Our research interest includes exploring wireless systems for smart interface between humans and machines. Technical areas of interest include, but are not limited to, microwaves, terahertz waves, and ultrasound. Based on core skills of analyzing and synthesizing distributed parameter systems, we apply those waves to transmit and receive signals and energies with high-spatial resolution, ultimately aiming at assisting human recognition and action.

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Terahertz Radar

Integrated Terahertz Radar Based on Leaky-Wave Coherence Tomography

The use of terahertz waves for detection and ranging offers a higher resolution and smaller aperture as compared to the microwave radar. However, despite the recently emerging terahertz sources and detectors applicable to radar front-ends, integration of a phased array radar system is still challenging due to the lack of phase shifters and circulators, the basic components for beam steering and input-output isolation. Here we demonstrate leaky-wave coherence tomography, a method to integrate a terahertz radar system using a pair of reversely connected leaky-wave antennas. With this architecture, we implement beam steering and homodyne detection in one package and thereby identify the direction and range toward targets without using phase shifters, circulators, half-mirrors, lenses, or mechanical scanners. Our work paves the way to a high resolution, penetrable, and compact radar system, which is suitable to be equipped even on mobile devices and drones for a wide range of applications. As an example, we demonstrate in-situ human heartbeat detection by measuring the small displacement of the chest of subjects through the clothes, which provides information as with a stethoscope but remotely.

Bessel Beamformer

We experimentally demonstrate terahertz Bessel beamforming based on the concept of plasmonics. The proposed planar structure is made of concentric metallic grooves with a subwavelength spacing that couple to a point source to create tightly confined surface waves or spoof surface plasmon polaritons. Concentric scatterers periodically incorporated at a wavelength scale allow for launching the surface waves into free space to define a Bessel beam. The Bessel beam defined at 0.29 THz has been characterized through terahertz time-domain spectroscopy. This approach is capable of generating Bessel beams with planar structures as opposed to bulky axicon lenses and can be readily integrated with solid-state terahertz sources.

  • Yasuaki Monnai, David Jahn, Withawat Withayachumnankul, Martin Koch, and Hiroyuki Shinoda, “Terahertz Plasmonic Bessel Beamformer,” Applied Physics Letters, vol.106, no.2, 021101, 2015. [PDF] (Front Cover, Research Highlights in Nature Photonics, vol.9, 141, 2015)

Mid-Air Haptic Touch Panel

We present HaptoMime, a mid-air interaction system that allows users to touch a floating virtual screen with handsfree tactile feedback. Floating images formed by tailored light beams are inherently lacking in tactile feedback. Here we propose a method to superpose hands-free tactile
feedback on such a floating image using ultrasounds. By tracking a fingertip with an electronically steerable ultrasonic beam, the fingertip encounters a mechanical force consistent with the floating image. We demonstrate and characterize the proposed transmission scheme and
discuss promising applications with an emphasis that it helps us ‘pantomime’ in mid-air.

ODMR Antenna for NV Centers

We report on a microwave planar ring antenna specifically designed for optically detected magnetic resonance (ODMR) of nitrogen-vacancy (NV) centers in diamond. It has the resonance frequency at around 2.87 GHz with the bandwidth of 400 MHz, ensuring that ODMR can be observed under external magnetic fields up to 100 G without the need of adjustment of the resonance frequency. It is also spatially uniform within the 1-mm-diameter center hole, enabling the magnetic-field imaging in the wide spatial range. These features facilitate the experiments on quantum sensing and imaging using NV centers at room temperature.

Terahertz Programmable Diffraction Grating

We propose a freely programmable THz diffraction grating based on an electrostatically actuated, computer controlled array of metallic cantilevers. Switching between different grating patterns enables tailoring spatio-temporal profiles of the THz waves. By characterizing the device with spatially resolved THz time domain spectroscopy, we demonstrate beam steering for a wide frequency band extending from 0.15 THz to 0.9 THz. The steerable range at 0.3 THz exceeds 40°. Focusing is also demonstrated by programming a chirped grating. The proposed approach could be employed to mimic arbitrary diffraction optics, enabling highly integrated and extremely flexible systems indispensable for THz stand-off imaging and communications.

Terahertz Beamforming Based on Plasmonic Waveguide Scattering

We demonstrate free-space focusing of terahertz (THz) radiation by scattering plasmonic surface-waves into the air. We use a grating of shallow holes which contains non-equidistant defects which act as scattering centers. The scattering occurs with defned phase delays such that the waves emitted in free-space interfere constructively to form a focus above the waveguide surface. In contrast to conventional lenses, this structure does not require any free-space on its backside and has great potential for integrated THz optics.