Yusuke Mori

Growth of a Two-Inch GaN Single Crystal Substrate Using the Na Flux Method

We succeeded in growing a GaN single crystal substrate with diameter of about two inches using the Na flux method. Our success is due to the development of a new apparatus for growing large GaN single crystals. The crystal grown in this study has a low dislocation density of 2.3×105 cm-2. The secondary ion mass spectrometry (SIMS) technique demonstrates that the Na element is difficult to be taken in the crystal in both the + and - c directions, resulting in a Na concentration of 4.2×1014 cm-3 in the crystal. Our success in growing a two-inch GaN substrate with a low impurity content and low dislocation density should pave the way for the Na flux method to become a practical application.

Growth of Protein Crystals in Hydrogels Prevents Osmotic Shock

High-throughput protein X-ray crystallography offers a significant opportunity to facilitate drug discovery. The most reliable approach is to determine the three-dimensional structure of the protein-ligand complex by soaking the ligand in apo crystals. However, protein apo crystals produced by conventional crystallization in a solution are fatally damaged by osmotic shock during soaking. To overcome this difficulty, we present a novel technique for growing protein crystals in a high-concentration hydrogel that is completely gellified and exhibits high strength. This technique allowed us essentially to increase the mechanical stability of the crystals, preventing serious damage to the crystals caused by osmotic shock. Thus, this method may accelerate structure-based drug discoveries.

Terahertz Comb Spectroscopy Traceable to Microwave Frequency Standard

The fine-structured spectrum of a terahertz (THz) frequency comb has been observed using asynchronous-optical-sampling THz time-domain spectroscopy with an extended time window covering multiple THz pulses. Fourier transformation of 10 consecutive THz pulses enables us to obtain the spectrum of THz comb mode having a linewidth of 25 MHz at intervals of 250 MHz. Further expansion of time window up to 100 consecutive THz pulses reduces the linewidth of THz comb mode down to 2.5 MHz while conserving interval of 250 MHz. The observed THz comb mode can be used as a precise and accurate frequency marker for broadband THz spectroscopy because it is phase-locked to a microwave frequency standard by laser control. The spectroscopy of pharmaceutical tablets and low-pressure molecular gas is used to demonstrate the utility of this approach, indicating a spectral resolution of 250 MHz. The proposed method enables frequency calibration of the THz spectrometer based on a microwave frequency standard.

Liquid Phase Epitaxy Growth of m-Plane GaN Substrate Using the Na Flux Method

We report the fabrication of {10-10} (m-plane) GaN single crystal substrates grown by the Na flux method. First, we investigated the growth conditions for which the m-plane is stable. Then, an m-plane GaN substrate, of size 10 ×10 ×0.5 mm3, was obtained employing the Na flux method and liquid phase epitaxy (LPE) technique using an m-plane GaN thin films grown by metal organic chemical vapor deposition (MOCVD) as a seed. The full width at half maximum (FWHM) of the rocking curve of the LPE m-plane GaN crystal was 231 arcsec, which was markedly improved compared with that of the MOCVD-grown m-plane GaN template (3248 arcsec) used as the seed crystal.

Bulk Crystal Growth of Stilbazolium Derivatives for Terahertz Waves Generation

The two stilbazolium derivatives, 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) and 4-dimethylamino-N-methyl-4-stilbazolium p-chlorobenzenesulfonate (DASC), are organic ionic nonlinear optical materials that have a tendency to form bulk crystals when grown in a mixed solvent of methanol and acetonitrile. We observed the generation of broadband high-power terahertz (THz) waves from the bulk crystals of DASC. DASC crystals have superior transmission characteristics in the THz band than DAST crystals, and it is expected that the THz waves generated using DASC crystals will have higher power than those generated using DAST crystals.

Fabrication of a-Plane GaN Substrate Using the Sr–Na Flux Liquid Phase Epitaxy Technique

We report the fabrication of a-plane GaN single crystal substrates grown by the Na flux method. In this research, Sr was added into the flux system as additive to control the crystal habit of GaN single crystals. As the amount of Sr in the melt increased, it was found that the GaN crystal shape changed from pyramidal to prismatic crystals elongated parallel to the direction. Additionally, liquid phase epitaxy (LPE) GaN crystals were grown in Sr–Na solution on a-plane GaN templates fabricated by metal organic chemical vapor deposition (MOCVD) on sapphire substrates. ω-scan X-ray diffraction measurements showed that the full width at half maximum (FWHM) of the (11-20) plane of the LPE GaN crystal was smaller (236 arcsec) than that of the a-plane of the GaN template (1152 arcsec).

Selective crystallization of metastable phase of acetaminophen by ultrasonic irradiation

A new method for selective crystallization of the metastable phase (form II) of acetaminophen is described. To obtain form II, we prepared a highly supersaturated solution (σI = 3.7) and then applied ultrasonic irradiation at different frequencies. Without ultrasonic irradiation, spontaneous crystallization did not occur within one month in the highly supersaturated condition (σI = 3.7). When ultrasonic irradiation at 28 kHz was applied, form II preferentially crystallized. Therefore, we conclude that ultrasonic irradiation can be an effective technique for selectively crystallizing the metastable phase.

Al Doping of CsLiB6O10 for High Resistance to Ultraviolet-Induced Degradation

We investigated the ultraviolet-induced degradation of a nonlinear optical crystal CsLiB6O10 (CLBO). High-repetition-rate pulsed lasers with an output wavelength of 266 nm induced degradation of CLBO at a lower peak power density than the bulk laser-induced damage threshold. This degradation behavior is similar to photorefractive damage. We found for the first time that Al-doped CLBO heated at 150 °C exhibits a longer lifetime for the degradation than undoped CLBO.

Highly efficient 355 nm UV generation with non-collinear phase-matching by a prism-coupled device based on CsLiB6O10

We report 355 nm third-harmonic generation (THG) of UV light using CsLiB6O10 (CLBO). To compensate for the walk-off angle in THG by Type II CLBO, we designed a prism-coupled device that takes advantage of non-collinear phase-matching. This new device, equipped with a Nd:YVO4 laser source, achieved THG with 1.86 W at 47.6% conversion efficiency from the second-harmonic input. This result marks a 1.43-fold improvement over a conventional CLBO crystal, thus validating the effectiveness of the design for walk-off compensation in Type II THG. This is also the first time THG has been achieved using CLBO.

Phase-Matching Properties at around 190 nm of Various Borate Crystals

We report on the phase-matching properties of borate crystals of LiB3O5 (LBO), CsB3O5 (CBO), and CsLiB6O10 (CLBO) in vacuum–ultraviolet (VUV) light generation at around 190 nm. A new phase-matching property in the VUV spectral range for CBO grown from a self-flux solution was found to have better agreement with new Sellmeiers equations developed in our laboratory. A 189 nm output of 11.4 mW was produced in a CLBO with a conversion efficiency of 7.3% from 213 nm input. CLBO is suitable for VUV light generation around 190 nm with the present laser system.