Table of contents

Photo-electrochemical (PEC) etching was used to fabricate deep trench structures in a GaN-on-GaN epilayer grown on n-GaN substrates. A 50-nm-thick layer of Ti used for an etching mask was not removed even after etching to a depth of >30 µm. The width of the side etching was less than 1 µm with high accuracy. The aspect ratio (depth/width) of a 3.3-µm-wide trench with a PEC etching depth of 24.3 µm was 7.3. These results demonstrate the excellent potential of PEC etching for fabricating deep trenches in vertical GaN devices.

The presence of an extra Al metal in an autoclave tremendously improved the overall quality of m-plane GaN single crystals grown by the acidic ammonothermal method using an NH₄F mineralizer. Although the growth rate was commonly decreased by adding an extra metal such as Al, Si, Ca, or Ti, the crystal coloration was mostly suppressed and the crystal mosaics were decreased, and the near-band-edge excitonic fine structure was observed in the low-temperature photoluminescence spectrum only when Al was present. The results likely indicate that the extra Al suppressed the incorporation of oxygen into m-planes of GaN owing to the oxygen gettering effect.

We report the internal structures and emission properties of GaN/AlN single- and multiple-quantum-well (QW) heterostructures with well widths of d_w = 1–4 monolayers (MLs), grown by plasma-assisted molecular-beam epitaxy on c-sapphire at metal-rich conditions and low temperatures (∼700 °C). The formation of plane QWs with abrupt symmetrical interfaces is confirmed by both scanning transmission electron microscopy and X-ray diffraction analysis. Pulse-scanning and continuous-wave output powers of 150 and 28 mW, respectively, at a peak emission wavelength of 235 nm were achieved at 300 K in an electron-beam-pumped deep-ultraviolet (1.5 ML-GaN/5.5 nm-AlN)₃₆₀ multiple-QW emitter with a maximum efficiency of 0.75%.

We explored the passive–active oxidation boundary for the thermal oxidation of a 4H-SiC(0001) surface. The O₂ partial pressure [P(O₂)] for passive–active transition was found to be around 0.03 and 0.3 kPa at 1500 and 1600 °C, respectively. We also found that the passive–active oxidation boundary for an Al-implanted surface shifted to a slightly higher P(O₂). A metal–oxide–semiconductor field-effect transistor with a gate oxide formed under P(O₂) of 0.3 kPa at 1600 °C demonstrated a field-effect mobility of 9.7 cm² V⁻¹ s⁻¹, which was three times higher than that for a reference sample oxidized in atmospheric oxygen ambient at a moderate temperature and a high threshold voltage stability.

Photocurrent in a 4H-SiC p–n junction diode under illumination with sub-bandgap light was investigated. Under a high reverse bias condition, the photocurrent significantly increased with an increase in the reverse bias voltage. We calculated the photocurrent taking into consideration the phonon-assisted optical absorption due to the Franz–Keldysh effect. The calculated photocurrent showed good agreement with the experimental results. The photocurrent also increased at elevated temperatures, which could be quantitatively explained by the redshift of the 4H-SiC absorption edge (the shrinkage of the bandgap) and the increase in the phonon occupation number with rising temperature.

The asymmetric light reflectance behavior arising from the Fano interference between Fresnel reflection and localized surface plasmons (LSPs) is investigated. Finite-difference time-domain (FDTD) simulation results demonstrate that, when light is incident from air, reflectance spectra show peaks at the LSP resonance wavelength regardless of the metal nanoparticle density. When light is incident from the substrate, reflectance spectra show typical Fano profiles. This phenomenon can be attributed to different reflectance phase shifts induced when light is incident from different directions. Experiments are conducted with Ag-nanoparticle-coated quartz wafers. The measured spectra are in good agreement with the simulated results.

A cylindrically symmetric plano-concave lens based on a one-dimensional (1D) Thue–Morse photonic quasicrystal (PQC) is proposed. Subwavelength focusing characteristics with an incident cylindrical vector beam are investigated. Numerical results reveal that the lens can exhibit polarization-independent, high-resolution, and low-side-lobe characteristics, simultaneously for near-field and non-near-field. Furthermore, the lens can achieve perfect focusing and size-independent characteristics with equal-proportion scaling of the size of the lens at the corresponding center frequency. The focusing mechanisms of the lens are explained by analyzing the transmission spectrum, dispersion relation of a 1D periodic photonic crystal, and transmission spectrum of the 1D Thue–Morse PQC.

We describe the fabrication of a large area of chromium nanogratings formed by laser focusing techniques. The focusing occurs in an elliptical laser standing-wave field, resulting in a line array covering an area of approximately 1.5 × 1.5 mm² with a spacing of 212.78 nm. The experimental arrangement for laser focusing is described. Improvements in the technique and potential applications are also mentioned.

We experimentally demonstrate a broadband terahertz (THz) intensity modulator operating at low current and frequency range. The modulator consists of VO₂ and meander–wire hybrid metamaterials. The measured 3-dB bandwidth for normal incidence is 0.5 THz, while the maximum modulation depth of 99% is achieved at 0.28 A. The physical mechanism of the device's electrical tunability is attributed to an ohmic-heating-induced transition. An equivalent circuit model is proposed for the upper cut-off frequency. This scheme paves the way for a full electric control of the intensity of a THz wave at a broad band, low current, and low frequency range.

