Published papers

© 2018 As the industrial utilization of polymeric materials is increasing owing to recent improvements in material durability and functionality, product safety becomes an important issue. Therefore, various inspection methods for polymeric materials are being developed. In this work, the rapid and non-destructive inspection of the averaged internal orientation of conductive carbon black fillers in rubber composites is demonstrated using a handmade high-speed terahertz polarization spectroscopy system that employs reflection geometry. From the birefringent information obtained from the black rubber sample (with arbitrary stretching condition), the internal orientation of the conductive fillers, which plays an essential role in the properties of the rubber composite, was determined. In addition, since data acquisition with the reflection method is fast, the spatially resolved measurement of the filler orientation could also be performed. The information of the internal orientation of the carbon black fillers allows verification of the condition of the rubber product.

Dual-comb spectroscopy (DCS), which uses two optical frequency combs (OFCs), requires an accurate knowledge of the mode number of each comb line to determine spectral features. We demonstrate a fast evaluation method of the absolute mode numbers of both OFCs used in DCS system. By measuring the interval between the peaks in the time-domain interferogram, it is possible to accurately determine the ratio of one OFC repetition frequency (frep) to the difference between the frep values of the two OFCs (∆frep). The absolute mode numbers can then be straightforwardly calculated using this ratio. This method is applicable to a broad range of ∆frep values down to several Hz without any additional instruments. For instance, the minimum required measurement time is estimated to be about 1 s for ∆frep ≈ 5.6 Hz and frep ≈ 60 MHz. The optical frequencies of the absorption lines of acetylene gas obtained by DCS with our method of mode number determination shows good agreement with the data from the HITRAN database.

The inhomogeneous broadening in the luminescence spectra of ordered arrays of pyramidal GaAs/AlGaAs semiconductor quantum dots (QDs) was studied as a function of the dot size. Dot arrays with inhomogeneous broadening as small as 4.1 meV and a corresponding ground state to first excited state transition separation of 28 meV were obtained. By evaluating the QD energy levels using a multiband k center dot p model, the authors estimated that the observed inhomogeneous broadening corresponds to dot height fluctuations of about 1-2 ML across the array.(c) 2007 American Institute of Physics.

Photoexcited dynamics of a spin-density-wave (SDW) state in an organic linear-chain compound bis-(tetramethyl-tetraselenafulvalene)hexafluorophosphate, (TMTSF)(2)PF6, is investigated by optical pump and terahertz probe experiments. After the ultrashort laser-pulse excitation, a sudden closing followed by a recovery of the SDW gap is clearly observed in the far-infrared spectrum. The recovery time is found to diverge toward the phase-transition temperature, T-SDW, while the gap remains open at T-SDW. The observed critical slowing down behavior is interpreted in terms of three-dimensional fluctuation which enlarges near the phase-transition temperature.

We report on the growth and optical properties of dense arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures with pitches as small as 300 nm. The samples were grown by organometallic chemical vapor deposition in dense inverted pyramids on {1 1 1}B GaAs substrate pre-patterned using electron beam lithography and wet chemical etching. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite differently from those employed with micron-size pyramids. Low-temperature micro-photoluminescence and cathodoluminescence spectra of the samples show distinct luminescence from the QDs with a linewidth of less than 1 meV and uniform emission energy for an ensemble of 900 QDs. The possibility of incorporating such QD arrays inside optical microcavity structures is also discussed. (C) 2003 Elsevier B.V. All rights reserved.

We report on the growth and optical properties of homogeneous, dense arrays of site-controlled, single GaAs/AlGaAs quantum dot (QD) heterostructures with periodicities as small as 300 nm. The samples were grown by organometallic chemical vapor deposition on (111)B GaAs substrates containing dense inverted pyramid recess patterns prepared by electron beam lithography and wet chemical etching. Low-temperature microphotoluminescence spectra of the samples show distinct luminescence from the QDs with 1-3 meV linewidth. Low-temperature cathodoluminescence spectrally resolved images reveal uniform emission energy within an ensemble of 900 QDs. (C) 2004 American Institute of Physics.

