Tadao Tanabe

Tunable terahertz wave generation in the 3- to 7-THz region from GaP

Following the generation of tunable terahertz waves from GaP in the 0.5- to 3-THz region, we extended the frequency region up to 7 THz, using an optical parametric oscillator and a YAG laser (1.064 μm). The tuning angle θin increased superlinearly in the 3- to 7-THz region, so that the total reflection took place at 5 THz, which was avoided by rotating the crystal relative to the incident optic axis. As a result, terahertz output peak powers of 100 mW at up to 5.6 THz and 3 mW at 7 THz were obtained, at pump and signal energies of 3 mJ, respectively.

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.

GaP THz wave generator and THz spectrometer using Cr:Forsterite lasers

We have developed a type of THz wave generator that uses Cr:Forsterite lasers as the pump and signal sources for difference frequency generation in GaP (Cr:F source system). We confirmed the generation of THz waves in the frequency range from 0.3to7.5THz, which is just similar to that obtained using the THz wave generator previously developed utilizing yttrium aluminum garnet and optical parametric oscillator (OPO) lasers (OPO source system). A peak output power of 100mW was obtained from 1.2to5THz when the power of the two input beams was 3mJ each, similar to the OPO source system. A wide measurable frequency range from below 0.6THz to over 6THz was obtained by using the Cr:F source system as the light source of a spectrometer, which has the merits of simple structure, easy maintenance, and low cost compared with the OPO source system. Although the linewidth of the Cr:F source system is greater than that of the OPO source system, the THz spectrometer still has sufficient resolution for measuring solids o...

Characteristics of terahertz-wave generation from GaSe crystals

Terahertz (THz) waves from 0.3 to 4.9 THz were generated from GaSe by difference-wave generation using a YAG laser and an optical parametric oscillator. The spectral THz intensity distribution matched the spectrum of the THz absorption coefficient well, which suggested the effect of imperfections in the crystal. The fairly narrow beam profile and narrow line width, together with the simplicity of the collinear configuration, make GaSe an effective THz-wave generator. It is compared with a GaP THz-wave generator.

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.

Terahertz-wave generation by GaP rib waveguides via collinear phase-matched difference-frequency mixing of near-infrared lasers

We constructed rib waveguides from GaP material using an inductively coupled plasma reactive ion etching technique based on Ar/Cl2 gas application. We obtained a waveguide with a rib height of 200 μm. Terahertz-wave generation from the GaP-crystal rib waveguides was demonstrated via collinear phase-matched difference-frequency mixing of near-infrared light. The terahertz output peak corresponding to the fundamental modes appeared around 0.75 THz for a 1-mm-wide rib waveguide. The position of the fundamental mode shifted to 1.32 THz for a 200-μm-wide waveguide, which is attributable to the two-dimensional confinement of the terahertz waves in the waveguide. The conversion efficiency was enhanced in the rib waveguide compared to that in both slab waveguides and bulk GaP crystals.

Enhancement of CW THz Wave Power Under Noncollinear Phase-Matching Conditions in Difference Frequency Generation

We investigated methods for improvement of continuous-wave (CW) terahertz (THz) output power by laser diode (LD) pumping in noncollinear phase-matched difference frequency generation (DFG). The effects of interaction length and beam spot size of input lasers (near-IR) in GaP crystals were studied. The THz wave power dependence on various sizes of GaP crystals was investigated and it was observed that an output power of 4 nW was obtained with a 20 mm long GaP crystal at 1.5 THz. Also, the THz wave absorption coefficient was dominant for longer GaP crystals at high frequencies (above 2.5 THz). The THz wave power dependence on beam spot size (1.2 mm-300 mum) of near-IR lasers at 1.62 THz was studied, and an improvement of THz wave power being seen with a 500 mum beam spot size, while the beam divergence effect was dominant for beam spot sizes below 500 mum.

Carbon monoxide adsorption on ultra-thin Fe film deposited on Cu(100)

This paper is mainly concerned with vibrational properties of CO adsorbed on fcc Fe(100) observed using IR reflection absorption spectroscopy. An epitaxial Fe film of eight monolayers has been grown at 300 K on Cu(100). At 0.1 L exposure only a single C-O stretch band appears at 1920 cm -1 which shifts to 1977 cm -1 at 0.55 L. This band further shifts to 1998 cm -1 and saturates in intensity at 0.88 L. In contrast, at 0.25 L a second C-O stretch band appears at 2020 cm - 1 which increases in intensity and shifts to 2048 cm -1 at 1.2 L. The initially observed band can be ascribed to the bridging CO and the second band to the on-top CO. Although the bridging CO was stable up to ca. 380 K, the band due to the on-top CO was less stable and disappeared above 313 K. The observed CO adsorption/desorption behavior on the epitaxial fcc Fe(100) surface is compared with that on a vacuum-evaporated bcc Fe polycrystalline film.

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.