A magnetically and electrically polarization-tunable terahertz emitter that integrates a ferromagnetic heterostructure and large-birefringence liquid crystals is demonstrated. The heterostructure and the liquid crystal cell act as the broadband terahertz source and the phase retarder, respectively. The polarization state is switched between linear and circular by changing the direction of the external magnetic field. The phase retardation for frequencies higher than 1 THz is continuously adjustable over a range of π/2 by applying a low voltage. This compact, broadband, economical, and easy-to-regulate terahertz emitter can be widely used in polarization-sensitive research and engineering applications.

We propose and experimentally demonstrate a mode-splitting device using a microring resonator (MRR) with a feedback coupled waveguide (FCW). For the proposed device, a pair of counter-propagating coupled modes exist in the MRR owing to the FCW, which is different from traditional ways of introducing roughness or center defects. We obtain the splitting of the transmission spectrum with a splitting wavelength of 0.37 nm and a notch depth of 21.87 dB. Moreover, the temporal coupled-mode theory is adopted to fit the experimental results and analyze the dependence of the transmission spectrum on the coupling coefficient κ. The experimental results are in good agreement with the simulations.

Traditional waveguide directional couplers based on coherent interferences of multiple elements suffer from a narrow operation band. Furthermore, most of them only operate for a specific linear polarization and cannot fulfill the bidirectional sorting function. In this study, using the spin–orbit interaction of a single optical catenary on an integrated silicon waveguide, we demonstrated an ultra-broadband directional bidirectional router for circular polarization, with an extinction ratio higher than 15 dB in the wavelength range of 1,450 to 1,600 nm and a peak value of 25 dB at 1,550 nm. By reversing the spin of incidence, the light flow can be routed to the opposite direction.

We investigate photo-induced lattice strains of a monoclinic VTe₂ thin flake using ultrafast electron diffraction. After photoexcitation by a 190 fs pulse, we observe diffraction intensity oscillations with periods of 35 and 75 ps, which indicate coherent acoustic phonons of two distinct branches. The oscillations of $9\bar{1}0$ and $\bar{9}10$ diffraction intensities have opposite signs, indicating the change of the diffraction angle due to the shear strain. By numerically simulating these diffraction intensities as a function of the monoclinic angle, we evaluate the amplitude of the photo-induced shear strain.

We mathematically model the thermal lens effect of Ti:sapphire for use in a high-power laser pulse amplifier. The model enables more accurate prediction with new interpretations and offers simplified equations for the optical path difference and thermally induced focal length. Our model is validated through comparisons with measurements of existing high-power laser facilities. Further, we apply the model to a 2 PW, 10 Hz Ti:sapphire laser amplifier design.

Quantum logic gates are important for quantum computation and quantum information processing in numerous physical systems. Although time-bin qubits are suitable for quantum communication over optical fiber, many essential quantum logic gates for them have not yet been realized. Here, we demonstrated a controlled-phase (C-Phase) gate for time-bin qubits that uses a 2 × 2 optical switch based on an electro-optic modulator. A Hong–Ou–Mandel interference measurement showed that the switch could work as a time-dependent beam splitter with a variable splitting ratio. We confirmed that two independent time-bin qubits were entangled as a result of C-Phase gate operation with the switch.

We studied the spin–orbit torque in heavy metal/ferromagnetic metal bilayers using the magneto-optical Kerr effect. A double-modulation technique is developed to separate signals from the spin–orbit torque and Joule heating. At a current density of ∼1 × 10¹⁰ A/m², we observe optical signals that scale linearly and quadratically with the current density, both in similar magnitude. The spin–orbit torque estimated using this technique is consistent with that evaluated using spin-transport measurements. We find that changes in the refractive index of the film with temperature are the main source of the heating-induced signal.

Candidates for new thermoelectric and superconducting materials, which have narrow band gaps and flat bands near band edges, were searched by high-throughput first-principles calculation from an inorganic materials database. The synthesized SnBi₂Se₄ among the target compounds showed a narrow band gap of ∼200 meV and a thermal conductivity of ∼1 W·K⁻¹·m⁻¹ at ambient pressure. The sample SnBi₂Se₄ showed a metal–insulator transition at 11.1 GPa, as predicted by theoretical estimation. Furthermore, two pressure-induced superconducting transitions were discovered under 20.2 and 47.3 GPa. The data-driven search is a promising approach to discovering new functional materials.

Critical temperatures (T_c) comparable to those of bulk materials were achieved by post-annealing MgB₂ thin films grown at a low temperature of 280 °C. The T_c was improved to 36.7 K under annealing conditions of 550 °C for 50 h or more. Under these annealing conditions, a critical current density several tens of times higher at than that of MgB₂ wires processed by a powder-in-tube method was achieved at 20 K under 5 T. This is the highest value reported in MgB₂ bulk wires and films. Film-based MgB₂ is a promising candidate for next-generation MgB₂ wires.

