Iwao Kawayama

Carbon nanotube terahertz polarizer.

We describe a film of highly aligned single-walled carbon nanotubes that acts as an excellent terahertz linear polarizer. There is virtually no attenuation (strong absorption) when the terahertz polarization is perpendicular (parallel) to the nanotube axis. From the data, the reduced linear dichrosim was calculated to be 3, corresponding to a nematic order parameter of 1, which demonstrates nearly perfect alignment as well as intrinsically anisotropic terahertz response of single-walled carbon nanotubes in the film.

Broadband Terahertz Polarizers with Ideal Performance Based on Aligned Carbon Nanotube Stacks

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 ~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.

Terahertz nonlinear superconducting metamaterials

We investigate the nonlinear effect of a planar superconducting metamaterial made from niobium nitride (NbN) at terahertz frequencies. As the variation of the incident intense terahertz field alters the intrinsic conductivity in the NbN, a consequent giant amplitude modulation is observed due to the strong nonlinearities. The high sensitivity of the chosen metamaterial even allows observing the nonlinear behaviors at various temperatures, but the resonance modulation induced by the nonlinear effect was distinct from that induced by the heating effect. The presented results illustrate a clever implementation of strongly enhanced nonlinearities and thus may bring nonlinear metamaterials into novel applications.

Planar-type spin valves based on low-molecular-weight organic materials with La0.67Sr0.33MnO3 electrodes

The spin injection and transport properties of low-molecular-weight organic semiconductors such as pentacene and bis(l,2,5-thiadiazolo)-p-quinobis(l,3-dithiole) (BTQBT) were investigated utilizing planar-type spin-valve devices with half-metallic La0.67Sr0.33MnO3 electrodes. The devices showed clear spin-valve characteristics with a magnetoresistance (MR) ratio of up to 29% at 5K. The MR ratio was found to depend on the gap spacing of the electrodes, the applied bias voltage, temperature, and the crystallinity of the films. It was also affected by gas adsorption onto the films, indicating that the spins were scattered by carriers and/or radical ions in the films generated through charge transfer from gas molecules.

Observation of TO1 soft mode in SrTiO3 films by terahertz time domain spectroscopy

We have measured the dielectric properties of pulsed-laser-deposited SrTiO3 (STO) thin films by terahertz (THz) time-domain spectroscopy in the frequency range from 80GHzto1.5THz at temperatures from 20to290K. The measured frequency dispersion of the dielectric constant of the STO thin films show a clear Lorentzian oscillator behavior below 180K and softening of the ferroelectric soft mode of the STO thin films takes place with decreasing temperature. Both eigen frequency and damping of the soft phonon mode show a peak nearly at 110K that can be attributed to the effect on the Ti–O–Ti bending in STO thin film due to the starting of tilting in oxygen octahedra near the cubic to tetragonal phase transition. Dielectric strength saturates near the temperature 60K, however, that is a point of further investigation.

Imaging of a Polycrystalline Silicon Solar Cell Using a Laser Terahertz Emission Microscope

We employed a laser terahertz (THz) emission microscope (LTEM) as a novel tool for evaluating solar cells. LTEM images are obtained by exciting a polycrystalline silicon solar cell with femtosecond laser illumination and visualizing the local distribution of the optical response. THz emission signals also provide various types of information, such as the screening effect of the built-in electrical field near pn junctions. These results indicate that this technique can be used to evaluate the local photoelectric conversion efficiency distribution and dynamic behavior of optically excited carriers in solar cells.

Chemical sensing plate with a laser-terahertz monitoring system

A new type of laser-terahertz emission system for noncontact investigations of chemical solutions has been developed. The system monitors terahertz emission from a sensing plate, which consists of silicon oxide and silicon thin film layers on a sapphire substrate. Sensing of chemical solutions with pH values between 1.68 and 10.01 was demonstrated. The amplitude of the terahertz emission from the sensing plate increased with increasing pH value. This change in the amplitude was caused by a change in the depletion layers of the silicon thin film when protons were adsorbed on the surface of the sensing plate. This study demonstrates that full noncontact monitoring of chemical solutions is possible using the laser-terahertz emission system.

A Terahertz Chemical Microscope to Visualize Chemical Concentrations in Microfluidic Chips

Here, a new type of terahertz chemical microscope (TCM) is proposed and developed, and the first demonstration of imaging the chemical concentration in fluid channels is reported. Fluid samples flow through channels possessing a semiconductor sensing plate as a bottom wall. Terahertz (THz) waves are radiated from the sensing plate as a result of femtosecond laser illumination. Because the amplitude of the THz radiation depends on the concentration of ions adsorbed on the surface of the plate, the ion distribution in the fluid channels can be visualized by scanning the laser across the plate. An image showing separated solutions with two different proton concentrations is successfully observed as the first demonstration of this instrument.

Nonlinear response of superconducting NbN thin film and NbN metamaterial induced by intense terahertz pulses

We present the nonlinear response of superconducting niobium nitride (NbN) thin film and NbN metamaterial with different thicknesses under intense terahertz pulses. For NbN thin film, nonlinearity emerges and superconductivity is suppressed with increasing incident terahertz electric field, and the suppression extent weakens as the film thickness increases from 15 to 50?nm. As the variation in intense terahertz fields alters the intrinsic conductivity in NbN, a consequent remarkable amplitude modulation in NbN metamaterial is observed due to the strong nonlinearity. Absorbed photo density in either NbN film or NbN metamaterial is estimated and used to understand the mechanism of nonlinear response. With a thicker NbN film element of 200?nm, the resonance of the metamaterial shows similar nonlinear modulation accompanied by a lower loss and a higher quality factor compared with a thinner NbN film element of 50?nm, which demonstrates the innovative implementation of strongly enhanced nonlinearity with thick superconducting film elements and the potential for novel applications using nonlinear metamaterial.

Study on terahertz emission and optical/terahertz pulse responses with superconductors

Recent progress in terahertz technology has enabled precise investigation of the ultrafast dynamics of excited carriers, nonequilibrium state and nonlinear response of superconductors, resulting in the proposal of novel optoelectronic device applications based on such ultrafast perturbation of supercarriers in the terahertz frequency region. In this paper, we focus on exploratory research in the field of superconductor terahertz science and technology, and present a review of superconducting terahertz sources and the response of superconductors excited by ultrashort electromagnetic pulses, including optical pulses and high-intensity THz pulses.