Kei Takeya

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.

Hydrogen transfer from guest molecule to radical in adjacent hydrate-cages

Electron spin resonance measurement of gamma-ray-irradiated propane hydrates shows that the normal propyl radical withdraws hydrogen from the adjacent propane molecule through the hexagonal planes of the hydrate cage without water molecule bridging.

Electron spin resonance study on γ-ray-induced methyl radicals in methane hydrates

Electron spin resonance (ESR) studies have been performed to investigate radicals induced in ethane hydrate irradiated by γ-rays at 77 K. Two ESR spectra are observed and identified as the induced ethyl radical (g=2.0031±0.0005, Aα⊥=2.2±0.1 mT, Aα|| =2.5±0.1 mT, Aβ=2.7±0.1 mT) and induced atomic hydrogen (g=2.0026±0.0005, A=50.5±0.1 mT). From the results of ESR analysis and gas mass spectroscopy, it is concluded that the ethyl radical decays into butane by dimerization in the first-order reaction in the temperature region of 250–265 K. The activation energy of the decay reaction is 73.1±6.3 kJ/mol, which is near the dissociation enthalpy change of ethane hydrate to liquid water and gaseous ethane. This finding implies that ethane hydrate does not dissociate into ice but supercooled water in the present temperature region, similar to the dissociation of methane hydrate in our previous study.

Bulk Crystal Growth of Stilbazolium Derivatives for Terahertz Waves Generation

The two stilbazolium derivatives, 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) and 4-dimethylamino-N-methyl-4-stilbazolium p-chlorobenzenesulfonate (DASC), are organic ionic nonlinear optical materials that have a tendency to form bulk crystals when grown in a mixed solvent of methanol and acetonitrile. We observed the generation of broadband high-power terahertz (THz) waves from the bulk crystals of DASC. DASC crystals have superior transmission characteristics in the THz band than DAST crystals, and it is expected that the THz waves generated using DASC crystals will have higher power than those generated using DAST crystals.

Broadband terahertz wave generation from a MgO:LiNbO 3 ridge waveguide pumped by a 1.5 μm femtosecond fiber laser

Cherenkov phase-matched terahertz (THz) wave generation from a MgO:LiNbO3 ridge waveguide was studied using optical rectification. Pumping was achieved using 20 and 60 fs laser pulses from a fiber laser centered at 1.56 μm. Time-domain spectroscopy (TDS) results showed a single-cycle pulse with 20 fs pulse pumping and a near-single-cycle pulse with 60 fs pulse pumping. The spectrum covered the range of 0.1-7 THz, with a signal-to-noise ratio of over 50 dB. The output power measured by a Si bolometer and a deuterated triglycine sulfate pyroelectric detector is shown and compared to that of a commercial photoconductive antenna. This system is believed to be a promising THz source for low-cost, compact, robust, and highly integrated TDS, THz imaging, and tomography systems.

Terahertz emission from coherent phonons in lithium ternary chalcopyrite crystals illuminated by 1560 nm femtosecond laser pulses

We have investigated terahertz (THz) emission from lithium ternary chalcopyrite crystals LiInSe2, LiGaSe2, LiInS2, and LiGaS2 that were illuminated by 1560 nm femtosecond pump laser pulses. Monocyclic THz emission caused by nonlinear optical effects was initially observed in all the illuminated crystals. Narrow-band THz emission from the coherent phonons were observed in LiInSe2 (2.87 THz) and LiGaSe2 (2.60 and 3.45 THz). These phonon modes were most likely caused by impulsive stimulated Raman scattering.

Wide Spectrum Terahertz-Wave Generation From Nonlinear Waveguides

We have obtained efficient terahertz (THz)-wave generation from surface-emitting or waveguide propagation using Cherenkov-type radiation with nonlinear optical (NLO) crystals. This approach presented the following advantages: 1) many crystals can be used as THz-wave emitters; 2) it is not necessary to satisfy the phase-matching condition inside the crystal; and 3) THz-wave generation is not suppressed by the absorption in the crystal. We demonstrated that the THz-wave generation was enhanced 50× by suppressing the phase mismatch with a surfing configuration for bulk lithium niobate (LiNbO3) crystals. Using the prism-coupled Cherenkov phase-matching method with organic crystal 4-dimethylamino-N-metyl-4-stilbazolium tosylate, we produced tunable THz radiation within ~0.1-10 THz. There was no evidence of significant absorption in the crystal. To show the advantages of the waveguide emitter, we demonstrated THz generation using a ridge-type LiNbO3 waveguide with a thickness of 3.8 μm. We obtained THz-wave generation with an ideal temporal half-cycle pulse and a wide tuning range, which is applied to reflection tomography imaging.

A Fiber-Laser Pumped, High-Power Terahertz Wave Source Based on Optical Rectification of Femtosecond Pulses in 4-Dimethylamino-N-methyl-4-stilbazolium Tosylate Crystal

We demonstrate a compact, broadband, and high-power terahertz wave-generation scheme via optical rectification of femtosecond pulses in a 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal. We used a femtosecond fiber laser with a high repetition rate and an average power of a few hundred milliwatts to illuminate the DAST crystal. The terahertz waves emitted from the DAST crystal were measured using a calibrated pyroelectric sensor. We obtained an average power of 18 µW, more than two orders of magnitude greater than the average terahertz power emitted by a typical photoconductive antenna.

Enhanced Cherenkov phase matching terahertz wave generation via a magnesium oxide doped lithium niobate ridged waveguide crystal

When combined with a nonlinear waveguide crystal, Cherenkov phase matching allows for highly effective generation of high power and broadband terahertz (THz) waves. Using a ridged Lithium Niobate (LiNbO3) waveguide coupled with a specially designed silicon lens, we successfully generated THz waves with intensity of approximately three orders of magnitude stronger than those from conventional photoconductive antenna. The broadband spectrum was from 0.1 THz to 7 THz with a maximum dynamic range of 80 dB. The temporal shape of time domain pulse is a regular single cycle which could be used for high depth resolution time of flight tomography. The generated THz wave can also be easily monitored by compact room-temperature THz camera, enabling us to determine the spatial characteristics of the THz propagation.