High-performance InGaN-channel high-electron-mobility transistors (HEMTs) are fabricated and investigated in detail. The transconductance exhibits a high stability over a wide range of gate voltages, indicating excellent operation linearity. The relative saturation output current densities are 81 and 68% when the temperature increases to 400 and 500 K, respectively, with respect to the value of 1128.2 mA/mm at 300 K. In addition, the breakdown voltage reaches 187 V at 300 K, which is comparable to that of a GaN-channel HEMT. The presented results demonstrate the large potentials of the InGaN-channel HEMT in high-frequency power applications.

The electronic structures and optical gain of GaAs_1−xN_x nanowires are calculated via the band anticrossing model together with the eight-band k · p theory. We find that the optical gain spectra show an obvious red shift, and the gain increases slightly with increasing nitrogen content. The transverse magnetic (TM) gain is approximately 8.5 times larger than the transverse electric (TE) gain when the radius R is 3 nm, which indicates that GaAs_1−xN_x nanowires can be used as TM linearly polarized lasers in the near-infrared range. However, when R is 6 nm, the TM gain approaches the corresponding TE gain. In this case, GaAs_1−xN_x nanowires are not suitable for linearly polarized lasers.

Isolated single metal atoms have attracted significant attention as catalytic materials with maximum surface areas as well as high activity and selectivity. However, a dense dispersion of single atoms on supported materials has not been achieved. We applied plasma sputtering in N₂ atmosphere to disperse single Pt atoms on graphene. Scanning transmission electron microscopy observations confirmed the high-density dispersion of single Pt atoms, without widespread sintering into three-dimensional nanoparticles. X-ray photoelectron spectroscopy measurements revealed that pyridinic nitrogen was selectively doped in graphene during the sputtering. Our novel method provides a simple approach to synthesize high-density single Pt atoms on graphene.

We theoretically investigated the variations in the characteristics of graphene-nanoribbon-based field-effect transistors (GNR FETs) using the nonequilibrium Green's function method. In this study, the drain current (I_d) was calculated as a function of gate voltage (V_g) for GNR FETs with various edge disorder concentrations (P). From the obtained I_d–V_g curves, we estimated the device characteristics. We found that the variations of these device characteristics became larger with increasing P, as evidenced by a dramatic change in the shapes of their histograms. Furthermore, we clarified that these variations were caused by Anderson localization originating from the edge disorder.

A heterostructure formed of topological and nontopological half-Heusler compounds provides a means to tune band inversion. In this work, we study the stabilities of [111] and [100] LuPtBi/LuAuSn half-Heusler superlattices. Since the [111] superlattice is more stable than the [100] one, we explore the band inversion in the [111] superlattice. By increasing the number of LuPtBi layers, the energy gap can be tuned from 0.63 to −1.50 eV, where the negative value indicates band inversion. The underlying mechanism involves band alignment, internal electric field, and quantum confinement. Our study offers an efficient means to obtain the desired band inversion in half-Heusler heterostructures.

Carrier recombination in lateral composition modulation (LCM) GaInP was probed in detail using time-resolved photoluminescence (TR-PL) and transmission electron microscopy (TEM). Upon SiO₂ passivation, the time-transient decay of the PL peak was slower, and carrier lifetime was significantly enhanced from 25 to 230 ps for passivated LCM GaInP in comparison with that of bulk GaInP. This is due to the suppressed surface recombination of the irregular and wavy surface of LCM GaInP observed by TEM. Temperature-dependent TR-PL also showed a large decrease in carrier lifetime with an increase in temperature, indicating the dominance of Shockley–Read–Hall recombination due to the nonperiodicity of the LCM structure.

We report on the systematic investigation of hot carrier dynamics in Ti₄O₇ by ultrafast time-resolved optical reflectivity. We find the transient indication for its two-step insulator–metal (I–M) transition, i.e., from the long-range ordered bipolaron low-temperature insulating phase to the disordered bipolaron high-temperature insulating phase at T_c1 and to the free-carrier metallic phase at T_c2. Our results reveal that photoexcitation can effectively reduce both T_c1 and T_c2 by increasing the pump fluence, allowing the light-control of the I–M transition. We address a phase diagram that provides a framework for the photoinduced I–M transition and helps the potential use of Ti₄O₇ for photoelectric and thermoelectric devices.

The hybrid photomagnetic (PMA) stimulation effect, which was found to be significantly stronger than stand-alone magnetic and photothermal responses, was characterized on magnetite/gold-nanoparticle-deposited dextran-covered carbon nanotube structures (DIGCNTs). The designed nanostructures demonstrated excellent biocompatibility on a neural cell model; however, the viability of tumor suppressor p53-deficient NE-4C cells was severely affected when activated by low-intensity PMA stimulation (50 Oe, 155 kHz; 400 mW/cm², 530 nm laser) at a concentration as low as 50 µg/mL. The acute toxicity of this unique hybrid actuation-hybrid material combination, coupled with excellent innate biocompatibility, may be extremely relevant in the development of in vivo hyperthermia.