Tomofumi Ikari

Component spatial pattern analysis of chemicals using terahertz spectroscopic imaging

We demonstrated the separation of the component spatial patterns of chemical samples in transillumination terahertz (THz) images using known spectral curves. The images and spectral data were measured between 1.3 and 1.8 THz, using a widely tunable coherent THz-wave parametric oscillator source. This method could be effective for analyzing spatial patterns and the concentrations of components with various chemical compositions.

Terahertz-wave near-field imaging with subwavelength resolution using surface-wave-assisted bow-tie aperture

We demonstrate the terahertz-wave near-field imaging with subwavelength resolution using a bow-tie shaped aperture surrounded by concentric periodic structures in a metal film. A subwavelength aperture with concentric periodic grooves, which are known as a bull’s eye structure, shows extremely large enhanced transmission beyond the diffraction limit caused by the resonant excitation of surface waves. Additionally, a bow-tie aperture exhibits extraordinary field enhancement at the sharp tips of the metal, which enhances the transmission and the subwavelength spatial resolution. We introduced a bow-tie aperture to the bull’s eye structure and achieved high spatial resolution (∼λ∕17) in the near-field region. The terahertz-wave near-field image of the subwavelength metal pattern (pattern width=20μm) was obtained for the wavelength of 207μm.

THz-wave parametric oscillator with a surface-emitted configuration

We propose a surface-emitted cavity configuration for a terahertz-wave parametric oscillator that allows THz wave emission perpendicular to the crystal surface without any output coupler. The oscillating idler and pump waves are reflected at the surface of a nonlinear crystal in a single resonance cavity, satisfying the noncollinear phasematching condition. The radiated THz wave has a Gaussian profile. The measured beam quality factors (M(2)) were 1.15 and 1.25 in the horizontal and vertical directions, respectively. The measured tunable range was 0.8-2.74 THz. A test of transmission imaging using a test pattern was demonstrated.

Highly sensitive coherent detection of terahertz waves at room temperature using a parametric process

We report the experimental results of coherent detection of narrow-linewidth nanosecond terahertz radiation at room temperature using frequency conversion in a nonlinear MgO:LiNbO3 crystal. Mixing the terahertz radiation with a near-infrared intense pump pulse results in the excitation and amplification of the difference-frequency component, which is detected with an InGaAs-based photodiode. We demonstrate this method in a fast and very sensitive terahertz wave detector. The detector is capable of capturing the temporal profile of terahertz pulses with nanosecond resolution, and is at least one order more sensitive than a typical liquid-He-cooled Si bolometer for detecting nanosecond pulsed terahertz wave beams.

Frequency-agile terahertz-wave parametric oscillator in a ring-cavity configuration

We demonstrate a frequency-agile terahertz wave parametric oscillator (TPO) in a ring-cavity configuration (ring-TPO). The TPO consists of three mirrors and a MgO:LiNbO(3) crystal under noncollinear phase-matching conditions. A novel, fast frequency-tuning method was realized by controlling a mirror of the three-mirror ring cavity. The wide tuning range between 0.93 and 2.7 THz was accomplished. For first demonstration using the ring-TPO, terahertz spectroscopy was performed as the verification of the frequency-agile performance, measuring the transmission spectrum of the monosaccharide glucose. The spectrum was obtained within about 8 s in good comparison to those of Fourier transform infrared spectrometer.

Spatial pattern separation of chemicals and frequency-independent components by terahertz spectroscopic imaging

We separated the component spatial patterns of frequency-dependent absorption in chemicals and frequency-independent components such as plastic, paper, and measurement noise in terahertz (THz) spectroscopic images, using known spectral curves. Our measurement system, which uses a widely tunable coherent THz-wave parametric oscillator source, can image at a specific frequency in the range 1-2 THz. The component patterns of chemicals can easily be extracted by use of the frequency-independent components. This method could be successfully used for nondestructive inspection for the detection of illegal drugs and devices of bioterrorism concealed, e.g., inside mail and packages.

Terahertz near-field imaging using enhanced transmission through a single subwavelength aperture

We demonstrate terahertz (THz) near-field imaging using resonantly enhanced transmission of THz-wave radiation (λ~200 µm) through a bulls eye structure (a single subwavelength aperture surrounded by concentric periodic grooves in a metal plate). The bulls eye structure shows extremely large enhanced transmission, which has the advantage for a single subwavelength aperture. The spatial resolution for the bulls eye structure (with an aperture diameter d=100 µm) is evaluated in the near-field region, and a resolution of 50 µm (corresponding to λ/4) is achieved. We obtain the THz near-field images of the subwavelength metal pattern with a spatial resolution below the diffraction limit.

Output power enhancement of a palmtop terahertz-wave parametric generator

Broadband terahertz (THz) waves were generated by optical parametric processes based on laser light scattering from the polariton mode of a nonlinear crystal. By using the parametric oscillation of a MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized a broadband, high-energy and compact THz-wave source. We report the development of a THz-wave parametric generator (TPG) using a small pump source with a short pulse width and a top-hat beam profile. These characteristics of the pump beam permit high-intensity pumping especially close to the output surface of the THz wave without thermal damage to the crystal surface. We also calculated the outcoupled THz wave for beams with two different intensity profiles: a top-hat beam (in this experiment) and a Gaussian beam (previously reported). The result shows the mechanism of the output energy and/or power enhancement.

Terahertz Wave Enhanced Transmission through a Single Subwavelength Aperture with Periodic Surface Structures

We demonstrate resonantly enhanced transmission of terahertz (THz) wave radiation (λ~200 µm) through a bulls eye structure (a single subwavelength aperture surrounded by concentric periodic grooves in a metal plate). The phenomenon is caused by the resonant excitation of surface waves, which are known as surface plasmon polaritons in the optical region, generated by the concentric periodic grooves. Strongly enhanced transmission of THz-wave radiation is observed through the single subwavelength circular aperture (diameter d = 100 µm). We also show that introducing the Bragg reflector to the bulls eye structure results in the further increase of the enhanced transmission.

Frequency-agile THz-wave generation and detection system using nonlinear frequency conversion at room temperature

A surface-emitting THz parametric oscillator is set up to generate a narrow-linewidth, nanosecond pulsed THz-wave radiation. The THz-wave radiation is coherently detected using the frequency up-conversion in MgO: LiNbO(3) crystal. Fast frequency tuning and automatic achromatic THz-wave detection are achieved through a special optical design, including a variable-angle mirror and 1:1 telescope devices in the pump and THz-wave beams. We demonstrate a frequency-agile THz-wave parametric generation and THz-wave coherent detection system. This system can be used as a frequency-domain THz-wave spectrometer operated at room-temperature, and there are a high possible to develop into a real-time two-dimensional THz spectral imaging system.