Shin’ichiro Hayashi

High-power, single-longitudinal-mode terahertz-wave generation pumped by a microchip Nd:YAG laser [Invited]

We report on the development of a high-peak-power, single-longitudinal-mode and tunable injection-seeded terahertz-wave parametric generator using MgO:LiNbO3, which operates at room temperature. The high peak power (> 120 W) is enough to allow easy detection by commercial and calibrated pyroelectric detectors, and the spectral resolution (< 10 GHz) is the Fourier transform limit of the sub-nanosecond terahertz-wave pulse. The tunability (1.2-2.8 THz) and the small footprint size (A3 paper, 29.7 × 42 cm) are suitable for a variety of applications.

Interference terahertz label-free imaging for protein detection on a membrane

We demonstrate a highly sensitive imaging method combined a terahertz time-domain spectroscopy and an interference effect for label-free protein detection on a polyvinylidene difluoride membrane. The method is based on terahertz time-domain spectroscopy and uses an interference effect. Biotin is linked to the membrane using poly ethylene glycol or poly ethylene glycol methyl ether to prevent it from being washed off. Binding of the biotin with streptavidin is then observed by measuring the terahertz signal change due to the variation of the membrane refractive index. We demonstrate the detection of the binding streptavidin protein in gradually decreasing concentrations, down to 27 ng mm-2, using the image recorded at 1.5 THz.

Terahertz-wave absorption in liquids measured using the evanescent field of a silicon waveguide

A technique is presented for the measurement of the absorption properties of liquids in the terahertz wave range, based on the interaction between the liquid and the evanescent wave covering the surface of a cylindrical waveguide made of high-resistivity silicon. The terahertz wave propagates inside the waveguide as the fundamental EH11 mode. The presence of a medium around the silicon rod has a measurable effect on the overall end-to-end transmission of the waveguide. As demonstration, we report the measurements of a D-glucose aqueous solution with a concentration ranging from 0% to 45%, using a backward-wave oscillator as the terahertz wave source.

Terahertz wave parametric amplifier

The importance of terahertz (THz) wave techniques has been demonstrated in various fields, and the range of applications is now expanding rapidly. However, the practical implementation of THz science to solve the real-world problems is restricted due to the lack not only of convenient high power THz wave emitters and sensitive detectors but also of efficient quasi-optical active devices such as amplifiers. In this work, we demonstrate the direct amplification of THz waves in room temperature using magnesium oxide-doped lithium niobate (MgO:LiNbO3) crystals as the nonlinear gain medium. The input THz wave is injected as a seed beam along with the pump beam into the nonlinear crystal and it is amplified by the optical parametric process. We report gain in excess of 30 dB with an input THz pulse energy of less than 1 pJ. We believe that this demonstration will contribute to the convenience and further applicability of THz frequency techniques.

Expansion of the tuning range of injection-seeded terahertz-wave parametric generator up to 5 THz

In this paper, we report the improvement of the frequency tuning range of an injection-seeded terahertz (THz)-wave parametric generator (is-TPG). A significant previous limitation was the high absorption coefficient in the higher-frequency region of a MgO:LiNbO3 crystal. Here, we inclined the crystal slightly, so that a fraction of the pump beam was internally reflected at the THz-wave exit surface of the crystal. In this configuration, it was easier for a higher-frequency THz wave to reach the crystal surface, because the center core region of the pump beam was closer to the exit surface. As a result, the upper limit of the frequency tuning range increased from 3.0 to 5.0 THz.

High-power, Single-longitudinal-mode Terahertz-wave Generation Pumped by a Microchip Nd:YAG Laser

We have developed injection-seeded terahertz-wave parametric generator pumped by a microchip Nd:YAG laser. This generated high peak power, tunable, narrow-linewidth terahertz wave with injection seeding by an external cavity diode laser. We observed terahertz wave, peak power of more than 30 W, tunable range from 0.9 to 3.1 THz, linewidth of less than 10 GHz.

Feasibility on the quality evaluation of agricultural products with terahertz electromagnetic wave

This study is aims at an application of terahertz (THz) waves to nondestructive evaluation of agricultural products. An experiment system with THz electromagnetic waves working in reflection geometry was built and applied to the quality evaluation of tomato fruits. This system was capable of nondestructive detection of defects derived from moisture difference on the thin surface layers of the tomato, which was difficult with visible or NIR light. From the experimental results, it was found that the assessment of the internal damage is feasible. Especially, it was clear that the difference between the damaged parts and normal parts could be easily distinguished by using THz waves in reflection. The complex refractive index of sugar solutions was also measured using the THz Time Domain Spectroscopy (THz-TDS) for obtaining its correlation with the measurable sugar content (Brix %). From the result, it was observed that higher sugar concentration relates to a lower absorption of the THz waves and that the real part of the complex refractive index did not change significantly, contrary to what is observed in the visible region.

Investigation of the non-thermal effects of exposing cells to 70–300 GHz irradiation using a widely tunable source

Abstract This study investigated the effects of millimeter wave (MMW) irradiation with a wide range of frequencies on the proliferation and activity of normal human skin fibroblast (NB1RBG) and human glioblastoma (A172) cells. Very few studies have focused on low-power, long-term irradiation of cells with a widely tunable source. Our research examined non-thermal effects on cells exposed to radiation at low power with tunable frequencies from 70 GHz to 300 GHz. A widely tunable MMW source was set within a cell culture incubator. To avoid the effect of heat generation due to irradiation, the intensity was maintained below 10 μW and the device was arranged such that the irradiation came from underneath the cells. Irradiation was performed by sweeping from 70 GHz to 300 GHz in 1.0 GHz steps. The MMW source was positioned 100 mm away from the container in which the cells were cultured. Cells were exposed to MMWs for either 3, 70 or 94 h. Measurements of cell proliferation were made using the alternating current measurement method. We found no difference in proliferation between cells exposed to MMWs and unexposed cells. A colorimetric method using novel tetrazolium compound: MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt] was used for cell activity and cytotoxicity assays. We found no difference in cellular activity or toxicity between MMW-exposed cells and sham cells. Our study thus found no non-thermal effect as a result of exposure of cells to 70 GHz to 300 GHz of radiation.

Multiwavelength terahertz-wave parametric generator for one-pulse spectroscopy

In this study, the simultaneous generation of multiwavelength terahertz (THz) waves by an injection-seeded THz parametric generator (is-TPG) was achieved for the first time. The output and stability of the multiwavelength THz waves were equivalent to those of the THz waves generated via a single-wavelength is-TPG. Spatial separation of frequencies and high-sensitivity detection were achieved by converting the THz waves to near-infrared detection beams. Furthermore, one-pulse spectroscopy of saccharides was realized, and a dynamic range of more than 60 dB was obtained. The results demonstrated the possibility of using the is-TPG to significantly shorten the measurement times of spectroscopic systems.

Topological Invariants and Corner States for Hamiltonians on a Three-Dimensional Lattice

We consider periodic Hamiltonians on a three-dimensional (3-D) lattice with a spectral gap not only on the bulk but also on two edges at the common Fermi level. By using K-theory applied for the quarter-plane Toeplitz extension, two topological invariants are defined. One is defined for the gapped bulk and edge Hamiltonians, and the non-triviality of the other means that the corner Hamiltonian is gapless. We prove a correspondence between these two invariants. Such gapped Hamiltonians can be constructed from Hamiltonians of 2-D type A and 1-D type AIII topological insulators, and its corner topological invariant is the product of topological invariants of these two phases.