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Abstract
We demonstrate the generation of THz waves (frequency 9.7 THz) using difference frequency generation in an InxGa1-xSe mixed crystal grown from In flux. The amount of indium and the lattice constant of the crystal were evaluated using electron micro probe analysis and X-ray diffraction, respectively. We believe that the Ga sites were substituted by In atoms in the InxGa1-xSe crystal because the In content, estimated according to the Vegard’s law, was similar to that measured by EPMA. The maximum power of the generated THz wave was 39 pJ and the conversion efficiency was 1.7×10−5 J−1. This conversion efficiency was 28 times larger than that reported for undoped GaSe crystal.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Gallium Selenide (GaSe) is one of the most promising layered chalcogenide semiconductors for future novel optoelectronic devices. In particular, two-dimensional (2D) GaSe exhibits attractive electrical and optical properties owing to its finite energy band gap, and has been widely investigated for the development of a variety of devices such as THz wave light sources [1–4] and spin-field-effect transistors [5].
Owing to its extremely asymmetric layered crystal structure, GaSe has a relatively high second order nonlinear optical (NLO) constant (d22 = 54 pm/V) [6]. Therefore, it can be used to generate THz waves via difference frequency generation (DFG), which is a second-order NLO effect. In addition, GaSe crystals are transparent over a wide range from infrared (IR) to THz frequencies, and in principle, it is possible to generate high frequency waves from THz to mid-IR (0.1-100 THz) region via DFG by using GaSe crystals [1–4]. Furthermore, GaSe crystal is a layered 2D semiconductor with high birefringence, facilitating the application of collinear phase matching for generating THz waves via DFG with a possibility of improving the conversion efficiency [7–9].
However, as the growth of GaSe is extremely anisotropic, stacking faults are easily generated, making it difficult to grow crystals of high optical quality [10]. This is because the inter-layers of GaSe are weakly bonded by van der Waals forces, even though its intra-layer atoms are strongly bonded by covalent forces. Van der Waals bonding forces were described theoretically by London in 1937 [11] and our group has directly measured these forces in GaSe crystals [12]. Furthermore, due to their poor mechanical properties, it is challenging to cut or polish GaSe crystals for using them as light sources.
Several earlier studies have attempted to resolve these issues by doping GaSe with various elements such as S [13,14], Te [15,16], In, Al [17], Er [18], Ti [19], and Ge [20]. These reports of doped GaSe crystals are reviewed in [21]. In particular, the second-order NLO constant [22,23] as well as the mechanical properties [24] of the crystal have been improved with In doping in second-harmonic generation (SHG) measurements. In addition, it is expected that the inter-layer bonding forces can be strengthened by In doping because interlayer bonding force of InSe is stronger than that of GaSe [25]. Although there have been many reports on improving the properties of GaSe crystals by doping them with In, the generation of THz wave in InxGa1-xSe mixed crystals via DFG has not been explored yet.
In this work, we have evaluated the THz wave generation capability of InxGa1-xSe mixed crystal, which is reported to have an improved second-order NLO constant and better mechanical strength than the undoped crystal. We generated THz waves via DFG in this mixed crystal grown from In flux and compared its THz wave generation properties with that of undoped GaSe crystal grown from Ga flux.
2. Experiments
InxGa1-xSe mixed crystal was grown using the traveling heater method. Indium (DOWA Electronics Materials Co., Ltd.) was used as the solvent and polycrystals of GaSe (Kojundo Chemical Lab. Co., Ltd.) were used as the source material. The purity of GaSe and In were 3N and 6N, respectively. We did not use a seed crystal in this process. The crystal was grown in two steps. First, the growth vessel was subjected to a temperature gradient for 7 days. After 7 days, the growth vessel was slowly pulled down toward lower temperature at a speed of 60 µm/hour. The range of growth temperature was 725 - 750 °C.
Electron probe microanalysis (EPMA) was used to measure the contents of Ga, Se, and In in the grown crystal using a field emission scanning electron microscope (FE-SEM; JXA-8530F, JEOL Ltd.). Measurements were also conducted for reference samples of undoped GaSe crystal grown by temperature difference method under controlled vapor pressure (TDM-CVP) [7] and an undoped InP wafer (Sumitomo Electric Industries, Ltd.). In content was measured at 7 points on each sample. We calculated the average and standard deviation of these 7 points. In addition, we also estimated the In content in the grown crystal from the lattice constant measured by X-ray diffraction (XRD) based on the variation in lattice constant due to In doping. Symmetric XRD patterns of InxGa1-xSe mixed crystal from (0 0 10) to (0 0 20) were measured by an automated multipurpose X-ray diffractometer (SmartLab, RIGAKU Ltd) with CuKα1 (λ=0.154056 nm) and CuKα2 (λ=0.154439 nm) radiation. The precise lattice constant was estimated by extrapolating the plot of ${\mathbf{cos}^2}{\boldsymbol \theta }$ vs. the lattice constant c to ${\mathbf{cos}^2}{\boldsymbol \theta }$ = 0.
