Open Access
Add to BibSonomyAbstract
Using the room-temperature-bonding technique, we have succeeded in fabricating a quasi-phase matching stack of thirty 106 µm-thick and 5.5 × 5.0 mm-aperture GaAs plates for high-power second-harmonic generation of CO2 lasers with the wavelength of 10.6 µm. Although its transmittance was lower than that of a single GaAs plate because of inclusion of small particles in the bonded interface, newly fabricated twenty stacked GaAs plates with the improved process show nearly the same transmittance with that of a single plate.
© 2017 Optical Society of America
1. Introduction
GaAs has attracted much attention as a wavelength-conversion material in the mid-infrared region owing to its transparency at the wavelengths from 1 to 17 µm, high thermal conductivity, and high optical nonlinearity [1, 2], Since GaAs is optically isotropic, realization of quasi-phase matching (QPM) is essential for highly efficient wavelength conversion. Although the sublattice reversal epitaxy [3] and the orientation patterned [4] QPM GaAs wavelength-conversion devices have been reported, high-power wavelength conversion is difficult because these methods have limitation of fabricating thick devices. On the other hand, diffusion bonded stacks of GaAs plates have been previously reported as a method for fabricating large-aperture QPM devices [5, 6]. However, since the diffusion bonding is a high-temperature process, dissociation of arsenic occurs and degrades the crystal quality, and free carrier absorption is also induced which reduces the transmittance and the conversion efficiency.
We recently proposed a new method, room-temperature bonding (RTB), for realizing QPM devices, and demonstrated a 12 plate-stacked structure of GaAs plates [7]. In addition, we fabricated a walk-off compensating β-BaB2O4 ultraviolet wavelength-conversion device using the room-temperature bonding to achieve 1.8 times higher efficiency than that of the bulk crystal [8].
In this work, we have applied the room-temperature-bonding process and succeeded in fabricating a 30 plate-stacked GaAs structure comprising 5.5 mm × 5.0 mm × 106 µm plates, which would realize first-order QPM second-harmonic generation (SHG) of CO2 lasers with the wavelength of 10.6 µm. CO2 lasers are one of standards for laser processing such as cutting or drilling because high power of kW level is available, and its wavelength of 10 µm is absorbed for many materials regardless of metals and non-metals. However, this wavelength makes difficult to reduce the spot diameter. High-power SHG devices for CO2 lasers would then realize more sophisticated laser processing with smaller spot sizes. Although CO lasers oscillating at 5 µm were recently developed [9], the wavelength would be switchable between 10 µm and 5 µm if high-power SHG devices are available. Moreover, the wavelength-conversion technique to be established by this work can also be applied to generating other mid-infrared wavelengths.
2. Fabrication of QPM stacks of GaAs plates
2.1 Room-temperature-bonding technique
Room-temperature bonding is also called surface activated bonding [10]. Figure 1 shows the process of room-temperature bonding. First, two GaAs plates are set in a vacuum chamber, the degree of vacuum of which is around ~10−5 Pa (Fig. 1(a)). Next, Ar atom beams are irradiated to the two surfaces. Then the adsorbed molecules and the oxidized layers on the surfaces are etched and the surfaces are activated (Fig. 1(b)). After the two plates are touched each other and pressed for a while, the surfaces are atomically bonded (Fig. 1(c)).
The room-temperature bonding does not need a high-temperature process so that it is possible to fabricate large-aperture high-power devices without degradation of crystal quality. Moreover, bonding can be made for variety of materials including semiconductors, metals, and ceramics.
2.2 Fabrication procedure
The GaAs plates used for bonding were semi-insulating (111) plates with the resistivity of larger than 1 × 107 Ω·cm. The both faces were optically polished, the flatness of which was less than 0.3 µm and the surface roughness, Ra, was less than 1 nm.
Fig. 2(a) shows the instrument for room-temperature bonding, and Fig. 3 shows the fabrication process of QPM GaAs stacks. First, one plate is set on the upper rod and several plates are aligned with the orientation reversed each other on the translation stage as shown in Fig. 2(b). Next, the two surfaces are irradiated with Ar atom beams (Fig. 3 (a)), and the two plates are touched and pressed (Fig. 3(b)). Then the rod is pulled up and the bonding was completed. Next, the translation stage was moved to supply another plate for bonding (Fig. 3(c)). This process was repeated and 10 stacked QPM GaAs was fabricated.
