Photonic approach in stacked slabs having periodic holes for enhancing photocatalytic activities.

Photonic approaches can improve the efficiencies of photo-electrochemical devices towards CO2 reduction and fossil fuel-free societies. In a system consisting of stacked dielectric slabs having periodic holes with each slab coated by photocatalyst layers at both sides, immersed in water, we show that an incident electromagnetic field is effectively confined in the photocatalyst layers, resulting in the enhancement of the photocatalytic activities. In addition, the antireflection effect was engineered by adjusting the distances between the photonic crystal slabs. Numerical results reveal an enhancement factor of 3 for the absorption of electromagnetic fields at the operation frequency in the 3rd band of the dispersion diagram, compared to the bulk photocatalyst. Our system has the feature of periodic holes allowing the movement of reaction products. An analytical model is developed using the revised plane wave method and perturbation theory, which captures the trends observed in numerical results.

Wirelessly powered motor operation in dynamic scenarios using non-Hermitian parity-time symmetry.

Motors arise as a heart of the mobility society, and wirelessly operated motors may improve our standard of living. Wireless power transfer in the kilohertz and megahertz range has been extensively explored, finding various potential applications in consumer electronics, electric vehicles, and medical implants. However, stable operation of wirelessly powered motors remains challenging due to voltage fluctuations for motors occurring in dynamic scenarios, e.g., the rotating speed of the motors is varied. Here, we theoretically and experimentally demonstrate the operation of a motor, where the power is wirelessly transferred via coils, is robust against the rotating speed by employing the analogy with non-Hermitian parity-time (PT) symmetry. In addition, our system is robust for misalignment of the coils. Our results open up opportunities for the robust operation of motors via wireless power transfer in dynamic scenarios towards autonomous vehicles.

Higher-order topological corner state in a reconfigurable breathing kagome lattice consisting of magnetically coupled LC resonators.

Higher-order topological insulators are attracting attention from fundamental interest to fascinating applications, owing to the topological properties with higher-order topological corner states. Breathing kagome lattice is a prospective platform which can support higher-order topological corner states. Here, we experimentally demonstrate that higher-order topological corner states are supported in a breathing kagome lattice consisting of magnetically coupled resonant coils. The winding direction of each coil is determined to hold C3 symmetry for each triangle unit cell, enabling to emerge higher-order topological corner states. In addition, topological and trivial phases can be switched by changing the distances between the coils. The emergence of corner states in the topological phase is experimentally observed through admittance measurements. As an illustration, wireless power transfer is performed between the corner states, and between the bulk and corner states. The proposed configuration is a promising platform for not only investigating topological properties of the breathing kagome lattice but also an alternative mechanism of selective wireless power transfer.

Near-field radiative heat transfer in three-body Weyl semimetals.

We investigate the near-field radiative heat transfer in a three-body system made of Weyl semimetals. At infinitesimal temperature gradient, the rotation of the middle and the right bodies leads to heat transfer suppression, enabling thermal switching with considerably enhanced heat flux but slightly smaller ratio than two-body system without the middle body, due to stronger cavity surface plasmon polariton modes and their mismatch caused by relative rotation. By further moving the middle body to induce asymmetric cavity sizes, the three-body system can achieve a switching ratio exceeding the two-body counterpart due to asymmetric cavity modes coupling. As the temperature gradient increases to 200 K, the highest switching ratio by optimally tuning the rotation and cavity size asymmetry decreases slightly yet still outperforms the two-body system. Our results provide important understanding of the near-field radiative heat transfer in many-body systems consisting of Weyl semimetals.

Near-field radiative heat transfer in three-body Weyl semimetals: erratum.

This erratum corrects a typographical error in Eq. (4) of our published paper [Opt. Express30(18), 31584 (2022).10.1364/OE.465017]. This misprint does not influence the results and conclusions presented in the original Article.

Substitutional 4d transition metal doping in atomically thin lead.

