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In this work, a series of chalcogenide glass ceramics were prepared by thermal treating a precursor glass in molar composition of Ge20Sn5Se75 with different duration. X-ray powder diffraction (XRD) measurements showed the precipitation of nano-crystals belonging to GeSe2 and SnSe2 phases in the heat-treated glasses, which was confirmed by Raman spectra as well. The crystallization in glass matrix caused the variation of optical properties (i.e. Optical band gap, Urbach energy) due to localized field effect of the nano-crystals, while the infrared transmittance was kept unchanged. Third-order optical nonlinear properties of the precursor glass and glass ceramic samples were investigated by femtosecond Z-scan technique at telecom wavelength of 1550 nm. The maximum nonlinear refraction (γ) of the Ge-Sn-Se glass ceramics reached 5.319 × 10−16 m2/W, which is almost one order higher than that of the precursor glass, demonstrating the significant influence of nano-crystals on optical nonlinear property of the chalcogenide glasses.
© 2016 Optical Society of America
1. Introduction
In the past few decades, exploring optical materials with high optical nonlinearities has attracted considerable attentions for their potential applications in fabrication of photonic devices in all-optical communication systems, such as all-optical switching, wavelength conversion and self-phase modulation etc [1–4]. On the other hand, with the rapid development of infrared photonic technologies, one of the research focuses on material science is to find optical materials with high infrared transmittance (especially at a few important atmosphere windows) [5] along with high optical nonlinearities to realize some critical photonic applications, such as infrared laser source, infrared sensor, super-continuum generation, et al. Therefore, optical nonlinearities of infrared optical materials, e.g. silicon, germanium crystal, and glasses, etc. have been studied [6, 7].
Recent studies of infrared optical materials focus on chalcogenide glasses (ChGs, i.e. amorphous semiconductors containing S, Se, Te) [8, 9] which were known as a category of fiber materials that can be integrated into infrared photonic system by taking advantage of their large third-order optical nonlinearity with ultra-short electronic response (<100 fs) [10–12]. Previous studies on nonlinear optical properties of ChGs had proved that number of atoms with lone-pair electrons (i.e. S, Se, Te) may be the key to the nonlinear performance [13, 14], and it also had been concluded that doping of heavy-metal species such as Bi, Pb, or Sb et al to ChGs could promote the nonlinear properties due to the large polarizability of the cations [15]. On the other hand, ChGs possess the flexibility of properties modification by composition changing or post processing. Zhang’s [16] study had found improved mechanical property in micro-crystallized Ge-Se chalcogenide glasses which had unchanged infrared transmittance. Lin et al [17] had shown that the formation of Ga2S3 nano-crystals in Ge-Ga-S chalcogenide glasses could promote the multiple-photon absorption behavior which leads to enhanced optical limiting performance of the glasses. Our previous had proved that the Ga2S3 nano-crystals could improve third-order optical nonlinearities of Ge-Ga-S chalcogenide glasses. However, detailed study on optical properties of chalcogenide glass ceramics remains limited, especially for the selenium-based chalcogenide glasses that possess considerably large third-order optical nonlinearities as well as high infrared transmittance, and they had been considered as a perfect candidate for mid-infrared photonic devices [11, 18, 19].
In this work, a precursor glass in molar composition of Ge20Sn5Se75 (GSS) was selected and prepared by melt-quenching method. The glass was then processed by one-step thermal treatment for crystallization in the glass network. Optical and mechanical properties of the GSS glass and the crystallized glasses (glass ceramics, GC) were investigated and compared. Third-order optical nonlinearities of the GSS glass and GCs were studied by Z-scan technique at the telecom wavelength of 1550 nm, and suitability of the samples for all-optical switching was evaluated by calculation of figure of merit.
2. Experimental
The Ge20Sn5Se75 glass was synthesized with high purity polycrystalline of germanium (5N), tin (5N) and selenium (5N). The raw materials were weighed carefully and mixed in sealed quartz ampoules in vacuum which were then put into rocking furnaces. The quartz ampoules were heated to 970 °C slowly in furnaces and then maintained for 12 h at this temperature. All of quartz ampoules were quenched in ice water very quickly to form bulk glasses. All samples were annealed at the temperature that 20 °C lower than the glass transition temperature (Tg) for 5 h to minimize the internal tension and slowly cooled to room temperature. To test optical characteristics of the samples, the glass rod was cut into disks with a thickness of 0.5 mm and polished to mirror smoothness on both sides for further testing. The GC samples were obtained by heat treating the GSS glass at 270 °C (50 °C above its Tg) for 3, 6, 12, 18 and 30 h which were labeled as GSS-3, 6, 12, 18 and 30 respectively, and the GSS glass was labeled as GSS-0.
