有川 敬

Abstract The capability for actual measurements—not just simulations—of the dynamical behavior of THz electromagnetic waves, including interactions with prevalent 3D objects, has become increasingly important not only for developments of various THz devices, but also for reliable evaluation of electromagnetic compatibility. We have obtained real-time visualizations of the spatial evolution of THz electromagnetic waves interacting with a single metal micro-helix. After the micro-helix is stimulated by a broadband pico-second pulse of THz electromagnetic waves, two types of anisotropic re-emissions can occur following overall inductive current oscillations in the micro-helix. They propagate in orthogonally crossed directions with different THz frequency spectra. This unique radiative feature can be very useful for the development of a smart antenna with broadband multiplexing/demultiplexing ability and directional adaptivity. In this way, we have demonstrated that our advanced measurement techniques can lead to the development of novel functional THz devices.

<title>Abstract</title>Fundamental understanding of the confinement of water in porous coordination polymers (PCPs) is important not only with respect to their application, such as in gas storage and separation, but also for exploring confinement effects in nanoscale spaces. Here, we report the observation of water in an exotic state in the well-designed hydrophilic nanopores of PCPs. Single-crystal X-ray diffraction finds that nanoconfined water has an ordered structure that is characteristic in ices, but infrared spectroscopy reveals a significant number of broken hydrogen bonds that is characteristic in liquids. We find that their structural properties are quite similar to those of solid-liquid supercritical water predicted in hydrophobic nanospace at extremely high pressure. Our results will open up not only new potential applications of water in an exotic state in PCPs to control chemical reactions, but also experimental systems to clarify the existence of solid-liquid critical points.

A resonant tunneling diode oscillator has been successfully' injection-locked to a continuous terahertz wave. The locking range is about 50 MHz when the RTD emission power and the injection power are set to 10 NV and 4 NV, respectively. The dependence of the locking range to the injection power is consistent with the Adler's theory.

We performed simulations for oscillation of series connected RTDs to realize higher output power. We found factors which determine whether oscillation or domain formation occurs. The variation of the fabricated fall areas should be within a few percent to realize the oscillation.

We perfoinled time-resolved terahertz near-field imaging of a gold disk with sub wavelength periodic grooves and successfully observed spoof localized surface plasmons. A selective excitation method is also demonstrated with orbital angular momentum of light.

Whether urea can serve as a kosmotrope or chaotrope has long been a topic of debate. In this study, broad-band THz spectroscopy (0.2-12 THz) of aqueous solutions of urea was used to characterize the hydration state and the hydrogen bond structure of water around urea. Three low-frequency vibration modes of urea were found around 2, 4, and above 12 THz. After eliminating the contribution of these modes, the "urea-vibration-free" complex dielectric constant was decomposed into the relaxation modes of bulk water and the oscillation modes of water. When hydration water is defined to be reorientationally retarded relative to bulk, our analysis revealed that the hydration number is 1.9 independent of urea concentrations up to 5 M, and this number is in close agreement with that of water constrained by strong acceptor hydrogen bonds of urea oxygen. Regarding the hydrogen bond structure, it was found that the tetrahedral-like water structure is mostly preserved (though the hydrogen bond lifetime is significantly shortened) but the population of non-hydrogen-bonded water molecules fragmented from the network is markedly increased, presumably due to urea's NH2 inversion. These experimental results point to the coexistence of apparently two contradictory aspects of urea: dynamical retardation (the kosmotropic aspect) by the -CO group and slight structural disturbance (the chaotropic aspect) by the -NH2 group.

We experimentally demonstrated focusing of light with orbital angular momentum (OAM) using an 8-element circular array of linear antennas A spiral phase plate was used to generate a vortex beam with an OAM of (h) over bar in the terahertz (THz) frequency region. We used THz near-field microscope to directly measure the phase vortex. A beam profile with a center dark spot and 2 pi phase rotation was observed in the small center gap region of the circular array antenna after the vortex beam excitation. The beam size is reduced by a factor of 3.4 +/- 0.2. Half-wave resonance of the antenna element is responsible for the focusing function, indicating the scalability of this method to other frequency regions. This method will enable deep subwavelength focusing of light with OAM and eliminate the obstacle for the observation of the dipole forbidden transition with finite OAM of the vortex beam. (C) 2017 Optical Society of America

An insulating bulk state is a prerequisite for the protection of topological edge states(1). In quantum Hall systems, the thermal excitation of delocalized electrons is the main route to breaking bulk insulation(2). In equilibrium, the only way to achieve a clear bulk gap is to use a high-quality crystal under high magnetic field at low temperature. However, bulk conduction could also be suppressed in a system driven out of equilibrium such that localized states in the Landau levels are selectively occupied. Here we report a transient suppression of bulk conduction induced by terahertz wave excitation between the Landau levels in a GaAs quantum Hall system. Strikingly, the Hall resistivity almost reaches the quantized value at a temperature where the exact quantization is normally disrupted by thermal fluctuations. The electron localization is realized by the long-range potential fluctuations, which are a unique and inherent feature of quantum Hall systems. Our results demonstrate a new means of effecting dynamical control of topology by manipulating bulk conduction using light.