To study the potential of plumbene as a dilute magnetic semiconductor, we computationally investigate the structural, electronic, and magnetic properties of 4d transition metal (TM) doped plumbene using density functional theory (DFT). These calculations show that Zr, Nb, Mo, Tc-doped plumbene systems are magnetic while no magnetic solution was found for Y, Ru, Rh, and Pd-doped cases. We also calculate the magnetic couplings between two TM impurities in the system with an impurity concentration of less than 2%. Strong exchange couplings and large magnetic anisotropic energies, indicate the potential for spintronics applications.

Fano-Like Acoustic Resonance for Subwavelength Directional Sensing: 0-360 Degree Measurement.

Directional sound sensing plays a critical role in many applications involving the localization of a sound source. However, the sensing range limit and fabrication difficulties of current acoustic sensing technologies pose challenges in realizing compact subwavelength direction sensors. Here, a subwavelength directional sensor is demonstrated, which can detect the angle of an incident wave in a full angle range (0°âˆ¼360°). The directional sensing is realized with acoustic coupling of Helmholtz resonators each supporting a monopolar resonance, which are monitored by conventional microphones. When these resonators scatter sound into free-space acoustic modes, the scattered waves from each resonator interfere, resulting in a Fano-like resonance where the spectral responses of the constituent resonators are drastically different from each other. This work provides a critical understanding of resonant coupling as well as a viable solution for directional sensing.

Magnetic properties of 3d transition metal (Sc-Ni) doped plumbene.

Recently, a synthesized two-dimensional layer structured material, so-called "plumbene", has attracted research interests because of its sizeable spin-orbit coupling. To study the potential of this material as a dilute magnetic semiconductor, we computationally investigate the structure, electronic, and magnetic properties of 3d transition metal (TM) doped plumbene using density functional theory (DFT). These calculations show that Ti, V, Cr, Mn, Fe, and Co-doped plumbene systems are magnetic while no magnetic solution was found for Sc and Ni. We also calculate the magnetic couplings between two TM impurities in the system with an impurity concentration of less than 2%. For V, Mn, Fe, Co-doped systems with short inter-impurity distances, we obtain a Curie temperature above room temperature using the mean-field approximation, indicating the potential for magnetic storage and spintronics applications.

Damped resonance for broadband acoustic absorption in one-port and two-port systems.

We demonstrate broadband perfect acoustic absorption by damped resonances through inclusion of lossy porous media. By minimally placing the lossy materials around the necks of single-resonance Helmholtz resonators, where acoustic energy is concentrated, we show an increase in absorption bandwidths (>100% of the resonance frequency). Using the damped resonance, we demonstrate three types of broadband acoustic absorbers in one-port and two-port systems: broadband absorbers (one-port), broadband sparse absorbers (two-port), and broadband duct absorbers (two-port). Our approach for broadband absorption allows to minimize the number of resonances for compact absorbers, while it is beneficial for practical applications owing to the minimum use of porous materials.

Modal approach to optical forces between waveguides as derived by transformation optics formalism.

Optical force between two lossless waveguides has been described by two approaches. One approach is the explicit description of the force by the Maxwell stress tensor. Another approach is to describe the modal force in terms of the derivative of the eigenmode frequency with respect to the distance variation. Here, we analytically prove the equivalence of these two approaches for lossless waveguides having arbitrary cross sections through the use of transformation optics formalism. Our derivation is applicable to both pressure and shear forces. We also show that these two approaches are not equivalent in the presence of loss.

High-aspect-ratio mushroom-like silica nanopillars immersed in air: epsilon-near-zero metamaterials mediated by a phonon-polaritonic anisotropy.

