Kota Ito

Design and characterization of a 256x64-pixel single-photon imager in CMOS for a MEMS-based laser scanning time-of-flight sensor

We introduce an optical time-of-flight image sensor taking advantage of a MEMS-based laser scanning device. Unlike previous approaches, our concept benefits from the high timing resolution and the digital signal flexibility of single-photon pixels in CMOS to allow for a nearly ideal cooperation between the image sensor and the scanning device. This technique enables a high signal-to-background light ratio to be obtained, while simultaneously relaxing the constraint on size of the MEMS mirror. These conditions are critical for devising practical and low-cost depth sensors intended to operate in uncontrolled environments, such as outdoors. A proof-of-concept prototype capable of operating in real-time was implemented. This paper focuses on the design and characterization of a 256 x 64-pixel image sensor, which also comprises an event-driven readout circuit, an array of 64 row-level high-throughput time-to-digital converters, and a 16 Gbit/s global readout circuit. Quantitative evaluation of the sensor under 2 klux of background light revealed a repeatability error of 13.5 cm throughout the distance range of 20 meters.

Experimental investigation of radiative thermal rectifier using vanadium dioxide

Vanadium dioxide (VO2) exhibits a phase-change behavior from the insulating state to the metallic state around 340 K. By using this effect, we experimentally demonstrate a radiative thermal rectifier in the far-field regime with a thin film VO2 deposited on the silicon wafer. A rectification contrast ratio as large as two is accurately obtained by utilizing a one-dimensional steady-state heat flux measurement system. We develop a theoretical model of the thermal rectifier with optical responses of the materials retrieved from the measured mid-infrared reflection spectra, which is cross-checked with experimentally measured heat flux. Furthermore, we tune the operating temperatures by doping the VO2 film with tungsten (W). These results open up prospects in the fields of thermal management and thermal information processing.

Parallel-plate submicron gap formed by micromachined low-density pillars for near-field radiative heat transfer

Near-field radiative heat transfer has been a subject of great interest due to the applicability to thermal management and energy conversion. In this letter, a submicron gap between a pair of diced fused quartz substrates is formed by using micromachined low-density pillars to obtain both the parallelism and small parasitic heat conduction. The gap uniformity is validated by the optical interferometry at four corners of the substrates. The heat flux across the gap is measured in a steady-state and is no greater than twice of theoretically predicted radiative heat flux, which indicates that the parasitic heat conduction is suppressed to the level of the radiative heat transfer or less. The heat conduction through the pillars is modeled, and it is found to be limited by the thermal contact resistance between the pillar top and the opposing substrate surface. The methodology to form and evaluate the gap promotes the near-field radiative heat transfer to various applications such as thermal rectification, thermal modulation, and thermophotovoltaics.

Multilevel radiative thermal memory realized by the hysteretic metal-insulator transition of vanadium dioxide

Thermal information processing is attracting much interest as an analog of electronic computing. We experimentally demonstrated a radiative thermal memory utilizing a phase change material. The hysteretic metal-insulator transition of vanadium dioxide (VO2) allows us to obtain a multilevel memory. We developed a Preisach model to explain the hysteretic radiative heat transfer between a VO2 film and a fused quartz substrate. The transient response of our memory predicted by the Preisach model agrees well with the measured response. Our multilevel thermal memory paves the way for thermal information processing as well as contactless thermal management.

System Design and Performance Characterization of a MEMS-Based Laser Scanning Time-of-Flight Sensor Based on a 256

This paper reports on a light detection and ranging (LIDAR) system that incorporates a microelectromechanical-system (MEMS) mirror scanner and a single-photon imager. The proposed architecture enables a high signal-to-background ratio due to pixel-level synchronization of the single-photon imager and the MEMS mirror. It also allows the receiving optics to feature a large aperture, yet utilizing a small MEMS device. The MEMS actuator achieves a mechanical scanning amplitude of ±4° horizontally and ±3° vertically, while the field of view of the overall sensor is 45 by 110. Distance images were acquired outdoors in order to qualitatively evaluate our sensor imaging capabilities. Quantitative ranging performance characterization carried out under 10 klx of ambient light revealed a precision of 14.5 cm throughout the distance range to 25 m, thus leading to a relative precision of 0.58%.