We studied adsorption state of water molecules in a porous coordination polymer (PCP) using infrared spectroscopy. We observed two absorbance peaks below 4% relative humidity (RH) that originate from OH stretch modes of adsorbed water molecules. Their peak frequencies and areas indicate that one of the OH is free, and the other is hydrogen bonded. We also found that Free-OH plays a role of water attraction above 4% RH, and discuss the water adsorption kinetics in detail.

We performed time-resolved terahertz (THz) near-field imaging of perfect electric conductor (PEC) disks with sub-wavelength periodic grooves. We successfully observed spoof localized surface plasmons (LSPs) whose resonance frequencies are solely determined by the groove depth. Controllable selective excitation method is demonstrated with orbital angular momentum of light. Our results pave the way to plasmonic applications in the THz frequency region.

光渦による軌道角運動量転写は、新たな物性制御の可能性を提示する。ナノ空間への電場の閉じ込めが実現可能な局在表面プラズモンはその一例であり、長波長近似下では励起不可能な多重極モードを、光渦により選択励起可能であることが理論的に示されている。但し、可視域における検証は実験的に困難であるため、我々は疑似局在表面プラズモンを利用し、近接場の時間・空間分解測定が可能なテラヘルツ領域で実験を行い、光渦励起により多重極モードが選択励起可能であることを実証した。

We demonstrate dramatic breakdown behavior in the current through GaAs/AlGaAs nanoconstrictions (NCs), and find that this exhibits multiple signatures characteristic of the Gunn effect. These include current fluctuations and hysteresis, and electroluminescence that are consistent with the formation of Gunn domains. An analytical model is developed to describe the current-voltage characteristics of the NCs prior to the onset of the breakdown, and reveals the conduction through them to be barrier limited under low bias. A comparison of the results of these calculations with experiment furthermore suggests that the Gunn effect in these devices is triggered, once the phenomenon of drain-induced barrier lowering becomes sufficiently developed to support the injection of large numbers of electrons into the NC. Our paper, therefore, demonstrates how the Gunn effect may be manipulated through nanoscale tailoring of semiconductors, a result that may have implications for the development of solid-state terahertz technology.

Plasmonics is part of the fascinating field of nanophotonics, which explores how electromagnetic fields can be confined over dimensions smaller than the wavelength. Given that THz waves may be generated and detected using ultra-short laser pulses, i.e. femtosecond (fs) laser, coherent time-domain THz measurement enable direct reading of the amplitude and phase information of the electric field. Combined with this capability, exploring the electromagnetic interactions of THz waves below diffraction limit is a powerful tool to understand the plasmonic nature of light at the metal/insulator interface. Here, we summarize on the recent advances in THz microscopy based on 2-dimensionnal electro optic (EO) imaging using an intense THz pulse source. These recent achievements in optimizing the spatial resolution and acquisition time will drastically accelerate our comprehension of the subwavelength light-matter interactions at THz frequencies and enable, among others, new sensing applications. Our discussion will include demonstrations on field enhancement in metallic-based resonators and potential avenues using THz vortex beam.

We report on the observation of collective radiative decay, or superradiance, of cyclotron resonance (CR) in high-mobility two-dimensional electron gases in GaAs quantum wells using time-domain terahertz magnetospectroscopy. The decay rate of coherent CR oscillations increases linearly with the electron density in a wide range, which is a hallmark of superradiant damping. Our fully quantum mechanical theory provides a universal formula for the decay rate, which reproduces our experimental data without any adjustable parameter. These results firmly establish the many-body nature of CR decoherence in this system, despite the fact that the CR frequency is immune to electron-electron interactions due to Kohn's theorem.

We study the coherent dynamics of cyclotron resonance in ultrahigh-mobility two-dimensional electron gases via time-domain terahertz magneto-spectroscopy. We show that superradiant damping is the dominant decoherence mechanism at low temperatures.

Anisotropy is ubiquitous in solids and enhanced in low-dimensional materials. In response to an electromagnetic wave, anisotropic absorptive and refractive properties result in dichroic and birefringent optical phenomena both in the linear and nonlinear optics regimes. Such material properties have led to a diverse array of useful polarization components in the visible and near-infrared, but mature technology is non-existent in the terahertz (THz). Here, we review several novel types of anisotropic material responses observed in the THz frequency range, including both linear and circular anisotropy, which have long-term implications for the development of THz polarization optics. We start with the extreme linear anisotropy of macroscopically aligned carbon nanotubes, arising from their intrinsically anisotropic dynamic conductivity. Magnetically induced anisotropy will then be reviewed, including the giant Faraday effects observed in semiconductors, semimetals, and two-dimensional electron systems.