有川 敬

Abstract As a key component for next-generation wireless communications (6 G and beyond), terahertz (THz) electronic oscillators are being actively developed. Precise and dynamic phase control of ultrafast THz waveforms is essential for high-speed beam steering and high-capacity data transmission. However, measurement and control of such ultrafast dynamic process is beyond the scope of electronics due to the limited bandwidth of the electronic equipment. Here we surpass this limit by applying photonic technology. Using a femtosecond laser, we generate offset-free THz pulses to phase-lock the electronic oscillators based on resonant tunneling diode. This enables us to perform phase-resolved measurement of the emitted THz electric field waveform in time-domain with sub-cycle time resolution. Ultrafast dynamic response such as anti-phase locking behaviour is observed, which is distinct from in-phase stimulated emission observed in laser oscillators. We also show that the dynamics follows the universal synchronization theory for limit cycle oscillators. This provides a basic guideline for dynamic phase control of THz electronic oscillators, enabling many key performance indicators to be achieved in the new era of 6 G and beyond.

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.

We performed simulations for injection locking of a submillimeter-wave RTD oscillator circuit. A wider locking range is realized with optimizing the position where the injection signal is applied.

<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.

We demonstrate a new technique for characterizing thin film electro-optic (EO) materials. This method is based on resolving the electric field distributions in the near-field region of a split ring resonator (SRR) designed for the terahertz (THz) frequency range. Our simulations are supported experimentally by THz near-field imaging of SRRs directly patterned in contact with a thin-film lithium niobate (LN) crystal as a sensor. Furthermore, we analytically present the effect of the various polarization of applied electric field and calculate the sensitivity of the sensor through different probe beam polarizations.

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.

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.

We study macroscopically aligned single-wall carbon nanotube arrays with uniform lengths via polarization-dependent terahertz and infrared transmission spectroscopy. Polarization anisotropy is extreme at frequencies less than similar to 100 cm(-1) with no sign of attenuation when the polarization is perpendicular to the alignment direction. The attenuation for both parallel and perpendicular polarizations increases with increasing frequency, exhibiting a pronounced and broad peak around 450 cm(-1) in the parallel case. We model the electromagnetic response of the sample by taking into account both radiative scattering and absorption losses. We show that our sample acts as an effective antenna due to the high degree of alignment, exhibiting much larger radiative scattering than absorption in the mid/far-infrared range. Our calculated attenuation spectrum clearly shows a non-Drude peak at similar to 450 cm(-1) in agreement with the experiment. DOI: 10.1103/PhysRevB.87.161401

We study the coherent dynamics of cyclotron resonance (CR) in ultrahigh-mobility two-dimensional electron gases via time-domain terahertz magneto-spectroscopy. The CR decoherence time increases with decreasing temperature but saturates below 10 K. We observe an increase in both CR decoherence rate and CR mass when holes are created via optical pumping; the latter indicates a breakdown of Kohn's theorem due to electron-hole Coulomb interactions.

We report on a giant Faraday effect in an electron plasma in n-InSb probed via polarization-resolved terahertz (THz) time-domain spectroscopy. Polarization rotation angles and ellipticities reach as large as pi/2 and 1, respectively, over a wide frequency range (0.3-2.5 THz) at magnetic fields of a few Tesla. The experimental results together with theoretical simulations show its promising ability to construct broadband and tunable THz polarization optics, such as a circular polarizer, half-wave plate, and polarization modulators. (C) 2012 Optical Society of America

We demonstrate a terahertz polarizer built with stacks of aligned single-walled carbon nanotubes (SWCNTs) exhibiting ideal broadband terahertz properties: 99.9% degree of polarization and extinction ratios of 10(-3) (or 30 dB) from similar to 0.4 to 2.2 THz. Compared to structurally tuned and fragile wire-grid systems, the performance in these polarizers is driven by the inherent anistropic absorption of SWCNTs that enables a physically robust structure. Supported by a scalable dry contact-transfer approach, these SWCNT-based polarizers are ideal for emerging terahertz applications.

We demonstrate coherent control of cyclotron resonance in a two-dimensional electron gas (2DEG). We use a sequence of terahertz (THz) pulses to control the amplitude of coherent cyclotron resonance oscillations in an arbitrary fashion via phase-dependent coherent interactions. We observe a self-interaction effect, where the 2DEG interacts with the THz field emitted by itself within the decoherence time, resulting in a revival and collapse of quantum coherence. These observations are accurately describable using single-particle optical Bloch equations, showing no signatures of electron-electron interactions, which verifies the validity of Kohn's theorem for cyclotron resonance in the coherent regime.

We determined the hydration numbers of 2-butoxyethanol (2BE) molecules in the monomeric and the micellar form using terahertz time-domain attenuated total reflection spectroscopy. The hydration number of the 2BE monomer is somewhat larger than that of the 2BE molecule in micelles due to the presence of immobilized water molecules around the hydrophobic surface of the 2BE monomer. The hydration number of the 2BE monomer decreases with increasing the temperature, whereas the hydration number of the 2BE micelle is almost unchanged up to 47 degrees C. This indicates that strongly bound hydration water molecules should exist and make a self-organized structure on the micelle surface. (c) 2009 Elsevier B.V. All rights reserved.

We determined the hydration numbers of 2-butoxyethanol (2BE) molecules in monomeric and micellar form using terahertz time-domain attenuated total reflection (THz TD-ATR) spectroscopy. Temperature- and concentration-dependences of hydration number indicate that strongly bound hydrated water molecules make a self-organized structure on the micelle surface.

We propose a novel method to characterize hydration states from dielectric responses of solutions in terahertz (THz) frequency region. Hydration states can be evaluated quantitatively from a complex dielectric function derived from a theory with a local field correction. We demonstrated this method in disaccharide (sucrose and trehalose) water solutions using the THz time-domain attenuated total reflection (TD-ATR) spectroscopy. The method successfully evaluates the hydration state and its concentration dependence, providing a new tool to investigate the picosecond dynamics of hydration directly. (C) 2008 Elsevier B.V. All rights reserved.

We demonstrate time-domain attenuated total reflection spectroscopy in terahertz frequency region. Geometry of the reflection measurement is well optimized to obtain accurate optical constants of water or aqueous biomolecular system. We determine the dielectric constants of distilled water and sucrose solutions with this technique. This technique will open new aspects in the research field of biological systems in water.

We demonstrate time-domain attenuated total reflection spectroscopy in terahertz frequency region to obtain accurate optical constants of water and water solutions. This technique will open new aspects in the research of biological systems in water. © 2007 Optical Society of America.

We measure complex dielectric constants of amino-acid solution with THz Time-domain attenuated total-reflection technique. The analysis with considering local field reveals intermolecular interaction between amino-acid molecules and water molecules. © 2005 Optical Society of America.

We measure optical constants of biomolecular solution in THz region with time-domain attenuated total reflection technique, which is the most sensitive method to obtain optical constants of water solution. The analysis with local-field correction gives us the information on hydrated molecule dynamics in water.

We have developed a novel technique of time-domain attenuated total reflection spectroscopy in THz frequency region. Complex dielectric constants in InAs, water, methanol and acetone are successfully determined. The advantage of this technique is that it can be used to access samples with various shapes and optical properties without changing the THz optical path. © 2004 IEEE.