Seizi Nishizawa

Dependence of emission of terahertz radiation on geometrical parameters of dipole photoconductive antennas

The dependence of the emission spectrum of terahertz (THz) radiation on the geometrical parameters of the dipole antenna, and the relationship between these parameters and the temporal characteristics of the transient current that generate THz radiation, is reported. The emission intensity and the peak frequency strongly depend on these geometrical parameters. This dependence varies significantly with changes in the temporal characteristics of the transient current. We have also found that the emission efficiency can be increased by increasing the aspect ratio for the dipole.

Terahertz time domain spectroscopy of complex dielectric constants of boson peaks

Abstract Using THz time domain spectroscopy we have observed both the real and the imaginary parts of complex dielectric constants in the far-infrared region for a glassy polymethyl methacrylate (PMMA). The intensity and phase of transmittance of PMMA were accurately measured in the frequency range between 5 and 110 cm−1. The strong absorption appeared around a boson peak (BP) frequency, which was also observed by Raman scattering. The broad peak of an imaginary part related to BP shows a similar line-shape to the reduced depolarized Raman spectrum.

New application of terahertz time-domain spectrometry (THz-TDS) to the phonon–polariton observation on ferroelectric crystals

A new instrument for terahertz time-domain spectroscopy (THz-TDS) has been developed. It consists of a composite THz-TDS system and a high throughput (Martin-Puplett) interferometer. The instrument is for use in the qualitative study of optoelectronic constants of materials. The spectral transmission intensity and phase shift related to phonon-polariton dispersion have been measured between 100 cm(-1) and 3 cm(-1) on ferroelectric crystals of industrial interest. These include bismuth titanate Bi4Ti3O12 (a key material for FeRAM), lithium niobate LiNbO3 (a typical nonlinear crystal for parametric oscillator applications) and lithium heptagermanate Li2Ge7O15 for surface elastic wave filter applications. The complex dielectric constants are well reproduced by the phonon-polariton dispersion relation based on the Kurosawa formula. The instrument details and phonon-polariton dispersion results are described.

A polarization-sensitive 4-contact detector for terahertz time-domain spectroscopy

A light polarization angle-sensitive photoconductive detector for terahertz time-domain spectroscopy is computer-modeled, microfabricated, and tested. The experimental results show good agreement with the linear angular response for an ideal detector. The detectors frequency, angular, and crosstalk responses are discussed in the context of theoretical and experimental considerations.

Fast-Scan Terahertz Time Domain Spectrometer Based on Laser Repetition Frequency Modulation

We propose a high-speed scanning terahertz (THz) time-domain spectrometer (TDS) based on optical sampling by repetition frequency modulation (OSREFM) using a repetition-frequency-tunable femtosecond fiber laser. Repetition frequency modulation is controlled by a highly stabilized external sweep generator. The fast scan of the time-delay between the pump and probe pulses is similar to an optical sampling with cavity tuning (OSCAT) THz-TDS system. However, the advantage of this technique is that it does not require physically moving components and uses only one laser system having an electronically modulated repetition rate. The time window and scanning time of this system depend on the modulation frequency range and the sweeping speed of the laser. In this study, we demonstrate the acquisition of a ~300 ps-window THz time waveform within 3 ms at a scan rate of 333 Hz.

Terahertz time-domain spectroscopy (THz-TDS) approach to the quality control on pharmaceutical products

Terahertz time-domain spectroscopy technique has recently been in progress with advantages over other conventional techniques and with a wide range of industrial applications. One of the latest industrial applications focusing on the quality control on pharmaceutical products will be described.

Emission of terahertz radiation from dipole photoconductive antennas with several geometrical parameters

The dependence of the emission spectrum of terahertz (THz) radiation on the geometrical parameters of the dipole antenna, and the relationship between these parameters and the temporal characteristics of the transient input field is reported. The emission intensity and the peak frequency strongly depend on these geometrical parameters.

Development of a new commercial THz-TDS instrument

A new commercial THz-TDS instrument has been developed with wide spectral wavenumber coverage. We have measured the absorption spectra of monosaccharides and disaccharides by the instrument. In the THz frequency region, almost all the absorption lines of disaccharide appear at different frequencies compared with those of the monosaccharides which form the disaccharide.

Far-infrared phonon-polariton dispersion studied by THz time-domain spectroscopy

We report the study of far-infrared dielectric properties of a polar crystal using the THz time domain spectroscopy. We measured the power and phase transmission spectra by coherent THz radiation. The anisotropy of dielectric properties has been investigated simply by rotating the polarization plane of an incident beam. The frequency dependence of complex dielectric constant has been determined without the Kramers-Kronig analysis. The complex dispersion relation of phonon polariton frequency and damping has been also determined. These far-infrared properties have been consistently reproduced by the damped harmonic oscillator model related to the lowest frequency polar mode.

Terahertz Time Domain Spectroscopy of Phonon-Polariton in Ferroelectric Bismuth Titanate

This paper reports the first application of the terahertz time domain spectroscopy to phonon polariton dispersion. In order to determine polariton-dispersion of a high-quality ferroelectric bismuth titanate crystal plate, the phase delay was determined minutely as a function of THz radiation frequency in the region between 3 cm m 1 and 100 cm m 1 . The polarization dependence of polariton dispersion was clearly observed on the c-plate only by switching the polarization of an incident beam from E//a to E//b. The observed polariton dispersion relations are consistently explained by the calculation using Kurosawas formula.