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Dielectric guided mode resonant gratings exhibit a sharp spectral and angular response of high reflectivity for propagation wave, and strong evanescent waves are excited. We show that in such a resonant grating positioned above the silicon carbide (SiC) plate, incident light is absorbed in the SiC plate via the evanescent wave coupling when the lateral wavenumber of a guided mode of the grating coincides with that of surface phonon polaritons on the SiC plate. This coupling scheme using the thermally transparent grating enables a sharp spectral and angular emission in the infrared region with capabilities of emissivity modulation and spatially asymmetric emissivity. Thermally transparent subwavelength structures electromagnetically coupled to polar material thermal bodies are crucial in enabling components for thermal emission control.

64-pixel Single-Photon Imager

Dynamic control of electromagnetic heat transfer without changing mechanical configuration opens possibilities in intelligent thermal management in nanoscale systems. We confirmed by experiment that the radiative heat transfer is dynamically modulated beyond the blackbody limit. The near-field electromagnetic heat exchange mediated by phonon-polariton is controlled by the metal-insulator transition of tungsten-doped vanadium dioxide. The functionalized heat flux is transferred over an area of 1.6 cm2 across a 370 nm gap, which is maintained by the microfabricated spacers and applied pressure. The uniformity of the gap is validated by optical interferometry, and the measured heat transfer is well modeled as the sum of the radiative and the parasitic conductive components. The presented methodology to form a nanometric gap with functional heat flux paves the way to the smart thermal management in various scenes ranging from highly integrated systems to macroscopic apparatus.

Thermal emission control by evanescent wave coupling between guided mode of resonant grating and surface phonon polariton on silicon carbide plate

We show that in a silicon double-groove grating with two different groove widths per period attached on top of a semi-infinite SiO2 substrate, almost 100% reflectivity is achieved for the -1st-order reflection with an incident angle of 60° in the Littrow mounting condition. The modal analysis reveals that modes propagating in the upward and downward directions have nearly the same amplitudes at resonance. They are added constructively for the -1st-order reflection and destructively for the 0th-order reflection and the -1st-order and 0th-order transmission. The asymmetric structure with a dielectric material poses a unique feature as a four port device.

Dynamic Modulation of Radiative Heat Transfer beyond the Blackbody Limit

The spectra of thermal radiation have been controlled for thermophotovoltaics and mid-infrared light sources, and the spectral heat flux has been shown to exceed the blackbody limit by utilizing near-field coupling. We show that a hyperbolic metamaterial layer enables quasi-monochromatic near-field radiative heat transfer between a metallic emitter and a dielectric receiver. The quasi-monochromatic transfer originates from the Fabry-Perot resonance in the hyperbolic layer, where evanescent waves in the vacuum gap become propagative. The Fabry-Perot resonance is excited in s and p polarizations, and the resonant condition is almost independent of the lateral wavenumber due to the large effective parallel permittivity of the hyperbolic metamaterial. The resonant frequency is tuned by the volume filling fraction and the thickness of the layer, while the frequency misalignment between polarizations is kept small. Furthermore, the resonant frequency is shown to be robust to the fluctuation of the gap width and...

Highly efficient -1st-order reflection in Littrow mounted dielectric double-groove grating

Metal-insulator-metal metamaterial thermal emitters or absorbers have been widely investigated, and the fundamental and higher-order modes are generally excited in these metamaterialresonators. In this paper, we propose a methodology to widen the frequency interval between the fundamental and the second-order modes by enhancing coupling between resonators in close-proximity. At the second-order mode, antiparallel magnetic fields are excited in the insulating layer of rectangular resonators. A diagonal arrangement of rectangles allows destructive interaction between neighboring resonators, resulting in higher second-order frequency. The maximum frequency interval between the two modes is achieved when resonators are shifted by half a period. Furthermore, we suggest a possibility to split the second-order mode by adjusting the arrangement of rectangles. Measured reflection spectra of fabricated metamaterial absorbers agree well with numerical simulations.