arXiv subjects

Yasutomo Ota

Publications and source records attributed to Yasutomo Ota.

At least 19 recordsLinked to original sources

arXiv2026

High-Q Magneto-Optical Microdisk Resonators Based on Monocrystalline Bismuth-substituted Yttrium Iron Garnets

Microresonators based on diverse material platforms have driven the progress of integrated photonics by enhancing light-matter interactions in compact volumes. Extending this strategy to magneto-optical (MO) media is highly attractive for nonreciprocal photonics, magnonics, and quantum transduction, yet compact high-Q resonators based on single-crystalline yttrium iron garnet (YIG) have remained elusive because of the difficulty of its nanofabrication. Here we demonstrate high-Q, small-mode-volume MO microdisk resonators based on monocrystalline Bi-substituted YIG (Bi:YIG). Low-loss Bi:YIG thin films prepared by bonding and thinning, combined with optimized Ar-plasma etching, enable submicron-thick, air-suspended microdisks that support telecom-band whispering-gallery modes with $Q = 1.52\times 10^5$, a mode volume $V \approx 200$ cubic wavelengths, and $Q/V \approx 760$, representing an orders-of-magnitude improvement for integrated high-Q YIG MO cavities. The device exhibits clear magnetic-field-dependent spectral responses, including reciprocal resonance shifts and nonreciprocal frequency splitting of counter-propagating modes. A perturbative analytical model quantitatively reproduces these responses and indicates an effective MO coefficient about twice the bulk value, suggesting a possible miniaturization-induced enhancement. These results establish monocrystalline YIG microdisks as a compact high-Q MO platform for integrated isolators, optomechanics, microcombs, and photon-magnon quantum interfaces.

physics.optics↗

arXiv2026

Photonic Crystal Defect Nanocavities Based on Monocrystalline Yttrium Iron Garnet

Monocrystalline yttrium iron garnet (YIG) is a key material for magneto optics and quantum magnonics owing to its high optical transparency, large magneto-optical (MO) effects at room temperature, and exceptionally-long spin coherence. While rich MO phenomena have been demonstrated in the microwave regime, extending these concepts to technologically important telecommunication wavelengths remains challenging due to the difficulty of fabricating high-quality YIG nanostructures. Here, we demonstrate photonic crystal (PhC) defect nanocavities based on monocrystalline Bi-substitudted YIG by developing a YIG-on-insulator platform and high-precision YIG nanopatterning. The fabricated nanocavities exhibit cavity resonances around lambda = 1500 nm with Q factors up to 1,800 and a mode volume V of 1.1(lambda/n)^3, corresponding to Q/V reaching around 10^3. The measured Q factor is primarily governed by intentionally introduced lattice modulations for out-of-plane light coupling, suggesting that further optimization of the cavity geometry and measurement configuration could yield an order-of-magnitude improvement of the experimental Q factor. YIG-based PhC nanocavities provide a platform for strongly confined light-magnetism interactions in the optical regime, opening pathways toward downsized nonreciprocal photonic devices and enhanced photon-magnon coupling.

physics.optics↗

arXiv2026

Arbitrary Polarization Generation in Magneto-optical Metasurfaces Enabled by Bound States in the Continuum

The generation of arbitrary polarization states of light is essential for optical communication and photonic information processing. Photonic crystal and metasurface platforms supporting bound states in the continuum (BICs) provide a powerful route for polarization engineering through tailoring the radiation from the resonant modes. However, existing approaches typically rely on static structural symmetry breaking or off-normal radiation, which limits continuous polarization tuning of vertical radiation. Here, we demonstrate a magnetooptical metasurface that generates arbitrary polarization states of light at normal radiation. By applying an external magnetic field with variable rientation, a symmetry-protected BIC is transformed into a quasi-BIC whose radiation polarization can be continuously tuned. The magneto-optical perturbation drives the controlled migration of polarization singularities in momentum space, allowing the emitted states to continuously span the entire Poincaré sphere without structural modification. This approach establishes a compact platform for actively tunable polarization sources and polarizationencoded photonic devices.

physics.optics↗

arXiv2026

Quantum dot single photon source on SiN integrated with coupled crossover waveguides