THz waves were generated via DFG from the InxGa1-xSe mixed crystal. The thickness of the crystal was 415 µm and its surface was treated by the tape peeling method. A schematic of the optical system for generating the THz waves is shown in Fig. 1. A near-infrared Nd:YAG laser (LOTIIS Inc.) with two channels was used to excite grating-based Cr:Foresterite lasers. These lasers, which contain gratings (1200 grooves/mm) for wavelength selection, achieve better spectral purity than the prism-based ones. In this study, we generated THz waves with a frequency of 9.7 THz using eoo collinear phase-matching conditions. The wavelength of the pump light (extra-ordinary light) and signal light (ordinary light) were 1203.0 nm and 1251.6 nm, respectively. The pulse width and repetition frequency were 22 ns and 10 Hz, respectively. The signal and pump light were incident on the (001) face of the InxGa1-xSe crystal and they propagated coaxially in the crystal. The diameter of aluminum aperture (Fig. 1) was 3 mm. Phase matching conditions were achieved by changing the angle at which the excitation light was incident on the crystal. The THz waves were detected by 4K-Si:Bolometer (Infrared Inc.). The near-IR excitation light was safely removed by the black polyethylene placed in front of the Si:Bolometer. Because the output power of the THz wave is proportional to the product of the powers of pump and signal waves, the conversion efficiency for generation of the THz waves can be expressed as
3. Results and discussion
3.1 Measurement of Indium content in the grown crystal by EPMA and XRD
Using EPMA, the average amounts of Ga, Se, and In in the crystal were obtained as 47.44 at%, 50.04 at%, and 2.52 at%, where the standard deviations are 0.0012 at%, 0.0097 at%, 0.012 at%, respectively.
Figure 2(a) shows the XRD peaks from (0 0 18), and Fig. 2(b) shows a plot of ${\mathbf{cos}^2}\theta $ vs. the lattice constant c calculated using the XRD peaks from (0 0 10) to (0 0 20). As shown in the Fig. 2(b), the estimated c of the grown InxGa1-xSe crystal was 15.9956 Å and 15.9954 Å based on the peaks of CuKα1 and CuKα2, respectively. According to Vegard’s law and considering the lattice constants for GaSe (c = 15.95 Å) and InSe (c = 16.70 Å) [26], the In content in the grown crystal was calculated as 3.04 at% and 3.03 at% from the lattice constants 15.996 Å and 15.995 Å, respectively, which are similar to the value measured by EPMA. Therefore, we believe that the InxGa1-xSe mixed crystal was grown from In flux and In atoms were substituted at Ga sites.
Fig. 2. (a) X-ray diffraction peaks from (0 0 18), (b) The plot of ${\textrm{cos}^2}\theta $ vs. the lattice constant c calculated using the diffraction peak from (0 0 10) to (0 0 20).
3.2 THz wave generation
Figure 3 shows the relationship between the output power of THz wave and the (0 0 1) in-plane angle of the InxGa1-xSe crystal. The thickness perpendicular to (0 0 1) plane of the crystal was 415 µm. Here, the output power of pump and signal lights were 0.8 mJ. As shown in Fig. 3, the output power of THz wave varied in 60° cycle of the in-plane angle, which can be attributed to the six-fold symmetric structure of the (0 0 l) plane in InxGa1-xSe crystal [27]. From this six-fold symmetric change, it is considered that stacking layers of InxGa1-xSe crystal used for the generation of THz wave were isotropically arranged, and the grown crystal was a single crystal.
Fig. 3. Variation of output power of the THz wave corresponding to six-fold symmetry of the (0 0 l) plane.
Figure 4(a) shows the conversion efficiency of the THz waves (9.7 THz) from the InxGa1-xSe crystal and undoped GaSe crystal. The (001) in-plane angle of the InxGa1-xSe crystal was set at 340° in Fig. 3, which corresponded to the generation of THz wave with the maximum intensity. Here, the output power of pump and signal light were 1.5 mJ. Interference peaks were observed in the measured conversion efficiency, so the squares show the average conversion efficiency between the corresponding peaks and troughs in Fig. 4(a). Accordingly, the maximum output power was obtained as 39 pJ and the maximum conversion efficiency of InxGa1-xSe crystal was 1.7 × 10−5 J−1. When the output power of the pump and signal light were 0.75 mJ, the maximum output power of the THz wave was 6.0 pJ. Therefore, it is confirmed that the THz output power was nearly proportional to the product of the pump and signal light intensity. The conversion efficiency of InxGa1-xSe crystal is 28 times larger than that for 9.41 THz waves generated from an undoped GaSe crystal grown from Ga flux reported in [3]. This result supports the earlier reports claiming that the second-order NLO constant is improved by doping GaSe with In [22,23]. Figure 4(b) shows the output power of the THz waves at 9.7 THz and 10.6 THz. As shown in the Fig. 4(b), the THz wave power at 10.6 THz was higher than that at 9.7 THz. Because this frequency dependence of the THz output power is same tendency with pure GaSe crystal [2], it is considered that InxGa1-xSe crystal has large absorption in 5–8 THz (60–37.5 µm) as well as GaSe crystal [7].
Fig. 4. (a) Conversion efficiency of THz wave at 9.7 THz as a function of the incident angle of pump wave. (Red line: InxGa1-xSe crystal, Red squares: The average conversion efficiency between the corresponding peaks and troughs, Dotted red line: Line connecting red squares, Black line: Undoped GaSe crystal), (b) Output power of the THz waves from the InxGa1-xSe crystal at 9.7 THz and 10.6 THz as a function of the incident angle of pump wave.
4. Summary
We generated THz waves at 9.7 THz via DFG using an InxGa1-xSe crystal grown from In flux, where the crystal thickness was 415 µm. The output power of the generated THz waves varied in 60° cycle, which was attributed to the six-fold symmetric structure of the (0 0 l) plane. The maximum THz output power and conversion efficiency were 39 pJ and 1.7 × 10−5 J−1, respectively. This conversion efficiency is 28 times larger than that reported for 9.41 THz waves generated from an undoped GaSe crystal grown from Ga flux. The In content of the In1-xGaxSe mixed crystal measured by EPMA was 2.52 at%. Moreover, because this value is close to that estimated using Vegard’s law from lattice constants measured by XRD, it is considered that the In atoms were substituted at Ga sites in the crystal.
Funding
Japan Society for the Promotion of Science (JP18J11396, JP19J20564).
Disclosures
The authors declare no conflicts of interest.
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