Epsilon-near-zero metamaterials offer opportunities for intriguing electromagnetic-wave phenomena. Here we experimentally demonstrate that silica perpendicular nanopillars immersed in air exhibit a uniaxial epsilon-near-zero response mediated by phonon polaritons in the mid-infrared range. Unique mushroom-shaped heads on nanopillars play a crucial role to realize SiO2 metamaterials over a large area in our self-assembled fabrication process with block copolymers, polystyrene-block-poly(dimethylsiloxane) (PS-b-PDMS). SiO2 nanopillars having heights of 80 nm, 200 nm, and 300 nm (aspect ratios up to ∼13) are obtained after calcination at 450 °C and the electromagnetic responses are evaluated using a mid-infrared ellipsometric apparatus. For nanopillars with 200 nm height, the permittivity of the perpendicular component ε ⊥ approaches to near zero (0.2) while the parallel component ε ‖ shows a value of 1.8. The measured uniaxial epsilon-near-zero responses are excellently reproduced by the effective medium theory. Our results, therefore, indicate that SiO2 nanopillars/air uniaxial epsilon-near-zero metamaterials in the mid-infrared range can be amenable to large scale fabrication.

Significant Enhancement of Near-Field Electromagnetic Heat Transfer in a Multilayer Structure through Multiple Surface-States Coupling.

We show that near-field electromagnetic heat transfer between multilayer thermal bodies can be significantly enhanced by the contributions of surface states at multiple surfaces. As a demonstration, we show that when one of the materials forming the multilayer structure is described by the Drude model, and the other one is a vacuum, at the same gap spacing the resulting heat transfer can be up to 40 times higher as compared to that between two semi-infinite materials described by the same Drude model. Moreover, this system can exhibit a nonmonotonic dependency in its heat transfer coefficient as a function of the middle gap spacing. The enhancement effect in the system persists for realistic materials.

Surface effects on ionic Coulomb blockade in nanometer-size pores.

Ionic Coulomb blockade in nanopores is a phenomenon that shares some similarities but also differences with its electronic counterpart. Here, we investigate this phenomenon extensively using all-atom molecular dynamics of ionic transport through nanopores of about one nanometer in diameter and up to several nanometers in length. Our goal is to better understand the role of atomic roughness and structure of the pore walls in the ionic Coulomb blockade. Our numerical results reveal the following general trends. First, the nanopore selectivity changes with its diameter, and the nanopore position in the membrane influences the current strength. Second, the ionic transport through the nanopore takes place in a hopping-like fashion over a set of discretized states caused by local electric fields due to membrane atoms. In some cases, this creates a slow-varying 'crystal-like' structure of ions inside the nanopore. Third, while at a given voltage, the resistance of the nanopore depends on its length, the slope of this dependence appears to be independent of the molarity of ions. An effective kinetic model that captures the ionic Coulomb blockade behavior observed in MD simulations is formulated.

Light trapping for photovoltaic cells using polarization-insensitive angle-selective filters under monochromatic illumination.

We have proposed a light-trapping concept for photovoltaic (PV) cells under monochromatic illumination with restricted incident angles. We employed a configuration consisting of a shortpass filter (SPF) on the front surface and a diffuse reflector on the rear surface of the cell. The SPF was designed so that it functioned as a polarization-insensitive angle-selective filter. We fabricated 30-80-µm-thick crystalline silicon samples for incident angles changing within 30°, and analyzed the measured results using a ray-trace simulation with the Monte Carlo method. The ratio of the absorbed intensity to the 1064 nm illumination intensity was 0.69-0.85, which was higher than those equipped with antireflection coatings instead of the SPFs by 0.19-0.13. Thus, we have proven the light-trapping concept of the SPF/diffuse reflector configuration for monochromatic illumination. The PV cells could be applied to wireless power supply, in particular from solar-pumped lasers.

Dynamic Modulation of Radiative Heat Transfer beyond the Blackbody Limit.

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.

Simple and scalable preparation of master mold for nanoimprint lithography.