Hybrid integration of InAs/GaAs quantum dot (QD) single-photon sources (SPSs) is a promising approach for introducing quantum light into SiN photonic integrated circuits. However, the large refractive-index mismatch between GaAs and SiN poses a challenge for efficient optical coupling. Here, we propose and experimentally demonstrate hybrid integration of an InAs/GaAs QD-SPS on SiN using a coupled crossover waveguide structure. A photonic crystal nanocavity is employed for coupling QD emission into a GaAs waveguide, which efficiently transfers photons to a SiN waveguide at the crossover section. We observed Purcell-enhanced single-photon emission, on-chip propagation, and outcoupling through a SiN grating coupler.

physics.optics↗

arXiv2025

BIC slow light waveguides based on interband coupling

Harnessing bound states in the continuum (BICs) for guiding light in leaky environments has unlocked new possibilities in photonic integrated circuits. BIC confinement enables low-loss waveguiding of leaky transverse-magnetic (TM) modes in etchless waveguides based on dielectric wires loaded on plane slabs. We have recently reported BIC slow light waveguides by introducing one-dimensional photonic crystals into such etchless waveguides. However, they were restricted to a high-symmetry point ($X$ point), limiting their applicability. In this Letter, we propose and numerically demonstrate BIC slow light waveguides at off-high-symmetry points by exploiting Friedrich-Wintgen BICs, arising from the interband coupling of two guided modes sharing a radiation continuum. We identified a systematic approach for tuning the loss minimum position in momentum space and simultaneously achieved a high group index over $100$ and a low propagation loss of less than $5 \times 10^{-2}~\mathrm{dB/cm}$ at an off-high-symmetry point. Our findings pave the way for advanced control of light-matter interactions in non-Hermitian photonic systems.

physics.optics↗

arXiv2025

Design of Ultrathin Faraday Rotators based on All-dielectric Magneto-optical Metasurfaces at the Telecommunication Band

Magneto-optical (MO) interactions offer a direct route to nonreciprocal optical devices but are intrinsically weak in the optical domain, posing a major challenge in downsizing MO functional devices. In this study, we present a design strategy for ultra-thin MO Faraday rotators based on all-dielectric metasurfaces supporting high-quality factor quasi-bound states in the continuum (QBIC) modes. Light trapping in QBIC modes induced by band folding significantly enhances MO interactions in a controllable manner, enabling a technologically relevant 45$^\circ$ Faraday rotation with a MO metasurface that is only a few hundred nanometers thick. The design also incorporates electromagnetically induced transparency via spectrally overlapping resonant modes to achieve high light transmittance reaching 80%. This approach not only enables compact yet practical MO Faraday rotator but also holds promises for advancing free-space magnetic sensors and MO modulators.

physics.optics↗

arXiv2025

Experimental realization of wide-mode-area slow light modes in valley photonic crystal heterostructure waveguides

We experimentally realized wide-mode-area slow-light modes in valley photonic crystals (VPhCs) heterostructure waveguides. The waveguides are fabricated on a silicon slab by inserting gapless photonic graphene layers with varying widths and modifying the unit cell spacing near the domain walls. By reducing the spacing between unit cells at the domain boundaries, slow-light guided modes are achieved in VPhCs heterostructure waveguides. The presence of wide-mode-area modes is verified by observing the radiation in light propagation of leaky guided modes above the light line. To characterize guided modes below the light line, we introduce air-slot terminations to induce out-of-plane scattering and measure intensity profiles. The results show that the mode widths are tunable for both fast-light and slow-light modes in VPhCs heterostructure waveguides by adjusting the number of photonic graphene layers. The ability to support wide-mode-area slow-light modes in VPhC heterostructures offers promising opportunities for the development of high-power, on-chip photonic integrated devices.

physics.optics↗

arXiv2025

Topological Corner States in Bilayer and Trilayer Systems with Vertically Stacked Topological Heterostructures

We investigate bilayer and trilayer systems composed of topologically distinct, vertically stacked layers, forming topological heterostructures based on the Benalcazar-Bernevig-Hughes model. We find that a topological phase transition induced by interlayer coupling significantly alters the number of corner states in these topological structures. Furthermore, we find that traditional nested Wilson loop analysis inaccurately classifies certain phases, leading us to evaluate multipole chiral numbers (MCNs) as a more appropriate topological invariant for this scenario. The MCNs not only enable accurate classification of topological phases but also directly correspond to the number of zero-energy corner states, effectively characterizing $\mathbb{Z}$-class HOTI phases. Our study proposes the novel concept of topological heterostructures, providing critical insights into the control of localized corner states within multilayer systems and expanding potential research directions.

