). Notably, the pyroelectric power probe introduces approximately 5% error because of thermal effects. In addition, the laser power fluctuation and scatter of the lens and glass samples add approximately 10% error. Therefore, the calculated results of TON refraction index (n₂) have approximately 15% error, which is within acceptable limits. The above measurements were conducted at room temperature.

3. Results and discussion

Figure 1(a) presents the absorption spectra of the Ge−Sn−Se ChGs. As the enlarged spectra of the absorption edge shown in the inset, the cut-off wavelength has a remarkable red-shift with the initial addition of Sn (Sn5) and then shifts to the short wavelengths with progressive increase in Sn content. As the Sn content reaches 16.5 mol % (Sn16.5, the MCN value is 2.63), the absorption cut-off edge shifts towards red again. By using Tauc’s method (see detailed calculation procedures in our previous work [18]), optical band gap (E_opg, ± 0.001 eV) of the glasses were calculated. The relationship between MCN and E_opg of the Ge-Sn-Se glasses is plotted in Fig. 1(b). It can be seen that the E_opg exhibits two transition points at MCN 2.4 and 2.63, which is consistent with the constraint theory predicting that property transition of ChGs constructed by covalent bonds would occur as the MCN value reached 2.4 or 2.6.

 

Fig. 1 (a) near-infrared absorption spectra of the Ge-Sn-Se glasses, inset is enlarged spectra of the absorption edge; (b) relationship between mean coordination number (MCN) and optical band gap (E_opg) of the Ge-Sn-Se glasses.

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Figure 2 shows the closed-aperture (CA) and open-aperture (OA) Z-scans of sample Sn5 at 3 and 3.5 μm, which represent typical qualitative behavior in the Z-scan measurements on all the ChGs prepared in this study. As shown in Figs. 2(a) and 2(b), the CA curves exhibit peak following valley configuration, indicating self-focusing; namely the nonlinear refractive indexes (n₂) of glasses have positive sign at the mid-infrared wavelengths.

 

Fig. 2 (a and b) Closed-aperture Z-scans and (c and d) open aperture Z-scans of the glass sample (Sn5) at 3.0 and 3.5 μm, respectively.

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According to Sheik-Bahae’s study [19], sign of n₂ of semiconductor materials is positive at incident photon energy (hv) below their half energy band gap (E_g), i.e., hv < 1/2E_g. For the Ge–Sn–Se ChGs known as amorphous semiconductors, their optical band gap (E_opg) is between 1.58 and 1.66 eV. Thus, a positive n₂ is correct in theory. In other words, the self-focusing behavior of ChGs can be obtained at mid-infrared wavelengths that have low photon energy (hv_3μm = 0.41 eV, hv_3.5μm = 0.35 eV). By fitting the CA Z-scan using a well-established fitting procedure [20, 21], n₂ values of the ChGs are obtained, and the variation of n₂ with MCN of the ChGs is shown in Fig. 3. The n₂ values of the ChGs change similarly with MCN at both wavelengths: n₂ increases after the introduction of Sn at MCN near 2.4 and then gradually decreases with the addition of Sn; as MCN reaches ~2.63, the n₂ increases again. Comparison between Fig. 1(b) and Fig. 3 indicated a good consistency between linear and nonlinear optical properties of the ChGs with varied MCN. The two jump point configurations of the figures are consistent with experimental results from the studies of Phillips, Thorpe, and Tanaka [22, 23] showing that transition of ChG network structure occurs near the critical MCN values, which significantly influenced the macroscopic behaviors of the glasses.

 

Fig. 3 Variation of nonlinear refractive index (n₂) with the mean coordination number (MCN) of the Ge–Sn–Se ChGs at wavelengths of 3 and 3.5 μm.

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Figure 4 shows the Raman spectra of the Ge–Sn–Se ChGs, and the variations in glass network structure with MCN (namely, introduction of Sn) can be clearly observed. The glass network of Sn0 consists of GeSe₂ tetrahedra (Raman peak at 195 cm⁻¹) as well as Se_n rings and chains (Raman peak at 260 cm⁻¹) [15]. After the introduction of Sn, part of Ge in [GeSe₄] tetrahedra was replaced by the former, and Se_n bonds were broken as well, which can be evidenced by the shift of GeSe₂ Raman peak to low frequency and the decrease of Raman intensity of Se_n bonds, resulting in significant evolution of the optical properties. The increase of n₂ and decrease of E_opg as Sn was initially incorporated to the Ge-Se glass is similar to the variation of optical properties of Ge-Sb-Se ChGs [7, 11] as Sb was introduced, thus such evolution can be attributed to the larger cation polarizability (a) of Sn atom compared with that of Ge (a_Sn = 0.479, a_Ge = 0.137) [24]. However, when MCN reached the first theoretical threshold of 2.4, the glass network became overconstrained, thus Se-Se bonds with non-paired electrons become the dominate contribution to the optical properties. Recent studies [6, 10] on selenium-based ChGs have shown that TON of the ChGs is positively correlated with the number of Se–Se homopolar bonds. For Ge–Sn–Se ChGs in this work, the Se_n structural units converted to Se_n−m (n > m) units after the further addition of Sn, and the characteristic Raman peak nearly disappeared as the MCN reached 2.6 (Sn15). These changes indicate that all the Se–Se homopolar bonds are converted to Sn–Se heteropolar bonds in the [SnSe₄] tetrahedra, resulting in the decrease of n₂ as well as the increase of E_opg. As MCN approaches 2.63, the Ge–Sn–Se glass network is turning to a stressed-rigid phase, which leads to the saturation of Sn in the glass network as signified by the presence of Raman peak belonging to Sn–Sn homopolar bonds [25] at 150 cm⁻¹ (inset of Fig. 4). The formation of such metal homopolar bonds resulted in a significant increase of the n₂ value, and the maximum n₂ at both testing wavelengths is obtained from the sample with highest Sn content (Sn18).