Nanoimprint lithography (NIL) is one of the most prominent bottom-up techniques for duplicating nanostructures with a high throughput. However, fabrication of starting master mold commonly requires expensive equipment of top-down techniques, or additional steps to transfer the fabricated patterns from bottom-up methods. Here we demonstrate that a SiO2 nanostructure manufactured from a self-assembled block copolymer, polystyrene-b-polydimethylsiloxane (PS-b-PDMS), directly serves as a master mold for NIL without further modification. A hexagonally aligned pattern over the entire substrate is established using a simple technique; solvent annealing and etching. Etching also plays an important role in endowing fluorine on the surface of SiO2, thus promoting smooth demolding upon imprinting. The obtained pattern of the SiO2 nanostructure is transferred to a polymer surface using UV nanoimprint. Identical patterns of the SiO2 nanostructure are elaborately reproduced on Ni and Cu nanodot arrays via electroplating on the polymer transcript, which was verified by morphological observations. The uniformity of the replicated Ni nanodot array is evaluated using spectroscopic ellipsometry. The measured optical response of the Ni nanodot is validated by electromagnetically simulated results, indicating that the pattern transfer is not limited to a small local area. In addition, the durability of the SiO2 mold pattern is corroborated after the imprinting process, thus guaranteeing the reusability of the fabricated nanostructure as a master mold. The proposed approach does not require any high-end lithographic techniques; this may result in significant cost and time reductions in future nanofabrication.

Invisibility cloak with image projection capability.

Investigations of invisibility cloaks have been led by rigorous theories and such cloak structures, in general, require extreme material parameters. Consequently, it is challenging to realize them, particularly in the full visible region. Due to the insensitivity of human eyes to the polarization and phase of light, cloaking a large object in the full visible region has been recently realized by a simplified theory. Here, we experimentally demonstrate a device concept where a large object can be concealed in a cloak structure and at the same time any images can be projected through it by utilizing a distinctively different approach; the cloaking via one polarization and the image projection via the other orthogonal polarization. Our device structure consists of commercially available optical components such as polarizers and mirrors, and therefore, provides a significant further step towards practical application scenarios such as transparent devices and see-through displays.

Extremely small wavevector regime in a one-dimensional photonic crystal heterostructure for angular transmission filtering.

One-dimensional photonic crystals (PCs), when operating near the band edge in the dispersion diagram, inherently possess nearly polarization-independent angular selectivity-an angular transmission window around the normal direction with reflection for other angles. However, the incident light is mostly reflected at the PC-air interface due to large impedance mismatch. We show that the reflection may be sufficiently suppressed by utilizing a specially designed antireflection structure consisting of a PC having a different pitch from that of the host PC. The underlying mechanism is that the interfaces of the antireflection PC with the host PC and the air structure are selected such that the transverse impedance has a real value, which is positioned at the center of the thickness of a material film. Moreover, our structure provides a high-throughput wide angular transmission window, including the normal direction in both s and p polarizations. We develop an analytical model that captures the angular selectivity observed in numerical results.

Densely-tiled metal-insulator-metal metamaterial resonators with quasi- monochromatic thermal emission.

Metal-insulator-metal metamaterial thermal emitters strongly radiate at multiple resonant wavelengths. The fundamental mode, whose wavelength is the longest among resonances, is generally utilized for selective emission. In this paper, we show that parasitic modes at shorter wavelengths are suppressed by newly employed densely-tiled resonators, and that the suppression enables quasi-monochromatic thermal emission. The second-order harmonics, which is excited at half the fundamental wavelength in conventional emitters, shifts toward shorter wavelength. The blue-shift reduces the amplitude of the second-order emission by taking a distance from the Wien wavelength. Other parasitic modes are eliminated by the small spacing between resonators. The densely-tiled resonators are fabricated, and the measured emission spectra agree well with numerical simulations. The methodology presented here for the suppression of parasitic modes adds flexibility to metamaterial thermal emitters.

Flat grating lens utilizing widely variable transmission-phase via guided-modes.

We experimentally demonstrate a polarization-independent flat grating lens in the near-infrared region. The grating lens consists of ridges in the square lattice arrangement, and the ridge dimensions are gradually changed to distribute a phase map with focusing ability. It is well known that guided modes in gratings offer unity-reflection at a resonance, and therefore the transmission phase is widely varied around the resonance. We employ such transmission phase behavior and show that high transmittance is obtained in each unit cell for wide variation range of the transmission phase at the operation wavelength by sharpening the resonance. This enables us to accomplish a highly efficient transmissive grating lens.