Powder X-ray diffraction (XRD) patterns were recorded with a Bruker AXS D2 PHASER diffractometer (voltage = 30 kV; current = 10 mA; Cu- Ka radiation) with a step width of 0.02° at room temperature to confirm the crystalline state of samples. Absorption spectra of samples were recorded in the range of 400~2500 nm using Perkin-Elmer-Lamda 950 UV-VIS-NIR spectrophotometer. Scanning electron microscopic (SEM) measurements was performed with a Tescan VEGA3 SB-Easyprobe scanning electron microscope, and the investigated samples were gold-sputtered on the surface before the SEM measurements. The infrared transmission spectra in the range of 2.5~25 µm were obtained using Nicolet 381 Fourier Transform Infrared spectrometer (FTIR). 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 a frequency resolution of ± 0.15 cm-1, in order to distinguish vibration energy of different bonds and structural units within the inner structure. Density of the samples was measured by Archimedes’ method with distilled water as immersion. Vickers-hardness of the samples was measured by a Everone MH-3 microhardness meter with a charge of 100 g for 5 s
Third-order nonlinearities (TONL) of the GSS glass and GCs at 1550 nm were measured by Z-scan technique utilizing a Mendocino fiber laser (CALMAR LASER FPL style) with 51 fs pulse width and 50 MHz repetition rate. The incident laser power was set at 60 mW.
3. Results and discussion
3.1 XRD and structural characterization
XRD patterns were used to confirm the nature of the GSS glass and GC samples, as presented in Fig. 1(a). The appearance of diffraction peaks in the patterns clearly illustrated the transformation of the GSS glass from amorphous state to crystal state after thermal treatment. Evident diffraction peaks started to appear in the spectrum of 12h treated sample (GSS-12) and grew in intensity with treatment duration. As shown in Fig. 1(b), the main diffraction peaks in GC samples can be attributed to the combination of GeSe2 monoclinic crystalline (JCPDF No. 71-117) and SnSe2 hexagonal crystalline (JCPDF No.89-2939). By using the well-known Scherrer formula as below, mean crystal size (D) in each GC sample can be calculated:
where K is a constant that equals to 0.899, λ is the wavelength of radiation (0.154nm), ω is the full width at half maximum (FWHM) and θ is the diffraction degree. According to Eq. (1), the mean size of crystal grains in GSS-12 was estimated to be 102 nm, and 654 and 25870 nm for sample GSS-18 and GSS-30, respectively.
Fig. 1 (a) XRD patterns of glass samples heat treated for different duration; (b) identification of the crystal phases in the crystallized samples.
Raman spectra of the GSS glass and GCs were presented in Fig. 2. The strongest Raman diffraction peak located at 192 cm−1, representing the combination vibration of Ge-Se bonds in the tetrahedron [Ge(Se1/2)4] and Sn-Se bonds in the tetrahedron [Sn(Se1/2)4] [20]. It can be seen that the full width at half maximum (FWHM) of the main peak decreased evidently in crystallized sample (GSS-12), which indicated uniform ordering of the network structure. To further confirm the structural change of glass network after thermal treatment, the Raman spectra were normalized to calculate the integrate area of the main peak from 170 to 210 cm−1. As shown in the inset of Fig. 2, the significant decrease of the integral area in GSS-12 could prove the presence of crystals in the network. Further, it can be seen that the integrate area of GSS-3 and GSS-6 is also lower than that of the precursor, which indicated that the crystallization had existed in all the heat-treated samples and the absence of diffraction signal in the corresponding XRD patterns can be attributed to two possible reasons: 1. small (< 10 nm) grain size; 2. low number of the grains.
Fig. 2 Normalized Raman spectra of the glass and glass-ceramics, inset is dependence of the integral area of the main peak from 170 to 210 cm−1 on the treatment duration.
The precipitation of crystalline phases in the GC samples was further confirmed by scanning electron microscope (SEM). Figure 3 presents the SEM images with two resolutions from the surface polished GSS glass and GCs. In Fig. 3(a) and 3(b), the clean and homogenous both in cross-section (left part) and surface (right part) indicated the amorphous nature of the GSS glass. In the surface of GSS-12 in Fig. 3(c) and 3(d), it is clear to observe some bright spots with water-drop shape on the sample surface which is the direct observation of the GeSe2 and SnSe2 combined crystals, and the size of ~100 nm is in consistent with the calculation value from Scherrer formula. For GSS-30, as can be seen in Fig. 3(e) and 3(f), this sample is highly crystallized, in accordance with the sharp and abundant diffraction peaks in the XRD pattern.
