Ryoichi Fukasawa

Terahertz imaging of silicon wafers

Silicon samples with and without implanted layers have been imaged with a standard time-domain terahertz (THz) imaging system. The carrier concentration and mobility of the substrate have been extracted from the frequency dependence of the THz transmittance using a simple model based on the Drude approximation. The carrier concentration of implanted layers could be determined simply from the relative amplitude of the main THz pluse with a spatial resolution of ≈1 mm. Both substrates and thin layers of a semiconductor were characterized with the same THz system.

Development of a THz spectroscopic imaging system.

We have developed a real-time THz imaging system based on the two-dimensional (2D) electro-optic (EO) sampling technique. Employing the 2D EO-sampling technique, we can obtain THz images using a CCD camera at a video rate of up to 30 frames per second. A spatial resolution of 1.4 mm was achieved. This resolution was reasonably close to the theoretical limit determined by diffraction. We observed not only static objects but also moving ones. To acquire spectroscopic information, time-domain images were collected. By processing these images on a computer, we can obtain spectroscopic images. Spectroscopy for silicon wafers was demonstrated.

Far-infrared reflectance study of coupled longitudinal-optical phonon-hole plasmon modes and transport properties in heavily doped p-type GaAs

We measured room-temperature far-infrared reflectance spectra of ten p-type, Be-doped, molecular-beam-epitaxy-grown GaAs films with hole densities from 6.3×1017 to 2.9×1019 cm-3 and examined the frequency-dependent dielectric function of coupled phonon-heavily damped hole plasmon modes in these samples without the complications that arise in Raman scattering experiments. Both a two-oscillator dielectric function e(ω), and Kukharskiis factorized form eK(ω) for the dielectric function, reproduce the data well. The plasmon-like modes clearly appear in the reflectance spectra, although they are suppressed in the Raman spectra, and we find that their frequencies are best given by finding the solutions of e(ω)=0 in the complex ω-plane. The infrared data also accurately yield the hole drift mobilities when we consider values for the Hall scattering factor in p-type GaAs.

Component spatial pattern analysis of chemicals by use of two-dimensional electro-optic terahertz imaging

We demonstrate a method of chemical mapping by using the transillumination terahertz (THz) images obtained by two-dimensional electro-optic THz imaging. The images and spectral data were measured between 0.1 and 1.0 THz. An experimental sample consisting of three chemicals was prepared, with one in two concentrations. By introducing the component spatial pattern analysis based on the least-squares method, the chemical composition, spatial distribution, and difference in concentration were clearly determined.

Terahertz emission spectroscopy of coherent phonons in semiconductors

We investigate infrared-active coherent phonons excited in Te, PbTe (un-doped, n-type and p-type), and CdTe by observing the terahertz (THz) emission from these samples. Coherent THz emission corresponding to the longitudinal- optical (LO)-phonon frequency has been observed for all the samples, while no significant THz emission is observed at the transverse-optical (TO)-phonon frequency even at high carrier densities (> 1018 cm-3). The absence of THz emission at the TO-phonon frequency strongly contradicts the observation in the transient reflectivity measurements, where the signal oscillations at the TO-phonon frequencies arising from the LO-phonon-plasmon coupling (L_ mode) have been observed at high carrier densities. It is found from a model calculation that for a dipole oscillator on a sample surface the THz emission around the TO-phonon frequency is strongly suppressed due to the large dielectric constant near this frequency.