arXiv subjects

Hisashi Sumikura

Publications and source records attributed to Hisashi Sumikura.

12 recordsLinked to original sources

arXiv2026

Mid-infrared spontaneous and stimulated emission dynamics in black phosphorus

Black phosphorus (BP) has attracted attention as a light-emitting layered semiconductor for mid-infrared (MIR) photonics owing to its direct and thickness-tunable bandgap energy, highly anisotropic optical transitions, and potentially suppressed Auger recombination. However, spontaneous and stimulated emission dynamics governed by carrier recombination in BP have remained unexplored because time-resolved emission spectroscopy is challenging in the MIR region. Here we develop a time-resolved MIR emission microscope combining wavelength upconversion with superconducting single photon detection. This approach enables observation of emission dynamics in photoexcited BP at a wavelength around 4.6 μm with sub-100-ps temporal resolution. Temperature-dependent measurements reveal a crossover from excitonic to electron-hole plasma emission at around 70 K, supported by independent results in the characteristic transition temperature, pump-fluence dependence, and rise and decay dynamics of MIR emission. In a suspended BP structure, we further observe a nonlinear increase in the emission intensity above a well-defined pump threshold. Spectral narrowing and intense picosecond emission pulses appear above the threshold, providing solid evidence of stimulated emission assisted by optical feedback from a Fabry-Pérot cavity. These results elucidate ultrafast carrier dynamics and optical gain formation in BP and establish time-resolved upconversion spectroscopy as a powerful approach for investigating MIR photonic materials and devices.

physics.optics↗

arXiv2026

Non-Centrosymmetric $γ$-Phase GaS Nanobelts for On-Chip Nonlinear Photonic Applications

Second-order nonlinear optical processes in van der Waals (vdW) semiconductors offer a compelling route toward compact, integrable photon-conversion platforms. Among III-VI vdW semiconductors, GaS is particularly attractive owing to its wide bandgap suppressing two-photon absorption under near-infrared laser excitation. However, bulk GaS typically crystallizes in the centrosymmetric $β$ phase, which eliminates second-order nonlinearity and severely limits its application in nonlinear photonics. Here, we demonstrate that GaS nanobelts synthesized via self-catalyzed vapor-liquid-solid growth predominantly crystallize in non-centrosymmetric $γ$-phase stacking. This behavior originates from edge-selective growth kinetics at the Ga catalyst interface, which stabilizes the $γ$ phase and enables deterministic in-plane dipole moment alignment. The GaS nanobelts exhibit strong second-harmonic generation (SHG) with intensities comparable to those of GaSe, a widely used nonlinear optical material. Furthermore, we integrate the nanobelts onto SiN waveguides and demonstrate efficient on-chip SHG and sum-frequency generation. These results establish $γ$-GaS nanobelts as a transferable one-dimensional nonlinear materials well suited for on-chip photonic integration and indicate their strong potential for nonlinear optical applications.

physics.optics↗

arXiv2026

Inverse-Designed Non-Hermitian Hollow Nanowire Cavity for Generating Optical Orbital Angular Momentum

We designed a gallium nitride hexagonal hollow nanowire whispering gallery mode cavity that generates an |m|=6 topological light with orbital angular momentum (OAM). OAM is generated by breaking the cross-sectional mirror symmetry of the nanowire, which creates a non-Hermitian system. This is achieved by replacing the central airhole of the hollow nanowire with a cluster of 6 overlapping circular air holes with rotational offset relative to the hexagonal cross-sectional profile of the nanowire. The design parameters were then further optimized in Finite Element Method using an inverse design method to maximize the normalized OAM order |l|. We were able to realize of a cavity mode with |l| = 5.7, a mode purity of about 97%, and a Q-factor of ~250. This marks the first OAM generating active photonic device design falling within a sub-micron footprint, with additional novelties of being single component and materialistically homogeneous.

physics.optics↗

arXiv2026

Silicon photonic optical-electrical-optical converters based on load-resistor and current-injection operation