We studied the uniformity of site-controlled, pyramidal InGaAs/AlGaAs semiconductor quantum dots (QDs) grown by organometallic chemical vapor deposition on prepatterned substrates. The inhomogeneous broadening of the QD ground state emission has been determined to be 7.6 meV by statistical single QD photoluminescence spectroscopy on a set of 120 individual QD structures. Taking into account the ground-to-excited state separation of 55 meV, such a small value has not yet been observed in QD systems where other growth mechanisms are employed. Moreover, a high reproducibility of the sharp QD emission features in the single exciton regime has been observed. (C) 2004 American Institute of Physics.

Spatially and spectrally resolved microscopic images of spontaneous and stimulated emissions are imaged at the mirror facets of a GaAs T-shaped quantum wire laser with high uniformity. Laser emission from the one-dimensional ground state reveals a circular image located at the core of a T-shaped optical waveguide but significantly smaller in area than the low power spontaneous emission from the same waveguide. These images unambiguously allow assignment of all spontaneous and laser emissions to the wire ground state and respective intersecting wells in the structure. (C) 2003 American Institute of Physics.

We report here on the fabrication by organometallic chemical vapour deposition of InxGa1-xAs quantum dots (QDs) with x up to 35% grown in pyramidal recess patterns with Al0.30Ga0.70As barriers. We show QD luminescence with narrow PL lines, tuning of the emission energies varying by more than 180 meV and ground state to first excited state separations as large as 80 meV.

We present a method of developing a high-speed terahertz time-domain polarimeter based on an electro-optic sampling setup, which consists of a rapid polarization analysis of the probe optical beam within 1 ms with an angular resolution of similar to 100 mrad using an electro-optic modulator (EOM). A fast optical delay line enabled us to obtain the vector waveform of the terahertz pulse within 100 ms. The setup does not utilize a manual/mechanical rotational stage, but the use of the polarization modulation technique employing the EOM ensures a high-speed and high-accuracy measurement of the state of polarization of terahertz waves. (C) 2014 The Japan Society of Applied Physics

This paper provides a detailed theoretical analysis of the unexpected transient divergent and rotational distributions of the focused electric field vector reported in Shibata et al (2015 Phys. Rev. A 92 053806). We reveal the physical origin of these distributions. More quantitatively, we derive the semi-analytic expressions and clarify how these distributions depend on the mirror size, offset angle, and the intensity distribution of the incident parallel light. We compare the formulas with numerical calculations and evaluate the area where linearity holds. If the wavelength and the mirror size are sufficiently shorter than the focal length, the radius of the linear area becomes longer than the wavelength. These formulas and evaluations are useful for studies, which require high spatio-temporal resolution.

Copyright © 2020 American Chemical Society. Stereocomplex poly(lactic acid) (sc-PLA) crystallites exhibit higher mechanical strength and thermal stability than PLA homocrystallites. In sc-PLA, a hydrogen bond that governs the physical properties is formed between two enantiomeric PLA chains during crystallization. Therefore, the investigation of this hydrogen bond is essential for a better understanding of the properties of sc-PLA. Here, we report on the terahertz spectroscopy of sc-PLA films with low crystallinities. The dielectric function in the terahertz regime allows us to estimate the crystallinity of sc-PLA because various intra- and intermolecular vibration modes lie in the terahertz frequency range. By comparison with X-ray diffraction measurements, the validity of the crystallinity evaluation based on terahertz spectroscopy is verified. Furthermore, a remarkable red-shift of the peak in the imaginary part of the dielectric function occurs as the crystallinity increases. By comparison with Fourier transform infrared spectroscopy, it is considered that this red-shift is caused by the anharmonic nature of the vibrational potential corresponding to the intermolecular libration mode of the sc-PLA crystallites. The hydrogen bond formation associated with the sc-PLA crystallization increases the anharmonicity of the vibrational potential of sc-PLA, and then the red-shift of the peak occurs. Our findings suggest that structural changes due to the stereocomplexation can be sensitively probed by terahertz spectroscopy.

We develop a method to precisely determine the polarization state of each comb tooth by a combination of dual-comb spectroscopy and rotating compensator polarimetry. This method is useful for measuring birefringent and dichroic materials.

