Tomoyuki Kimura

Frequency-tunable high-power terahertz wave generation from GaP

A frequency-tunable terahertz wave was generated from GaP crystals using an optical parametric oscillator as the pump source and a YAG laser (1.064 μm) as the signal source. By tuning the very small angle, θin, between the pump and signal light beam directions, tunable terahertz waves over the frequency range from 0.5 to 3 THz were obtained. The THz frequency changed almost linearly with the angle θin. The precise noncollinear phase matching condition is discussed. The pulsed peak power of the THz wave was as high as 480 mW at 1.3 THz.

Spectral measurement of terahertz vibrations of biomolecules using a GaP terahertz-wave generator with automatic scanning control

Having previously generated frequency-tunable terahertz (THz) waves from GaP, we constructed a THz spectral measurement system with an automatic scanning control. We demonstrate the use of this THz spectrometer for measuring the transmittance spectra of D-(+)-glucose, 2-deoxy-D-glucose, D-(−)-fructose, and D-(+)-sucrose in the 0.8 to 5.5 THz (25 to 185 cm−1) frequency region with a spectral resolution of 3.2 GHz (0.1 cm−1), using a pyroelectric detector operated at room temperature. Each crystalline saccharide had different spectral features.

Frequency-tunable terahertz wave generation via excitation of phonon-polaritons in GaP

High-power, wide-frequency-tunable terahertz waves were generated based on difference-frequency generation in GaP crystals with small-angle noncollinear phase matching. The tunable frequency range was as wide as 0.5–7 THz, and the peak power remained high, near 100 mW, over most of the frequency region. The tuning properties were well described by the dispersion relationship for the phonon-polariton mode of GaP up to 6 THz. We measured the spectra of crystal polyethylene and crystal quartz with high resolution using this THz-wave source.

Semiconductor Raman amplifier for terahertz bandwidth optical communication

Semiconductor Raman amplifiers are useful for frequency selection in terahertz bandwidth and wavelength division multiplexing (WDM) systems with terabit capacity, as well as direct terabit optical communication systems. We have developed GaP-AlGaP Raman waveguides with micrometer-size cross sections. We have reduced residual optical loss of the waveguide by improvement of the fabrication process and realized a low-loss waveguide that is 10-mm long, which has a continuous wave (CW) Raman gain of 3.7 dB. Also, the time-gated amplification with 80-ps pulse pumping is performed and 20-dB gain is obtained. These performances are very suitable for light frequency selection in terahertz bandwidth and WDM optical communication systems.

Raman gain and optical loss in GaP–AlGaP waveguides

The stimulated Raman gain coefficient and the optical loss in GaP/AlGaP semiconductor Raman amplifiers are measured for both double path and single path structures. For the double path structure, the highest gain per waveguide length is 3.27 dB/cm for a waveguide with a cross-sectional area of 4 μm2, under continuous wave pumping. A long waveguide with a length of 10 mm is demonstrated to have the highest gain of 2.57 dB. For a single path structure, the gains of backward scattering and forward scattering are separately measured and it is found that the gain of backward scattering is dominant. From these results, the backward gain coefficient is estimated to be 12.3×10−8 W cm−1. This value is about five times that of CS2 and LiNbO3, which is known to have the highest Raman scattering gain. However, the optical losses of the waveguides are estimated from the finesse measurement. The lowest one way loss corresponds to an effective absorption coefficient of αeff=0.28 cm−1.

THz Generation From GaP Rod-Type Waveguides

Terahertz (THz) generation was demonstrated from GaP rod-type waveguides via difference-frequency-mixing of near-infrared light using a collinear phase-matching condition. THz output peaks were observed, and appeared at frequencies corresponding to the fundamental and high-order waveguide modes. Interestingly, the position of the fundamental mode shifted to a higher frequency for a smaller waveguide cross-section, which is attributed to the waveguide confinement of the THz wave. The conversion efficiency was enhanced in the waveguide with a cross section of 200 mumtimes160 mum as compared to that in bulk GaP crystals

Characteristics of time-gated Raman amplification in GaP–AlGaP semiconductor waveguides

Time-gated Raman amplification in the GaP–AlGaP waveguide is investigated using mode-locked Ti–sapphire pump source with 80 ps pulse width. Logarithmic Raman gain linearly increases with increasing the pump power density as long as the gain is less than about 10 dB. However, with further increasing the pump power it becomes nearly proportional to the square root of the pump power density. This is due to the fact that the equivalent linewidth of the pump pulse is comparable to the spectral full width half maximum of the Raman gain coefficient (24 GHz). Another point is that the amplified pulse broadens as the waveguide length exceeds the optical length corresponding to the pump pulse width because Raman amplification occurs mainly due to backward scattering.

Buried‐heterostructure semiconductor Raman laser with threshold pump power less than 1 W

The low‐power operation of a semiconductor buried‐heterostructure Raman laser is reported. We are developing these devices for very wide‐band optical communication in the terahertz frequency region. It has a structure with a GaP active layer and AlxGa1−xP cladding layers, which are grown by the temperature‐difference method under controlled vapor pressure. By making the stripe width 30–40 μm, we have obtained a threshold pump power of 500 mW. A low‐threshold semiconductor Raman laser can be pumped by semiconductor injection lasers. We have measured the optical loss of the waveguide and detected the contribution from scattering and leakage at heterointerfaces.

The structure and maximal gain of CW-pumped GaP-AlGaP semiconductor Raman amplifier with tapers on both sides

GaP-AlGaP waveguide semiconductor Raman amplifiers (SRAs) tapered on both sides were fabricated by high-quality GaP-AlGaP liquid phase epitaxial growth using the temperature difference method with controlled vapor pressure (TDM-CVP), photolithography patterning, and reactive ion etching with PCl/sub 3/ gas. Although the finesse of the both-sides-tapered waveguide SRA is lower than previous values for straight or one-side-tapered waveguides, the CW-pumped gain was maximized, and a maximal gain of 4.2 dB was obtained. This letter presents the effect of tapered structures in SRA with CW pumping amplification.

Heteroepitaxy of GaP-AlxGa1−xP system by the Temperature Difference Method under Controlled Vapor Pressure (TDM-CVP)

Abstract Multilayer growth of Al x Ga 1− x P and GaP on a GaP substratewith narrow buried stripe structures has been successfully performed by the temperature difference method under controlled vapor pressure (TDM-CVP). We have observed cross-hatch patterns and strain-induced optical anisotropy, although the lattice misfitting is as small as that of the Al x Ga 1- x As-GaAs system. Also we have measured the optical transmission of the GaP waveguiding stripes and confirmed sufficient transparency and optical confinement.