Optical-electrical-optical (OEO) converters are key primitives for low-latency, energy-efficient photonic computing because they enable nonlinear activation and optical signal regeneration on chip. We report two monolithically integrated silicon-photonic OEO converters-load-resistor (high-speed variant) and current-injection (high-gain variant) types-fabricated at a silicon photonics foundry. Each device combines a germanium photodetector with a micro-ring modulator (MRM). The converters exhibit reconfigurable nonlinear transfer functions and measurable on-chip RF OEO gain. The RF OEO gain scales linearly with the MRM bias power, with slopes of 0.10 mW^-1 (load-resistor of 10 kΩ) and 1.4 mW^-1 (current-injection), enabling a gain > 1 region at practical bias powers (~10 mW and ~1 mW, respectively). Eye diagrams confirm clear openings up to 4 Gb/s for a high-speed load-resistor variant with a 500-Ω load. To the best of our knowledge, this is the first experimental demonstration of a monolithically integrated, foundry-fabricated silicon-photonic load-resistor type OEO converter exhibiting reconfigurable nonlinear transfer and on-chip RF OEO gain. In the carrier-injection device, the activation slope exceeds unity, yielding 3.9 dB extinction-ratio regeneration. Short-pulse measurements yield 3-dB bandwidths of 1.49 GHz, 160 MHz (load-resistor of 500 Ω and 10 kΩ), and 76 MHz (current-injection), consistent with the RF data. Energy analysis shows an energy-bandwidth trade-off (RC-limited for load-resistor vs. lifetime-limited for injection) and outline routes to sub-pJ/bit operation via reduced capacitance and improved EO efficiency. These results establish silicon-photonic OEO converters as compact, foundry-compatible building blocks for scalable optoelectronic computing and optical neural networks.

physics.optics↗

arXiv2025

Low loss switchable topological photonic crystal enabled by submicron-scale patterning and phase-change of Sb2Se3

Photonic topological insulators (PTIs) offer robust platforms for light manipulation, but reconfigurable control of their topological properties without degrading performance remains a major challenge. While phase-change materials (PCMs) provide large refractive index modulation, widely used materials such as Ge2Sb2Te5 (GST) have been successfully deployed in commercial applications including optical data storage. However, they exhibit significant optical absorption in their crystalline state, which poses a challenge for transmissive photonic devices such as PTIs where high transparency is essential. Here, we overcome this fundamental limitation by integrating the ultra-low-loss PCM antimony triselenide (Sb2Se3) onto a silicon-based 2D PTI. We achieve submicron-scale selective patterning of Sb2Se3 on a photonic crystal for the first time, and demonstrate a topological phase transition induced by the material phase change. Owing to the transparency of Sb2Se3 in both its amorphous and crystalline states, a high Q-factor on the order of 10^3 is preserved-representing nearly an order-of-magnitude improvement over previous GST-based devices. This work resolves the absorption-loss bottleneck in reconfigurable PTIs and paves the way for practical, low-loss, tunable topological photonic devices.

physics.optics↗

arXiv2025

Observation of non-Hermitian point gap in photonic crystals

Non-Hermitian point gap (NHPG) is a unique phenomenon in non-Hermitian systems and induces non-Hermitian skin effect (NHSE). In photonic crystals, NHPG and the NHSE have previously been explored mainly through material loss, where the typically low $Q$ factors make direct observation of complex frequencies challenging. Here, we demonstrate the direct experimental observation of an NHPG by using a radiation-loss-based non-Hermitian photonic crystal. Radiation loss can be engineered through structural design, enabling control of the imaginary part of the complex frequency and allowing relatively high $Q$ factors. This approach is compatible with widely used absorption-free silicon-slab photonic crystals. We developed a measurement system that can measure photonic bands along arbitrary lines in $k$-space. Our measurements demonstrated direct observation of the NHPG in photonic crystals, and the reversal of non-Hermitian topology through the flip of loop rotation in a complex plane. Our platform, which requires neither gain media nor synthetic dimensions, establishes radiation-loss engineering as a simple and versatile route for photonic functionality using an NHSE in nanophotonic systems.

physics.optics↗

arXiv2025

On-chip room-temperature CW lasing from a III-V nanowire integrated with a Si photonic crystal platform

We report the demonstration of continuous-wave (CW) lasing at room temperature from a III-V semiconductor nanowire integrated into a Si photonic crystal (PhC) cavity. Conventional hybrid nanowire lasers [M. Takiguchi. et.al., APL Photonics, 2, 046106 (2017)], which typically feature circular nanowire-cross-sections, suffer from a weak optical confinement, preventing CW lasing under ambient conditions. To overcome this limitation, we fabricated nanowires with rectangular cross-sections via dry etching and integrated them into the air trenches of Si PhC cavities formed using atomic force microscope tips. This configuration forms a hybrid photonic crystal cavity with an improved optical confinement. As a result, we achieved room-temperature CW oscillation from a single nanowire, representing a significant step toward on-chip nanophotonic light sources. This unique in-plane integration of the nanolaser in the same plane as the Si slab rather than on top of the substrate will contribute to the development of compact, scalable, and CMOS-compatible photonic circuits.