 

Fig. 4 Raman spectra of the Ge–Sn–Se ChGs. Inset shows the enlarged region between 140 and 160 cm⁻¹ showing the presence of Sn–Sn homopolar bonds.

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Furthermore, n₂ value of the ChGs at 3 μm is much lower than that at 3.5 μm (Fig. 3), and the average decrease rate of n₂ from 3.5 to 3 μm reached 66%, which is inconsistent with the dispersion of bound electronic TON of ChGs that depends inversely on wavelength of incident laser [8, 26]. Wang’s recent study [27] found exactly the same behavior in TON property of silicon at the mid-infrared wavelengths, but no reasonable explanation was given. As the FTIR spectra of the Ge-Sn-Se ChGs shown in Fig. 5, it is clear that the glasses have an evident impurity absorption due to hydroxyl (–OH) group at ~3 µm which can be considered as the reason for the low n₂ value at the corresponding wavelength. To verify such assumption, an As₂Se₃ glass prepared by distilled chemicals was used and the number of –OH group present by absorption intensity at 3 µm has been significantly reduced as shown in Fig. 5. Using the same CA Z-scan measurement, the n₂ values of As₂Se₃ glass at 3 and 3.5 μm were calculated to be 1.23 and 1.24 × 10⁻¹⁷ m²/W, respectively. The former remains slightly lower than the latter probably because the –OH group cannot be completely eliminated. However, the decreased amplitude of n₂ from 3.5 to 3 μm in As₂Se₃ glass with purification is much smaller than those in Ge–Sn–Se glasses without purification. Therefore, it can be concluded that distillation is important for both linear and nonlinear optical performance of ChGs at mid-infrared regions. For ChG compositions with chemicals unsuitable for distillation, such as the Ge–Sn–Se glasses in the present study, acceptable TON performance can also be obtained at wavelengths without impurity absorption. More importantly, the largest n₂ value of the Ge–Sn–Se ChGs (Sample Sn18) reached 2.43 × 10⁻¹⁷ m²/W at 3.5 μm, which is twice that of the As₂Se₃ glass, indicating greater potential to be applied in TON-based photonic devices.

 

Fig. 5 FTIR transmittance spectra of the Ge-Sn-Se glasses as well as the As₂Se₃ glass.

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OA Z-scan signal of the ChGs was not obtained and remained constant as the average laser power increased from 1 to 30 µW, as confirmed by the flat signal lines in Figs. 2(c) and 2(d). By comparing the E_opg of the ChGs with the incident photon energy, four and five photon absorption (4PA and 5PA) could be the main contributions to nonlinear absorption at mid-infrared wavelengths [28], but the nonlinear absorption coefficient at the corresponding wavelengths was too low to be detected by the present OA Z-scan method. The absence of MPA that went against optical propagation is expected to be beneficial for the performance of n₂-based photonic applications, such as all-optical switching and supercontinuum generation. Thus, lower power dissipation and smaller size can be expected as the corresponding devices operated at the mid-infrared wavelengths.

4. Conclusions

In summary, TON properties of Ge-Sn-Se ChGs were investigated at mid-infrared window of 3 and 3.5 μm. It is found that the impurities (–OH group) exhibited a negative influence on nonlinear refraction (n₂) of the ChGs. Larger n₂ values were obtained at 3.5 μm, where no impurity absorption occurs. Nonlinear absorption was either absent or below the sensitivity of the OA Z-scan measurement at both mid-infrared wavelengths. This result indicates that higher TON performance of ChGs can be obtained at mid-infrared wavelengths compared with that at near-infrared wavelengths, where MPA is significant. The ChGs in molar composition of Ge₁₅Sn₁₈Se₆₇ (Sn18) exhibited the highest TON performance at 3.5 μm. The n₂ value of the ChGs reached 2.43 × 10⁻¹⁷ m²/W, which is two times higher than that of the As₂Se₃ glass, as a result of the presence of Sn–Sn homopolar bonds in the glass network. Thus, ChGs have great potential to be applied in photonic devices operated at the mid-infrared region.