Welcome to Ken Araki's World

Arizona State University, Tempe AZ (Jan. 2023 - Present)

• Part-time Experimental Postdoctoral Researcher
• Mentor: Dr. Liping Wang

University of North Texas, Remote (Dec. 2023 - Aug. 2024)

• Part-time Theoretical Postdoctoral Researcher
• Mentor: Dr. Richard Zhang

University of North Texas, Denton TX (Aug. 2019 - Dec. 2022)

• Research Assistant, Teaching Assistant (Fall 2021: Thermodynamics 1, Spring 2022 & Fall 2022: Heat Transfer)
• Dissertation Advisor: Dr. Richard Zhang
• Dissertation: "Tunable Effect on Thermal Radiative Emission of Gratings and Multilayers" Oct. 28th, 2022 12 PM - 1:20 PM (Committee members: Dr. Richard Zhang, Dr. Bo Zhao (University of Houston), Dr. Jeffry Kelber (Department of Chemistry), Dr. Wonbong Choi, Dr. Weihuan Zhao). link

Niigata University, Niigata, Japan (Apr. 2018 - Mar. 2019)

• Research Advisor: Dr. Atsushi Sakurai
• Research topics: Beam splitter for PV-TEG hybrid system, Exterior tuning for radiative cooling device
• Thesis: "Exterior Tuning for Radiative Cooling Device"

National Institute of Technology Gunma College, Gunma, Japan (Apr. 2016 - Mar. 2017)

• Advisor: Dr. Masashi Kurose
• Thesis: "Effect of Grinding Speed of Work Hardening of Austenitic Stainless Steel"

• JSME Student Membership (2016 - 2019)
• IEEE Membership (2021 - Present)
• ASME Membership (2022 - Present)
• SPIE  Membership (2022 - Present)

MATLAB, Python, C++, Machine Learning (Bayesian COMBO, Genetic Algorithm), SOLIDWORKS, Rigorous Coupled-Wave Analysis (RCWA), Transfer Matrix Method (TMM), Finite Difference Time Domain (FDTD), Fourier-Transform Infrared Spectroscopy (FTIR), Variable Angle Spectroscopic Ellipsometer (VASE), E-Beam Evaporation, MiniBrute Thermal Furnace, Oxford PECVD, Cambridge ALD, DC/RF Sputtering. Tunable Light Source (TLS), Keithley Digital Multimeter (DMM) 6500, Keithley Source Meter 2400.

• San Diego, California
• Ohtawara, Tochigi, Japan
• Vancouver, Washington
• Takasaki, Gunma, Japan
• Niigata, Niigata, Japan 
• Denton, Texas
• Tempe, Arizona

(6) Resonant-mode metasurface thermal super mirror by deep learning assisted optimization algorithms (link)

A “super-mirror” having ultrahigh infrared reflectance is achieved by an optimized photonic contrast grating metasurface. Finding ways to achieve this exceptional performance can be enabled by implementing global optimization and machine learning elements, such as Bayesian optimization and genetic algorithm. Here, we acquired an optimized grating design made of high-index Germanium, which excites resonances that result in ultralow emittance at certain wavelengths. Our optimizations assisted in the discovery of hybridized coupling of Fabry-Perot modes and guided modes in a monolithic microscale multilayered coating. We demonstrate constraints in the given geometric variable ranges improves the overall performance of algorithms. We also show the enhanced performance of a deep learning Feedforward Neural Network, which is implemented as the inverse design using the network trained with dataset obtained from Bayesian and genetic approaches. The performance of the Neural Network-assisted design produced normal emissivity difference by only +3.5%, with lower sensitivity to grating dimensional parameter variations. The improvement is achieved by predicting and understanding the optical physics of resonant gratings. The proposed coating suggested by machine learning can be applied to space components, enclosures, and vessels to prevent thermal radiation loss.

(5) Infrared Radiative Switching With Thermally and Electronically Tunable Transition Metal Oxides-Based Plasmonic Grating (link)

Tunability of optical radiative properties are provided through material properties such as thermally tunable negative thermal expansion of graphene and phase transition of vanadium dioxide. In contrast, the optical properties can be manipulated via electronically. One can tune the chemical potential of graphene to shift the plasmonic excitation. Likewise, voltage supply with positive ion insertion can transition its dielectric function, known as electrochromic materials including tungsten trioxide (WO3) and molybdenum trioxide (MoO3) which only requires small ranges of DC voltage supply of plus minus 2 to 3 V.   Both WO3 and MoO3 transition to metallic phase as voltage is applied so that its dielectric function can be modeled by Drude model. Interestingly, MoO3 is also temperature dependent where phase transition occurs between 498-623 K. In this work, tunable plasmonic grating is compared between three different transition metal oxides, VO2, cWO3 (crystalline), and MoO3.  