Fig. 2 (a) The instrument for room-temperature bonding and (b) the translation stage in the instrument which successively supplies plates to be bonded.
It took a considerable time to bond multiple plates since we previously opened the chamber to atmosphere every time to set another plate to be bonded. We then introduced a translation stage that can successively supply the plates, which enabled us to fabricate many-plate stacked structures in a short time. Moreover, we optimized the process conditions such as the pressure applied to the sample (3.2 kg/cm2), the irradiation time by the Ar beams (10 min.). As a result, we succeeded in fabricating a 30 plate-stacked structure by bonding 3 sets of 10 stacks, the photograph of which is shown in Fig. 4. The whole thickness is 3.18 mm.
3. Optical characterization of the QPM stacked GaAs
As characterization of the fabricated QPM stacked GaAs, we first measured the bonding strength using a tensile force gauge. The tensile strength of a 7 plate stack was 20 kg/cm2, which is enough for practical wavelength-conversion devices.Next, we measured the transmittance spectra of 20 stacked QPM GaAs plates using FT-IR (JASCO, FT/IR-6300) with the resolution of 16 cm−1. The beam size was about 3 mm x 3 mm. The transmission spectrum (green line) and that of a single GaAs plate (red line) are shown in Fig. 5. It is noted that the transmittance of the single plate agrees with that calculated from the refractive indices [11], and the multiple-reflection effects inside the thin single GaAs plate appear in the wavelength region from 5 to 7 µm. The transmittance was 10 to 20% lower than that of the single GaAs plate. The reason for lower transmittance of the stacked GaAs plates was found to be the scattering loss at the bonded interfaces which accidentally include dusts and the air gaps induced by the dusts due to imperfection of the process, as shown in Fig. 6 (data taken with Zygo NewView 7300). Although it is difficult to directly determine the size of the dusts and the gap area from the profile, long-period oscillations are observed for the stacked plates (green line) in Fig. 5. They are supposed to be the interference at the air gap, the height of which is estimated to be as large as around 30 μm from the oscillation period in this case.
Fig. 5 Transmission spectra of a single GaAs plate (red line), the 20 stacked GaAs plates fabricated with the previous successive bonding process (green line), and that with the one-by-one bonding process (blue line).
Fig. 6 (a) Surface profile of the top GaAs plate of the 20-plate stack fabricated with the previous successive bonding process. Inclusion of small particles at the bonded interface (surrounded with dashed circles) are observed, the profile of which along the arrow line is shown in (b). gap, the height of which is estimated to be as large as around 30 μm from the oscillation period in this case.
On the other hand, we also fabricated a 20 plate-stacked structure by venting the vacuum chamber, setting only one GaAs plate on the translation stage, and cleaning its surface every time before bonding. Its transmission spectrum is the blue line in Fig. 5, which was improved to nearly the same with that of a single GaAs plate. This indicates that setting of plates directly on the translation stage made of stainless steel could introduce small particles onto the surfaces of plates. Then we covered the translation stage with elastomer sheets and put the GaAs plates on them to keep their surfaces away from dusts, and fabricated a stack of 9 plates by the successive bonding process. Figure 7 shows the surface profiles of the top plate, from which no inclusion of dusts are observed. This is confirmed by the transmission spectra as shown in Fig. 8, in which the transmittance of the 9 stacked plates with the improved successive bonding process is nearly the same with that of a single plate.
Fig. 7 Surface profile of the top GaAs plate of the 9-plate stack fabricated with the improved successive bonding process.
Fig. 8 Transmission spectra of a single GaAs plate (red line), 10 stacked GaAs plates fabricated with the previous successive bonding process (green line), and the 9 stacked GaAs plates with the improved successive bonding process (blue line).
4. SHG measurements
We performed the preliminary SHG measurement using the newly fabricated 9-plate stacked GaAs-QPM structure. A linearly polarized laser beam from a cw CO2 laser (Universal laser Systems, ULR-10) oscillating at the wavelength of 10.6 μm, was focused to the sample with the beam radius of around 150 μm. The fundamental beam was cut with Si and sapphire plates after the sample, and only the generated second-harmonic (SH) wave was detected with an InSb photovoltaic detector (Hamamatsu, P5968-100) because its power was much lower than 1 mW with such a small number of plates.