A compact and rigid terahertz time domain spectroscopy system is presented. The size of the device is 20 mm diameter times 61 mm length with four parabolic-shaped concave mirrors dug in it to effectively focus a terahertz beam on a sample. The device has no chromatic aberration over the whole bandwidth of the beam (0.3-2 THz), and an effective numerical aperture of about 0.45 is achieved, which has a capability to image the structure whose size is almost the same as the wavelength. Frequency resolved images clearly show this performance. We also show that quantitative retrieval of the complex refractive index of the structure as small as twice of the wavelength is possible. (C) 2007 American Institute of Physics.

We investigated the draw ratio (DR) dependence of the anisotropic dielectric function and conductivity of styrene butadiene rubbers (SBRs) with different carbon black (CB) concentrations by polarization-sensitive terahertz time-domain spectroscopy. From the frequency dependence of the conductivity in the unstretched SBRs ranging from direct current to terahertz frequencies, it is found that the SBR with a CB concentration above 30 wt. % exhibits percolation conductivity. We investigated the spectral shape of the dielectric function and conductivity of the SBR samples below and above the percolation threshold for two representative DRs in the terahertz frequency region. We found that the DR dependence of the spectral shape is well explained by the effective medium approximation, except for the sample with the CB concentration above 30 wt. % under the unstretched condition. The conductivity in that sample remarkably changes in the low terahertz frequency region, which suggests a change in the CB network by deformation. The investigation of the dielectric anisotropy and percolation conductivity using our polarization technique can be applied to a wide range of elastomer composites. Published by AIP Publishing.

We report a correlation between the dielectric property and structure of stretched poly(lactic acid) (PLA) films, revealed by polarization-sensitive terahertz time domain spectroscopy and two-dimensional (2D) wide-angle X-ray scattering (WAXS). The experiments evidence that the dielectric function of the PLA film becomes more anisotropic with increasing draw ratio (DR). This behavior is explained by a classical Lorentz oscillator model assuming polarization-dependent absorption. The birefringence can be systematically altered from 0 to 0.13 by controlling DR The combination of terahertz spectroscopy and 2D WAXS measurement reveals a clear correlation between the birefringence in the terahertz frequency domain and the degree of orientation of the PLA molecular chains. These findings imply that the birefringence is a result of the orientation of the PLA chains with anisotropic macromolecular vibration modes. Because of a good controllability of the birefringenco, polymer-based materials will provide an attractive materials system for phase retarders in the terahertz frequency range.

Elastomers are one of the most important materials in modern society because of the inherent viscoelastic properties due to their cross-linked polymer chains. Their vibration-absorbing and adhesive properties are especially useful and thus utilized in various applications, for example, tires in automobiles and bicycles, seismic dampers in buildings, and seals in a space shuttle. Thus, the nondestructive inspection of their internal states such as the internal deformation is essential in safety. Generally, industrial elastomers include various kinds of additives, such as carbon blacks for reinforcing them. The additives make most of them opaque in a wide spectral range from visible to mid-infrared, resulting in that the nondestructive inspection of the internal deformation is quite difficult. Here, we demonstrate transmission terahertz polarization spectroscopy as a powerful technique for investigating the internal optical anisotropy in optically opaque elastomers with conductive additives, which are transparent only in the terahertz frequency region. The internal deformation can be probed through the polarization changes inside the material due to the anisotropic dielectric response of the conductive additives. Our study about the polarization-dependent terahertz response of elastomers with conductive additives provides novel knowledge for in situ, nondestructive evaluation of their internal deformation.

We report on a drawing-ratio dependence of the refractive index difference between the two optic axes (Delta n) of poly lactic acid films in terahertz frequency range. We found that Delta n can be controlled in a range of 0-0.04 by changing the drawing ratio. The result shows that we can design a terahertz wave plate with arbitral retardation, which will be useful for establishing polymer-based terahertz optical component with low cost and easy implementation in mass production.

We investigated the polarization-dependent terahertz optical conductivity of rubber compounds with carbon black (CB) fillers under cyclic deformation to detect deformation-induced changes of the internal CB structure in a contactless manner. It is clearly observed that the conductivity during the unloading is reduced from that during the loading at the same draw ratios. To interpret this hysteresis of the conductivity, we perform theoretical calculation based on the effective medium theory. Comparing with the calculation, we attribute the hysteresis of the conductivity to the reduction of the anisotropy of the CB aggregates, which might result from the deformation-induced rupture of them.