To find the influence of the Ge(Sn)Se2 crystals on mechanical strength of the samples, Vicker’s hardness (Hv) were measured and given in Table 1. In good agreement with previous studies [16, 21], presence of the nano-crystals did increase the Hv value in the first two GC samples (GSS-3 and GSS-6). However, the Hv decreased as the treating duration passed 6h, indicating that the improvement in mechanical performance of GSS glasses by the precipitation of nano-crystals requires appropriate crystal number and size. Excess grains formed in glass would destroy the original connectivity of glass network, which consequently leads to a decrease of the mechanical strength.
Table 1. Physical, optical and TONL parameters of the GSS glass and GC samples
3.2 Spectral properties
The absorption spectra of GSS glass and GC samples are presented in Fig. 4(a). It can be seen that the absorption cut-off edge of GSS-12 red-shifted apparently as compared to that of the GSS glass due to the precipitation of Ge(Sn)Se2 nano-crystals. As shown in enlarged spectral region in inset of Fig. 4(a), similar behavior can also be observed in GSS-3 and GSS-6, proving the presence of nano-crystals with smaller size in the samples. On the other hand, as the FTIR spectra shown in Fig. 4(b), the presence of crystals could influence the mid-infrared transmission [22], which depends on duration of thermal treatment. In general, the Ge(Sn)Se2 crystals with size < 100 nm have relatively small influence to the infrared transmittance, but scatting loss becomes apparent when the crystal size grows over 0.6 μm (GSS-18). It can be seen that GSS-18 is partially transparent in mid and far-infrared region due to the large grain size, and GSS-30 is completely opaque in the whole spectral region due to the crystallized nature of this sample which had been illustrated by its SEM images in Fig. 3(e) and 3(f).
To study the influence of Ge(Sn)Se2 crystals to spectral properties of the GSS glass, optical band gap (Eopg) and Urbach energy (Ee) [23, 24] of both GSS glass and GCs were calculated. The Eopg of the studied samples was defined as the wavelength where the linear absorption coefficient α is 100 cm−1. As the calculation results given in Table 1, Eopg gradually decreased with the increasing time of thermal treatment, which can be attributed to the precipitation of Ge(Sn)Se2 crystals in the GSS glass network. For crystalline materials especially semiconductors, Eopg presents the energy gap between valence band and conduction band. Chalcogenide glasses are known to possess amorphous semiconductor behavior, and the “valence band” is related to the amount of lone-pair electrons provided by chalcogen atoms in glass, and the “conduction band” lies on anti-bonding band [25]. After heat treatment, the total number of chalcogen atoms (i.e. Se) in the samples kept constant so that the width of valence band is unchanged, while the formation of Ge(Sn)Se2 crystals causes the presence of localized states which gives rise to the extension of anti-bonding band [26]. Consequently, growth of the Ge(Sn)Se2 crystals narrows the gap between the valence and conduction band, leading to the decrease of Eopg.
The Urbach tail located at spectral range between the sharp absorption edge and absorption-zero region, where the α value has exponential dependence on wavelength. As seen in Fig. 4(a), absorption of the Urbach tail enhanced after heat treatment, which is associated with the precipitation of the Ge(Sn)Se2 nano-crystals. The Urbach energy (Ee) ison behalf of the width of the tail of localized states between conduction and valence band, and it can be calculated by the following equation:
where α0 is a constant, hv is the photon energy and α(v) is the linear absorption coefficient with wavelength dependence. By plotting ln(α) versus photon energy (hv) as shown in Fig. 5, Ee of the samples can be obtained from the linear fitting, As the data given in Table 1, the variation of Ee with thermal treatment shows an opposite tendency to that of Eopg. It is known that Urbach tail is the unique characteristic of amorphous materials, and the Ee value depends on the degree of network disordering of the materials [24]. Since present GSS glass is not in chemical stoichiometric, the precipitation of the nano-scale Ge(Sn)Se2 crystals results in the formation of defects (unsaturated bonds), which enhanced the disordering of the glass network. The growth of Ge(Sn)Se2 crystals in both size and number promoted the tailing in bandgap state and consequently increased the Ee value.3.3 Third-order optical nonlinearity