The generated wave was confirmed to be the SH wave because its power increased with the square of the fundamental power, as shown in Fig. 9. Moreover, the SH power from the 9-plate stack fabricated with the improved successive bonding process (open circles) was found to be larger than that from the 10-plate stack with the previous process (open squares) owing to its higher transmittance as shown in Fig. 8.
Fig. 9 Second-harmonic power as a function of the fundamental power for the 9 stacked QPM-GaAs plates fabricated with the improved process (circles) and the 10 stacked plates with the previous process (squares).
5. Summary
We have succeeded in fabricating multple-plate stacked GaAs-QPM structures using a new technique: room-temperature bonding. The stacked structure shows nearly the same transmittance with a single GaAs plate by optimizing the bonding process. It was also confirmed that the second-harmonic power from the 9-plate GaAs stack with the improved process was larger than that form the 10-plate stack with the previous process because of higher transmittance. We are now trying to fabricate 100-plate or more stacked GaAs-QPM structures for realizing highly efficient and high-power wavelength conversion.
References and links
1. I. Shoji, T. Kondo, A. Kitamoto, M. Shirane, and R. Ito, “Absolute scale of second-order nonlinear-optical coefficients,” J. Opt. Soc. Am. B 14(9), 2268–2294 (1997). [CrossRef]
2. I. Shoji, T. Kondo, and R. Ito, “Second-order nonlinear susceptibilities of various dielectric and semiconductor materials,” Opt. Quantum Electron. 34(8), 797–833 (2002). [CrossRef]
3. S. Koh, T. Kondo, M. Ebihara, T. Ishiwada, H. Sawada, H. Ichinose, I. Shoji, and R. Ito, “GaAs/Ge/GaAs sublattice reversal epitaxy on GaAs (100) and (111) substrates for nonlinear optical devices,” Jpn. J. Appl. Phys. 38(2), L508–L511 (1999). [CrossRef]
4. C. B. Ebert, L. A. Eyres, M. M. Fejer, and J. S. Harris Jr., “MBE growth of antiphase GaAs films using GaAs/Ge/GaAs heteroepitaxy,” J. Cryst. Growth 201–202, 187–193 (1999). [CrossRef]
5. L. A. Gordon, G. L. Woods, R. C. Eckardt, R. K. Route, R. S. Feigelson, M. M. Fejer, and R. L. Byer, “Diffusion-bonded stacked GaAs for quasiphase-matched second-harmonic generation of a carbon dioxide laser,” Electron. Lett. 29(22), 1942 (1993). [CrossRef]
6. E. Lallier, M. Brevignon, and J. Lehoux, “Efficient second-harmonic generation of a CO2 laser with a quasi-phase-matched GaAs crystal,” Opt. Lett. 23(19), 1511–1513 (1998). [CrossRef] [PubMed]
7. M Kawaji, K Imura, T Yaguchi, and I Shoji, “Fabrication of quasi-phase-matched devices by use of the room-temperature-bonding technique” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (Optical Society of America, 2009), paper TuB24.
8. K. Hara, S. Matsumoto, T. Onda, W. Nagashima, and I. Shoji, “Efficient ultraviolet second-harmonic generation from a walk-off-compensating β-BaB2O4 device with a new structure fabricated by room-temperature bonding,” Appl. Phys. Express 5(5), 052201 (2012). [CrossRef]
9. A. Held, “CO Lasers from Lab to Fab,” Laser Tech. J. 13(3), 15–17 (2016). [CrossRef]
10. T. Suga, Y. Takahashi, H. Takagi, B. Gibbesch, and G. Elssner, “Structure of Al-Al and Al-Si3N4 interfaces bonded at room temperature by means of the surface activation method,” Acta Metall. Mater. 40, S133–S137 (1992). [CrossRef]
11. A. N. Pikhtin and A. D. Yas’kov, “Dispersion of the refractive index of semiconductors with diamond and zinc-blende structures,” Sov. Phys. Semicond. 12, 622–626 (1978).
