Takehiko Hidaka

Generation of coherent terahertz radiation by photomixing of dual-mode lasers

We have reviewed and discussed the generation of continuous-wave terahertz radiation by the photomixing of dual-mode lasers, which includes a multi-mode laser diode (LD), a dual-mode microchip laser, a monolithic dual-mode DBR LD, and a widely tunable dual-mode external cavity LD. It is demonstrated by several experiments that the beat frequency of a dual-mode laser is more stable than the original laser modes because of the common-mode rejection effect. We also suggest a novel scheme by which the optical beat is stabilized to less than 1 kHz and its frequency can be precisely determined as well.

Ferroelectric PVDF cladding terahertz waveguide

A new type of flexible terahertz-wave hollow waveguide has been developed using ferroelectric polymer poly vinylidene fluoride (PVDF) as the cladding material. The transmission efficiency of the PVDF waveguide was better than a metal hollow waveguide with the same size and structure as the PVDF waveguide. This paper also reports the bent-angle dependencies of the transmission efficiency of the hollow PVDF.

Adaptive optics instrumentation in submillimeter/terahertz spectroscopy with a flexible polyvinylidene fluoride cladding hollow waveguide

A simple instrument has been developed to carry out temperature dependent submillimeter/terahertz-wave spectroscopy using a polyvinylidene fluoride flexible hollow waveguide and an eggplant-shape launching lens.

Soft x-ray transmission characteristics of hollow-core fused-quartz fibers

Transmission factors of hollow-core fused-quartz fibers were measured as a function of curvature angle (0–90°) and photon wavelength (25–75A). They were compared with the results obtained by a simple calculation based on geometrical optics. Both the measured and calculated transmission factors decreased exponentially with increasing curvature angle and showed a slight dependence on the wavelength.

Ultrafast Coherent Control of Excitons Using Pulse-Shaping Technique

In this paper, we report on the ultrafast coherent control of excitons in quantum wells using a phase-locked pulse sequence generated by pulse-shaping techniques. The pulse-shaping system with a double liquid-crystal spatial light modulator (SLM), and a Ti3+-sapphire laser is used to generate phase-locked pulses. The ultrafast coherent control of the exciton population and polarization is demonstrated by the observation of the reflectivity change in the pump-probe and the diffracted power in the degenerate-four-wave-mixing (DFWM) measurements. In single quantum wells, good coherent control characteristics with 87% coherent carrier destruction are demonstrated at the low excitation power of 0.15 mW (~1.1×1010 photons/cm2). In addition, the preliminary manipulation of the exciton population is demonstrated by the phase-locked triple pulses. In the case of coupled quantum wells, the modulation of the SLM is carefully controlled and the coherent control of both the exciton population and the polarization is demonstrated in the two different coupled quantum-well samples.

Generation of a Diffraction-Free Laser Beam Using a Specific Fresnel Zone Plate

A diffraction-free, high-efficiency light beam has been generated using a specific Fresnel zone plate and a suitable convex lens. The principle and experimental results are given.

Electrically Tunable and Dual-Wavelength Semiconductor Laser with a Liquid Crystal Display

An electrically tunable semiconductor laser with a liquid crystal display (LCD) and a commercially available laser diode is presented. We tested two types of LCD under strong laser light and, on the basis of the results, selected the supertwist-nematic (STN) LCD with a passive matrix. Tuning is achieved up to 5 nm by moving a single stripe pattern displayed on the LCD. The wavelength shifts in 0.553 nm/cell, which agrees well with the calculated resolution derived from the diffraction grating formula. Dual-wavelength oscillation is also achieved up to a wavelength separation of 4.62 nm using the double stripe pattern.

Generation and Homodyne Detection of Continuous Terahertz Waves Using Single Photoconductive Antenna

The generation and homodyne detection of continuous terahertz (THz) waves using a single photoconductive antenna excited by dual-wavelength continuous-wave laser light have been carried out. The THz waves emitted from the photoconductive antenna are returned and focused onto the same photoconductive antenna via a mirror. Sinusoidal variation of the homodyne current corresponding to the mirror displacement was observed. The homodyne current is caused by THz waves with the frequency of 1028 ±5 GHz. The homodyne current amplitude of 0.5–1.5 nA with a signal-to-noise ratio of 10–25 is obtained with the incident laser power of 4 mW and bias voltage of 10–30 V applied to the photoconductive antenna. The homodyne current amplitude is proportional to the square of the incident laser power up to 2 mW, which can be explained by considering the dependences of the dc photocurrent and dc photoconductance on the irradiating laser power.

Ferroelectric liquid crystal spatial-light modulator for location-based communication with higher data transfer rate

Reflectivity modulating spatial optical communication has been researched using liquid crystal light modulator and corner-reflecting device for an adequate human-supporting information service environment. Conventional polymer dispersed liquid crystal light modulator had a subject of its response speed. In this paper, characteristics of a ferroelectric liquid crystal light modulator device are reported to improve its communicating data rate. A technique for higher data rate using a high-speed vision chip is also mentioned.

Investigation of ultrafast carrier dynamics in quantum wire by terahertz time-domain spectroscopy

We have investigated ultrafast carrier dynamics in a semiconductor crescent-shaped quantum wire by terahertz (THz) wave time-domain spectroscopy (THz-TDS). THz waves are generated by the motion of photoexcited carriers in semiconductor nanostructures, so that the time evolution of coherent polarization in the nanostructures is obtained by integration of observed THz waves. THz waves were generated by ultrashort optical pulse excitation of the quantum wire and detected using a free-space electrooptic (EO) sampling method. The dephasing time of coherent polarization in the quantum wire was varied by increasing the excitation energy. It was estimated that the dephasing time of coherent polarization is 300 fs at an excitation energy of 1.534 eV.