Yasuaki Monnai / Research

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Terahertz-Induced Ultrasound

We have proposed and demonstrated a non-contact method for generating in-vivo ultrasound by irradiating the body surface with modulated terahertz waves. Unlike traditional ultrasound imaging, this approach allows for the creation of in-vivo ultrasound without the need to attach a transducer to the skin using gel. While terahertz waves are generally thought to be unsuitable for in-vivo applications due to their strong absorption by water, we are actively leveraging this absorptive property for a photoacoustic effect. Looking ahead, we aim to develop in-vivo ultrasound technologies that can seamlessly integrate into daily life and sports activities, with the goal of contributing to preventive medicine and enhancing physical skills.

Non-Contact Underwater Communication

We have developed a non-contact method to generate ultrasound in water by irradiating terahertz waves without making direct contact with the water or its tank. We have shown the proof-of-concept of remote control of underwater drones based on the proposed technique. We believe that it could potentially enable communication with in-vivo sensors/robots such as capsule endoscopes in the future. Furthermore, our paper clarifies through analysis that efficient generation of ultrasound is attained when the tank wall meets specific conditions, i.e. having an acoustic impedance close to that of water and a thickness approximately equal to a quarter wavelength of the ultrasound.

Terahertz Radar

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.

THz Beam Steering

In the realm of communication and measurement using terahertz waves, two-dimensional beam scanning in the air is essential. However, the implementation remains challenging in the terahertz frequency band, where no low-loss, broadband phase shifters are available. In this study, we have proposed and demonstrated a new technique for realizing two-dimensional beam scanning with terahertz waves. Specifically, we utilized two degrees of freedom (2-DOF) in terahertz beam steering by radiating a formed terahertz wavefront into external space as a leaky-wave beam. We achieved this by controlling both the frequency sweep and the gradient of the conductor plate. Initially, we substituted the upper flat plate with a mesh layer to allow for wave leakage. We then demonstrated that the beam could be scanned vertically by sweeping the frequency and horizontally by adjusting the relative inclination between the plates, exploiting the spatial gradient of the effective refractive index. By integrating these two methods, we have proven that the beam can be scanned with two degrees of freedom.

Ultrasound Compression

We have proposed and demonstrated a technique for enhancing the peak power of airborne ultrasound. Generally, the efficiency of generating airborne ultrasonic waves via transducers suffers due to issues with acoustic impedance mismatch. We have shown that by inputting continuous ultrasonic waves from a conventional transducer into an acoustic resonator, and then instantaneously releasing the stored energy, it is possible to emit airborne ultrasonic pulses with higher peak power than the original input. Further improvements are anticipated by increasing the Q-factor of the resonator. While the method inherently limits the repetition rate, it has the potential to improve the output power in applications like airborne ultrasonic haptic displays, which have garnered much recent research and development. In such systems, the importance lies not only in the continuous emission of ultrasonic waves but also in their low-frequency modulation.

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.