(4) Simultaneous solar rejection and infrared emission switching using an integrated dielectrics-on-VO2 metasurface link

Passive infrared emittance switching can be achieved with a metal-to-insulating phase transition material vanadium dioxide (VO2), but its non-transitioning bandgap results in high absorptance in the visible wavelength range. To achieve a half-order reduction of absorptance in the visible to near-infrared region, we design integrated dielectric photonic metasurface structures on monolithic VO2 coatings. This combination of nano/micro-patterned dielectric diffractive and resonant gratings with a multilayer VO2 structure preserves the terrestrial thermal wavelength emission switching capabilities. We demonstrate a periodic microscale diffractive prism array, comparing the reflectance provided by either infrared-transparent germanium (Ge) or silicon (Si). Despite the advantage of total internal reflection in the broad near-infrared region, some bandgap absorption limits the performance in the visible wavelengths. A better theoretical means to reflect broadband light via waveguide-like Fabry–Pérot resonance are near-wavelength 1D and 2D High Contrast Grating (HCG) high-index metasurface structures surrounded by a low-index host medium. This HCG metasurface allows broadband high-quality reflection within the dual-mode (or tri-mode) region from 1.0 to 2.2 µm wavelengths for HCG with a refractive index of 4.0, which corresponds to Ge. This study investigates the advantages and disadvantages along with the thermal performance of these metasurface augments aimed to enable thermally switchable passive radiative cooling—thermal emission exceeding solar absorption—of solar cells, terrestrial buildings, and energy storage devices.

(3) An optimized self-adaptive thermal radiation turn-down coating with vanadium dioxide nanowire array link

Insulator-to-metal temperature phase transition Vanadium Dioxide (VO2) can enable radiative property switching in the mid- to far-infrared wavelengths. With computational optimization of grating arrangement and layer thickness parameters, we identify a monolithic high-performance turn-down thermal emittance coating of no more than 2 μm thick, consisting of a VO2 sub-wavelength nanowire grating array on an index-matched Fabry-Perot dielectric thin film on an additional absorbing VO2 sublayer. The working principles of this optimized VO2 structure are its gradient refractive index allowing high through-coating transmittance in the cold state, and its near-unity emissivity from semi-metal-insulator-metal plasmonic coupling in the hot state. This anisotropic patterned structure also considers performance over polarized incident light. A survey of other Fabry-Perot cavity materials with refractive index matching points to higher turn-down performances given an optimal VO2 nanowire volume filling ratio. With 24-hour solar and environmental analysis in comparison to other VO2 metasurfaces and multilayers, this coating enables responsive passive radiative cooling at high temperatures exceeding transition. This nano/micro-patterned coating could potentially impact self-cooling of the solar cells, batteries, and electrical devices where risk presents at high temperatures.

(2) Mechano-Optical Resonant Emission by Edge Angle Modulation of Wrinkled Graphene on Plasmonic Metal Gratings link

Graphene coated on top of photonic-plasmonic metasurfaces can produce resonant radiative emission in the mid-infrared region. Narrowband emission peaks are observed through folding graphene into “origami” ridges over metal grating grooves, creating a complementary cavity mode above the trench. This geometrically tuned phenomenon of graphene surface plasmon excitation along the folded sheet enhances the emission when added to magnetic polariton (MP) resonance induced within the plasmonic grating groove. Our analytical models describe how this graphene surface plasmon polariton (SPPG) is a function of folded graphene geometric parameters, and most importantly, the graphene edge angle that distends from the grating surface. The frequency-dependent phase shift of SPPG is fitted to grating parameters, and a modified inductor-capacitor circuit model was developed for predicting MP resonance mode with graphene influence. It was found that the edge angle of wrinkled graphene blue-shifts the groove MP resonance and SPPG resonant emission peak in both wrinkled graphene alone and with the grating substrate. The understanding of geometrically modulated graphene adhered on plasmonic gratings impacts the design and capability of narrowband cavity emitters and contributes toward the development of mechanical-optical environmental sensors.

(1) Plasmon-resonance emission tailoring of “origami” graphene-covered photonic gratings link

Due to the negative coefficient of thermal expansion of graphene, temperature changes of graphene-coated photonic surfaces could induce resonant mode shifts in diffractive optical absorptance and emission. This study focuses on the modification of optical properties through folding, or “origami,” of graphene covering a plasmonic metal channel grating. This work is especially critical to understanding tailored deep plasmon emission from geometrically-modulated conducting sheets such as graphene. Conformational changes in graphene on gratings are found to tailor cavity resonance emission and plasmonic oscillations such as magnetic polaritons (MPs) and surface plasmon polaritons (SPPs), respectively. Up to 46% reduction in radiative absorptance was observed through retarded MP. Excited SPP modes can increase narrowband absorptance of 0.5 through folding of graphene. Tailoring of optical absorptance can be used for applications such as photodetectors and thermal emitters.

(1) R.Z. Zhang and K. Araki, Thermal High-Contrast Metamaterials, Thermal Plasmonics and Metamaterials for a Low-Carbon Society (2024) link.