Shintaro Kasai

High-resolution time-of-flight terahertz tomography using a femtosecond fiber laser

High-resolution tomographic imaging is demonstrated using a reflection-type terahertz time-domain spectroscopy. To realize a practical system for general use, a robust all-fiber laser was used as the pump light source. Broadband terahertz waves were generated with the combination of optical pulses compressed to 17 fs using optical fibers and a DAST crystal. Using deconvolution signal processing, the wideband spectrum of the generated terahertz waves provided high-axial resolution leading to successful imaging of a multilayered structure containing a 2-microm-thin GaAs layer. To our knowledge, this is the first demonstration of terahertz tomographic imaging of such a thin layer.

Terahertz spectroscopy of native-conformation and thermally denatured bovine serum albumin (BSA).

Proteins are expected to exhibit collective vibrational modes at terahertz frequencies. We have developed a promising approach to measure these motions by using a membrane device to hold samples. Samples of bovine serum albumin (BSA) in native and thermally denatured conformations were measured using terahertz time-domain spectroscopy. Clear differences were observed in transmittance and phase between native-conformation BSA samples and thermally denatured BSA samples. Time-domain data shows that samples exhibited relative time shifts when compared with a standard. Results suggest that there were differences in dielectric responses in the BSA samples, and these are probably associated with molecular conformational changes in the membrane device.

Micro Strip Line-Based On-Chip Terahertz Integrated Devices for High Sensitivity Biosensors

We have achieved label free DNA hybridization detection at the level of a few femto-moles using on-chip terahertz (THz) integrated devices. The devices consist of two photoconductive switches for the generation and detection of THz pulses. A DNA specimen placed in a reservoir on the micro strip line (MSL) connecting the switches delays the propagating THz pulses. Different delay times were observed depending on the DNA conformation. The high sensitivity achieved is due to MSL design adjustment and the use of a specimen reservoir. Both simulations and experimental results indicate that devices with a narrower MSL have higher sensitivity.

Reduction of phonon resonant terahertz wave absorption in photoconductive switches using epitaxial layer transfer

A broadband terahertz time-domain spectroscopy (TDS) system for frequencies of up to 15 THz, including the phonon resonance frequency range, has been developed using a transferred thin-film photoconductive switch (PCS) detector. The thin-film PCSs, based on low-temperature-grown GaAs, were fabricated using epitaxial layer transfer onto high-resistivity Si substrates. We observed a reduction of phonon resonant absorption, including between 7 and 10 THz, in a forward radiation configuration. Numerically calculated absorption spectra show good agreement with our experimental results. This technique will provide compact, broadband TDS systems.

Improved Terahertz Wave Intensity in Photoconductive Antennas Formed of Annealed Low-Temperature Grown GaAs

We investigated the dependence of terahertz (THz) wave intensity on ex situ annealing temperature of low-temperature-grown gallium arsenide (LT-GaAs). THz waves excited by femtosecond laser pulses were emitted from photoconductive antennas (bow-tie type) formed on LT-GaAs, and the radiation intensity was measured with a Si bolometer. LT-GaAs ex situ annealed at temperatures of 700 °C and above generated 1.4 times higher radiation intensity than unannealed LT-GaAs. Transmission electron microscopy images revealed apparently improved crystal quality, explaining the observed higher carrier mobility and associated radiation intensity.

Terahertz Integrated Transmission Line Sensors Using a Bonded Epitaxial GaAs Layer on Silicon Substrates

We have developed a novel process of transmission line type terahertz (THz) devices for biological sensing. A low-temperature-grown (LT) GaAs layer side is bonded onto silicon substrates using Au-Sn solder, then the GaAs substrate is removed by selective wet etching. Photoconduetive switch elements for generating and detecting THz signals are fabricated on the transferred LT-GaAs layer. This process results in robust integrated devices against biological solutions. Sequential measurement of THz time domain spectroscopy is available for biological molecules such as DNA samples. We observe clear difference in delay time shifts according to change of quantity and the state of applied molecules.

Utility of micro-filter membrane in THz spectrum of molecules in solution

We report the experimental results of physiologically active substances including hormones utilizing Terahertz (THz) spectrum technology. Various analytical techniques have been employed for the determination of physiologically active substances. Most of methods used oxidation reagent and sodium sulfite stabilization reagent or fluorescent reagent. These methods had a good selectivity, but the stability of samples was not satisfactory. The direct detection method has not been advanced yet. In this study, the THz characteristics of physiologically active substances were measured directly. We found all of samples have their vibrational features like signature peaks either in pellet and/or sample solution. A membrane device was used to hold the sample solution in this study. This device allows samples to be prepared in solutions and measured easily with THz measurement system after dried. Results suggest that this membrane device is sensitive for detecting the physiologically active substances in THz ranges. THz spectrum technology has the potential to be a useful tool in clinical applications. This approach promotes the understanding of the relationships between biomolecules with THz radiation.

Terahertz tomography system using fiber lasers and applications

High depth-resolution terahertz three-dimensional tomography was developed by monocycle-like terahertz pulses using ultra-short pulse fiber lasers. We designed novel photoconductive devices such as a thin-film LT-GaAs detector on Si substrates and an LT-InGaAs emitter for the monocycle waveforms. The depth resolution exhibited under 20 µm in the air and three-dimensional imaging was successfully obtained for multi-layer semiconductor devices.

Application of a Membrane Device for Biosensing with Terahertz Time Domain Spectroscopy

We propose a novel device relying on a membrane used in life sciences for analyzing the terahertz (THz)-wave transmission characteristics of biomolecular samples. This device allows small amounts of sample to be prepared easier than current methods. We prepared several sugar samples adsorbed on the membrane, and measured them using THz time domain spectroscopy. The THz absorption data were found to be different depending on the concentration of samples, which demonstrates the potential of the membrane device in THz-wave biosensing applications.

High-resolution terahertz tomography using 17-fs ultrashort-pulse fiber laser

We describe the generation and detection of coherent broadband terahertz waves using ultrashort-pulse fiber laser, and its application to terahertz tomography. A pulse compression technique using nonlinear and dispersion effects of optical fibers leads to the development of high-peak-power 17- fs ultrashort- pulse fiber laser. With the optical rectification in an organic crystal, 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), ultra-broad terahertz pulses are successfully generated. The spectrum ranges widely from 0.1 to 25 THz. By applying these broadband THz pulses to THz tomography, the thickness of 5- mum thin Teflon film is measured correctly.