physics.optics↗

arXiv2023

Vector beam generation from standing hollow GaN nanowire lasers on sapphire substrate

We fabricated GaN based hollow nanowires standing upright on a sapphire substrate by the sublimation method and found that they exhibit laser oscillation at room temperature. These very long, hollow, nano-sized structures cannot be fabricated by other means. Furthermore, we determined the condition under which the fundamental mode is azimuthally polarized by investigating the dispersion of the hollow structure. Examination of the measured emission properties indicates that the hollow nanowire operates as a topological, vector-beam, light source.

physics.optics↗

arXiv2019

Ultrafast and energy-efficient all-optical switching with graphene-loaded deep-subwavelength plasmonic waveguides

All-optical switches have attracted attention because they can potentially overcome the speed limitation of electric switches. However, ultrafast, energy-efficient all-optical switches have been challenging to realize due to the intrinsically small optical nonlinearity in existing materials. As a solution, we propose graphene-loaded deep-subwavelength plasmonic waveguides (30 nm x 20 nm). Thanks to extreme light confinement, we have significantly enhanced optical nonlinear absorption in graphene, and achieved ultrafast all-optical switching with a switching energy of 35 fJ and a switching time of 260 fs. The switching energy is four orders of magnitudes smaller than that in previous graphene-based devices and is the smallest value ever reported for any all-optical switch operating at a few picoseconds or less. This device can be efficiently connected to conventional Si waveguides and employed in Si photonic integrated circuits. We believe that this graphene-based device will pave the way towards on-chip ultrafast and energy-efficient photonic processing.

physics.optics↗

arXiv2019

Simulation technique of quantum optical emission process from multiple two-level atoms based on classical numerical method

In this paper, we report a numerical method for analyzing optical radiation from a two-level atom. The proposed method can consistently consider the optical emission and absorption process of an atom, and also the interaction between atoms through their interaction with a radiation field. The numerical model is based on a damping oscillator description of a dipole current, which is a classical model of atomic transition and is implemented with a finite-difference time-domain method. Using the method, we successfully simulate the spontaneous emission phenomena in a vacuum, where the interaction between an atom and a radiated field plays an important role. We also simulate the radiation from an atom embedded in a photonic crystal (PhC) cavity. As a result, an atom-cavity field interaction is sucessfuly incorporated in the simulation, and the enhancement of the optical emission rate of an excited atom is explained. The method considers the effect of the interaction between atoms through the radiated field. We simulate the optical emission process of the multiple atoms and show that an enhancement of the emission rate can occur owing to the an atom-atom interaction (superradiance)(R. H. Dicke, Phys. Rev. {\bf 93}, 99[1954]). We also show that the emission rate is suppressed by the effect of the destructive dipole-dipole interaction under an out-of-phase excitation condition (subradiance).

physics.optics↗

arXiv2019

All-optical dynamic modulation of spontaneous emission rate in hybrid optomechanical cavity quantum electrodynamics systems

Recent nanofabrication technologies have miniaturized optical and mechanical resonators, and have led to a variety of novel optomechanical systems in which optical and mechanical modes are strongly coupled. Here we hybridize an optomechanical resonator with two-level emitters and successfully demonstrate all-optical dynamic control of optical transition in the two-level system by the mechanical oscillation via the cavity quantum-electrodynamics (CQED) effect. Employing copper-doped silicon nanobeam optomechanical resonators, we have observed that the spontaneous emission rate of excitons bound to copper atoms is dynamically modulated by the optically-driven mechanical oscillation within the time scale much shorter than the emission lifetime. The result is explained very well with an analytical model including the dynamic modulation of the Purcell effect and the exciton population. To the best of our knowledge, this is the first demonstration of a dynamic modulation of the spontaneous emission rate by mechanical oscillations. Our achievement will open up a novel field of hybrid optomechanical CQED systems in which three body--optical transitions, optical resonance modes, and mechanical resonance modes--are strongly coupled and will pave the way for novel hybrid quantum systems.

physics.optics↗

arXiv2010

All-silicon sub-Gb/s telecom detector with low dark current and high quantum efficiency on chip

We demonstrate channel selective 0.1-Gb/s photo-receiver operation at telecom wavelength using a silicon high-Q photonic crystal nanocavity with a laterally integrated p-i-n diode. Due to the good crystal property of silicon the measured dark current is only 15 pA. The linear and nonlinear characteristics are investigated in detail, in which we found that the photo-current is enhanced of more than 100,000 due to the ultrahigh-Q (>100,000). With the help of two-photon absorption, which is visible at a surprisingly low input power of 10 nW, the quantum efficiency of this device reaches about 10%.

physics.optics↗