Published papers

We have developed a real-time terahertz time-domain polarization analyzer by using 80-MHz repetition-rate femtosecond laser pulses. Our technique is based on the spinning electro-optic sensor method, which we recently proposed and demonstrated by using a regenerative amplifier laser system; here we improve the detection scheme in order to be able to use it with a femtosecond laser oscillator with laser pulses of a much higher repetition rate. This improvement brings great advantages for realizing broadband, compact and stable real-time terahertz time-domain polarization measurement systems for scientific and industrial applications.

We investigated the structural anisotropy of poly(lactic acid) (PLA) film samples by polarizationsensitive(PS) terahertz time-domain spectroscopy (THz-TDS). The dielectric function of the PLA filmbecomes more anisotropic with the sample's increased draw ratio. This behavior is explained by theclassical Lorentz oscillator model that takes into account the orientation of PLA chains along thestretching direction. For further understanding of structural anisotropy in PLA, we performed twodimensional(2D) wide-angle X-ray scattering (WAXS) experiments and found that anisotropy isstrongly correlated to the orientation of the crystalline PLA chains. Our results suggest that PS THz-TDS is more suitable for examining the anisotropy of amorphous PLA chains.

今回、我々は可視光から赤外光領域に至るまで不透明な導電性添加物を含んだゴムに対してテラヘルツ偏光測定を行い、大きな光学異方性を観測した。誘電関数の異方性はMaxwell-Granett理論によってよく記述され、導電性添加物の誘電関数の異方性がゴム全体の異方性の起源であることを明らかにした。この結果は、テラヘルツ領域における二元混合物の光学応答の好例であり、また実用性も高い重要な成果である。

テラヘルツ周波数領域には高分子特有の振動モードが存在しており、テラヘルツ光を用いた高分子材料の物性測定に、注目が集まっている。我々は高分子ポリ乳酸薄膜を延伸することで、ポリ乳酸に複屈折が生じることを発見した。また、複屈折の延伸率依存性をテラヘルツ帯の回転検光子型偏光計で調べた。さらにテラヘルツ時間領域分光法を用いて、延伸ポリ乳酸薄膜の複素屈折率が持つ偏光依存性を調べた。

We determined complex refractive index and thickness of a silicon wafer with unprecedented precision using dual-comb spectroscopy. The standard deviations of the refractive index and the thickness are 8.9×10-6 and 1.0 nm, respectively.

We determined complex refractive index and thickness of a silicon wafer with unprecedented precision using dual-comb spectroscopy. The standard deviations of the refractive index and the thickness are 8.9×10-6 and 1.0 nm, respectively.

© OSA 2018 We present a novel method for imaging terahertz electric-field vector using a rotating polarizer technique. We investigate the polarization-dependent propagation of terahertz surface waves on an indium tin oxide metal film.

© OSA 2018 We succeed to develop the internal strain imaging technique for visibly-opaque black rubbers using terahertz polarization spectroscopy. Our novel method allows the nondestructive and contactless evaluation of the internal triaxial strain without any preprocesses.

© 2018 IEEE. We recently demonstrated the nondestructive and contactless inspection of visibly-opaque black rubbers with a conductive carbon fillers, which is required for the rubber industrial community, by terahertz polarization spectroscopy. Using our developed technique, in this work, the terahertz optical and mechanical properties of the black rubbers were simultaneously measured under cyclic deformation. The black rubbers show the hysteresis in the draw ratio dependence of the terahertz birefringence as well as the well-known hysteresis behavior in the stress. The result implies that the terahertz optical response of black rubbers have correlation with their mechanical properties.

© 2018 OSA - The Optical Society. All rights reserved. Electro-optic sampling (EOS) is a technique widely used to measure electric field waveform in ultra-broadband frequency range from terahertz (THz), mid- (MIR) [1], and near infrared (NIR) [2]. Since EOS signal depends on the relative orientations between probe, THz and crystal axis, the modulation of EOS signal upon crystal rotation provides polarization information of the infrared pulses [3, 4]. This so-called polarization-sensitive (PS-) EOS has an advantage that no additional polarization optics such as wire-grid polarizer is needed to measure polarization. In this talk we extend PS-EOS for the first time to the MIR region, wherein fruitful application possibilities such as vibrational circular dichroism [5] are expected.

© 2020 OSA. We demonstrate ultrafast coherent control of higher-order spin precession modes in the permalloy thin film by means of optical techniques. We can selectively excite one of the higher spin precession modes using double pulse excitation.

CLEO 2020 © OSA 2020. We demonstrate ultrafast coherent control of higher-order spin precession modes in the permalloy thin film by means of optical techniques. We can selectively excite one of the higher spin precession modes using double pulse excitation.

© 2019 IEEE Dual-comb spectroscopy (DCS) is a modern method using two frequency combs with slightly different repetition rates [1,2]. The DCS down-converts a material response in the optical frequency domain to a rf signal that is easy to handle. It provides significant advantages over conventional spectroscopy such as high spectral resolution, broad spectral range, frequency precision, short measurement time, and so on. The DCS has mainly been applied to precise molecular gas spectroscopy [3]. There are several attempts to expand its application to solid state physics. One of the applications is the determination of complex refractive index of materials [4]. As changes in both the amplitude and phase inside the material can be measured by DCS, one can determine both the real and imaginary part of the refractive index without utilizing the Kramers-Kronig relation. In addition, we consider physically interesting materials such as low-dimensional nanostructures and polymers with polarization-dependent complex refractive index (optical anisotropy). For investigating their physical property, it is important to add polarization sensitiveness to the DCS. So far, we have developed a polarization-sensitive (PS) DCS using a rotating compensator polarimetry [5].

© 2019 IEEE. Dual-comb spectroscopy (DCS) is a modern method using two frequency combs with slightly different repetition rates [1,2]. The DCS down-converts a material response in the optical frequency domain to a rf signal that is easy to handle. It provides significant advantages over conventional spectroscopy such as high spectral resolution, broad spectral range, frequency precision, short measurement time, and so on. The DCS has mainly been applied to precise molecular gas spectroscopy [3]. There are several attempts to expand its application to solid state physics. One of the applications is the determination of complex refractive index of materials [4]. As changes in both the amplitude and phase inside the material can be measured by DCS, one can determine both the real and imaginary part of the refractive index without utilizing the Kramers-Kronig relation. In addition, we consider physically interesting materials such as low-dimensional nanostructures and polymers with polarization-dependent complex refractive index (optical anisotropy). For investigating their physical property, it is important to add polarization sensitiveness to the DCS. So far, we have developed a polarization-sensitive (PS) DCS using a rotating compensator polarimetry [5].