Yasuaki Monnai

Broadband, large-area microwave antenna for optically detected magnetic resonance of nitrogen-vacancy centers in diamond

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.

Low-Profile Terahertz Radar Based on Broadband Leaky-Wave Beam Steering

We demonstrate short-range terahertz radar based on a leaky-wave antenna with a beam steering capability. As a proof of concept, we develop a microstrip-based periodic leaky-wave antenna driven by a vector network analyzer. By sweeping the frequency from 235 to 325 GHz, beam steering from -23° to +15° across the broadside can be achieved with a nearly constant beam width of 4°. Small target detection is demonstrated by locating a metal cylinder with a diameter of 12 mm placed 46-86 mm in front of the antenna with a mean error of 2.4 mm. The use of a leaky-wave antenna can pave the way for developing a low-loss, low-profile, and wide-aperture terahertz radar. Importantly, it can be integrated with a solid-state source and a detector. The proposed approach is particularly promising for use with emerging small devices such as drones or wearable devices, where millimeter-wave radar is not suitable in terms of the resolution and system footprint.

Focused terahertz waves generated by a phase velocity gradient in a parallel-plate waveguide

We demonstrate the focusing of a free-space THz beam emerging from a leaky parallel-plate waveguide (PPWG). Focusing is accomplished by grading the launch angle of the leaky wave using a PPWG with gradient plate separation. Inside the PPWG, the phase velocity of the guided TE1 mode exceeds the vacuum light speed, allowing the wave to leak into free space from a slit cut along the top plate. Since the leaky wave angle changes as the plate separation decreases, the beam divergence can be controlled by grading the plate separation along the propagation axis. We experimentally demonstrate focusing of the leaky wave at a selected location at frequencies of 100 GHz and 170 GHz, and compare our measurements with numerical simulations. The proposed concept can be valuable for implementing a flat and wide-aperture beam-former for THz communications systems.

Polarization- and frequency-tunable microwave circuit for selective excitation of nitrogen-vacancy spins in diamond

We report on a planar microwave resonator providing arbitrarily polarized oscillating magnetic fields that enable selective excitation of the electronic spins of nitrogen-vacancy centers in diamond. The polarization plane is parallel to the surface of diamond, which makes the resonator fully compatible with (111)-oriented diamond. The field distribution is spatially uniform in a circular area with a diameter of 4u2009mm, and a near-perfect circular polarization is achieved. We also demonstrate that the original resonance frequency of 2.8 GHz can be varied in the range of 2–3.2u2009GHz by introducing varactor diodes that serve as variable capacitors.

Tutorial: Terahertz beamforming, from concepts to realizations

The terahertz range possesses significant untapped potential for applications including high-volume wireless communications, noninvasive medical imaging, sensing, and safe security screening. However, due to the unique characteristics and constraints of terahertz waves, the vast majority of these applications are entirely dependent upon the availability of beam control techniques. Thus, the development of advanced terahertz-range beam control techniques yields a range of useful and unparalleled applications. This article provides an overview and tutorial on terahertz beam control. The underlying principles of wavefront engineering include array antenna theory and diffraction optics, which are drawn from the neighboring microwave and optical regimes, respectively. As both principles are applicable across the electromagnetic spectrum, they are reconciled in this overview. This provides a useful foundation for investigations into beam control in the terahertz range, which lies between microwaves and infrared...

Current-induced magnetization switching using an electrically insulating spin-torque generator

This paper reports the first demonstration of current-induced magnetization switching using an ordinary insulator. Current-induced magnetization switching through spin-orbit torques is the fundamental building block of spin-orbitronics, which promises high-performance, low-power memory and logic devices. The spin-orbit torques generally arise from spin-orbit coupling of heavy metals. However, even in a heterostructure where a metallic magnet is sandwiched by two different insulators, a nonzero spin-orbit torque is expected because of the broken inversion symmetry; an electrical insulator can be a source of the spin-orbit torques. We demonstrate current-induced magnetization switching using an insulator. We show that oxygen incorporation into the most widely used spintronic material, Pt, turns the heavy metal into an electrically insulating generator of the spin-orbit torques, which enables the electrical switching of perpendicular magnetization in a ferrimagnet sandwiched by insulating oxides. We also show that the spin-orbit torques generated from the Pt oxide can be controlled electrically through voltage-driven oxygen migration. These findings open a route toward energy-efficient, voltage-programmable spin-orbit devices based on insulating metal oxides.

Dual Circularly Polarized Series-Fed Microstrip Patch Array With Coplanar Proximity Coupling

We demonstrate a concept of compact and highly efficient circular traveling wave series-fed microstrip patch array working at 10xa0GHz. The patch elements are excited by a microstrip line through coplanar proximity coupling. A dual-port feeding allows us to select the direction of the traveling waves and, consequently, the sense of circular polarization of the patches. Simultaneous feeding to both ports with proper amplitude and phase difference can result in radiated waves of arbitrary polarization. Owing to the circular array arrangement and the strong proximity coupling, the antenna exhibits a compact structure and achieves a high antenna efficiency of 90% with seven patches. Based on reciprocity, the antenna can readily be also utilized for polarimetry applications for which the signal amplitude at each port is proportional to the amplitude of each individual sense of polarization. Measured results of an antenna prototype successfully validate the concept.

Integrated terahertz beamformers based on leaky-wave approaches

We review recent approaches to terahertz (THz) wavefront engineering using leaky-wave antennas (LWAs). The use of LWAs offers high gain, low loss, and flat profile antenna implementation. Here we consider two types of LWAs: slow- and fast-wave types. We use a metal corrugation for the former and parallel metal plates for the latter, both reasonable for the THz regime. Further, we extend the beamforming concept using LWA to mimic arbitrary diffraction optics and demonstrate focusing and Bessel beamforming. The proposed approach benefits most real-world THz applications in sensing and communication.

Midair touch display

We demonstrate a system which enables users with bare hands and naked eyes to tactually interact with midair three-dimensional objects. The displayed midair images are three-dimensional and can be seen and touched from multiple angles. By creating neatly controlled and designed ultrasound fields, our system can create rich tactile textures added onto them with no devices to be worn by the users, all of which are programmable.