To study the third-order optical nonlinearity (TONL) of the GSS glass and GCs, femtosecond Z-scan technique at the telecom wavelength of 1550 nm was performed. The measurements were calibrated by using an As2Se3 glass. Nonlinear refractive index (γ, defined as the coefficient of intensity (I) dependent refractive index as n(I) = n0 + γI) of the As2Se3 glass was estimated to be 1.57 × 10−17 m2/W at the off-resonant wavelength of 1550 nm, which is in good agreement with the value reported previously [4], confirming the accuracy of the present Z-scan measurements. Figure 6 gives the closed-aperture (CA) and open-aperture (OA) Z-scan curves of the GSS glass and GCs, and the variation of TONL performance of the glass after heat treatment can be clearly illustrated by the change of signal intensity [27]. As the the CA Z-scans shown in Fig. 6(a), the curves show peak following valley configuration indicating the self-focusing behavior of all samples under the high laser irradiance, namely positive sign of γ which can be calculated from the transmittance gap between the peak and valley by fitting the CA Z-scans. As the calculated results listed in Table 1, γ value of the GC samples is significantly larger than that of the pure GSS glass, and the maximum γ of 5.3 × 10−16 m2/W was obtained from GSS-6, and the value is nearly one order higher than that of the GSS glass. The evident increment of γ can be attributed to the local field effect (LFE) which depends on the intensity of crystalline field around the Ge(Sn)Se2 nano-crystals, and it can be considered that the γ in GC samples with crystal size below 10 nm had already been resonantly enhanced by the LFE at a high level. Further, the γ decrease as the treatment duration reached 12h (GSS-12), indicating that the size confinement effect from the crystals is the main contribution to the LFE. As confirmed by the SEM images of GSS-12, size of Ge(Sn)Se2 crystals in this sample is ~100 nm which had resulted in weak confinement of the polar fields and consequently the decrease of the nonlinear refractive behavior.
Fig. 6 Z-scan curves of the GSS glass and GCs: (a) closed aperture Z-scans; (b) open aperture Z-scans.
On the other hand, the valley in center region of the OA Z-scans of GSS glass and GC samples illustrated the presence of nonlinear absorption. According the Tanaka’s study [28], such nonlinear absorption can be attributed to two-photon absorption (TPA) because the incident photon energy (0.83 eV at 1550 nm) located at half band gap (1/2Eopg) of the samples. By fitting the OA Z-scans using a TPA model, the TPA coefficient (β, defined as the coefficient of intensity dependent absorption coefficient as α(I) = α0 + βI) was calculated and shown in Table 1. Similar to the nonlinear refractive behavior, the β value of the glass ceramics is much higher than that of the glass samples as a result of LFE. The growth of the crystals gave rise to localized states, which promoted the possibility of electron transitions from valence to conduction band by mean of TPA. However, unlike the variation of γ, β of the samples keeps increasing with increase of treatment duration, indicating that the shifting of absorption edge, namely variation of Eopg needs to be considered. According to Wang’s recent study [4], TPA behavior of chalcogenide glasses can be described by Dinu’s theoretical mode which indicated that β of chalcogenide glasses is inversely proportional to the cubic power of the band gap energy (Eopg3), which means that small decrease of Eopg could cause significant increase of β value.
Figure of merit (FOM, FOM = γ/βλ) was utilized to evaluate the suitability of present samples for photonic devices limited by TPA at the telecom wavelength. As the calculated result listed in Table 1, FOM of the GSS glass and GCs (except GSS-12 with huge β) are comparable, and the values over 10 indicated that the samples are desirable for efficient all-optical devices [29]. However, the GSS GSs in this study can be considered as a more promising candidate for all-optic photonic devices for their larger γ which would facilitate devices with smaller size or lower energy consumption.
4. Conclusions
In summary, a significant effect of thermal treatment on optical properties of the Ge-Sn-Se (GSS) chalcogenide glasses was observed. The treatment caused the precipitation of nano-scale crystals in Ge(Sn)Se2 phases within the GSS network, and the crystals grew in size with increase of treatment duration as confirmed by XRD and Raman spectra. The presence of Ge(Sn)Se2 crystals led to decrease of optical band gap, while that of Urbach energy behaved oppositely. By employing femtosecond Z-scan technique at the telecom wavelength of 1550 nm, resonant enhancement of third-order optical nonlinearities due to the localized field effect of nano-crystals were observed in the GSS GCs, and the maximum nonlinear refraction (γ) of 5.3 × 10−16 m2/W was obtained. Calculation of figure of merit confirmed the suitability of the GSS GCs for photonic devices in all-optic communication system.
Acknowledgments
This work was partially supported by National Natural Science Foundation of China (Grant Nos. 61435009, 61308094). It was also sponsored by K.C. Wong Magna Fund in Ningbo University.
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