cond-mat.mes-hall↗

arXiv2025

Slow Light Waveguides based on Bound States in the Continuum

The concept of bound states in the continuum (BIC) has been advancing light confinement technology in leaky environments. In this letter, we propose and numerically demonstrate a slow light waveguide based on a BIC mode. We considered a waveguide with a polymer core loaded on a plane slab, which supports a leaky guided mode coupled to the radiation continuum in the slab. We found that periodic modulation of the polymer core along the propagation direction can result in a high group index mode with a low propagation loss due to BIC confinement. The introduction of one-dimensional photonic crystals into the BIC waveguides will largely expand its functionality and applications in integrated photonics.

physics.optics↗

arXiv2024

On-Chip Optical Skyrmionic Beam Generators

Optical skyrmion beams, which encompass two-dimensional topology in their spatial structures, are promising for ultra-dense optical communications and advanced matter manipulation. Generating such light beams via a chip-based approach will vastly broaden their applications and promote the advancement of untapped fundamental science. Here, we present a breakthrough in chip-based technology by experimentally demonstrating on-chip devices capable of generating optical skyrmions with tailored topological invariants. These devices, fabricated with high precision, exhibit behavior that closely aligns with theoretical predictions and numerical simulations. The realization of on-chip optical skyrmion beam generators ushers a new dawn in optical and material science.

physics.optics↗

arXiv2024

Space-Time Hopfion Crystals

Hopfions, higher-dimensional topological quasiparticles with sophisticated 3D knotted spin textures discovered in condensed matter and photonic systems, show promise in high-density data storage and transfer. Here we present crystalline structures of hopfions lying in space-time constructed by spatiotemporally structured light. A practical methodology using bichromatic structured light beams or dipole arrays to assemble 1D and higher dimensional hopfion lattices is proposed and a technique for tailoring topological orders is elucidated. The birth of photonic hopfion crystals heralds a new era in high-dimensional, condensed, and robust topological information processing.

physics.optics↗

arXiv2024

Topological Laser in Anomalous Quadrupole Topological Phases

Topological photonics shows considerable promise in revolutionizing photonic devices through the use of topological phases, leading to innovations like topological lasers that enhance light control. One of recent breakthroughs is reducing the size of these systems by utilizing lower-dimensional boundary states, notably via higher-order topological phases. This paper presents the first experimental demonstration of topological laser in anomalous quadrupole topological phase, an instance of higher-order phases. To facilitate this, a topological nanocavity with quality factor near 6,000 is engineered through a twisting operation. The topological nature of our system is validated by calculation of nested Wannier center and the emergency condition of corner states. Our experimental observations reveal the manifestation of corner states and the achievement of single-mode pulsed laser, driven by optical gain from multiple quantum wells at telecommunication wavelengths and at a temperature of 4 K. A lasing threshold of 23 uW and a cold quality factor of 1,500 are deduce through rate equation. Our work gives a new potential in the application of topological principles to advance nanophotonic technologies.

physics.optics↗

arXiv2024

Wide-mode-area slow light waveguides in valley photonic crystal heterostructures

We designed slow-light waveguides with a wide mode area based on slab-type valley photonic crystal (VPhC) heterostructures which are composed of a graphene-like PhC sandwiched by two topologically distinct VPhCs. The group velocity of the topological guided mode hosted in a VPhC heterostructure can be slowed down by shifting the VPhC lattice toward the graphene-like PhC at the domain interfaces. Simultaneously, the mode width of the slow-light topological guided mode can be widened by increasing the size of the graphene-like PhC domain. We found that employing the graphene-like structure at the center domain is crucial for realizing a topological single-guided mode in such heterostructures. Furthermore, the impact of random fluctuations in air-hole size in the graphene-like domain was numerically investigated. Our simulation results demonstrate that the transmittance for the slow-light states can be kept high as far as the size fluctuation is small although it drops faster than that for fast-light states when the disorder level increases. The designed wide-mode-area slow-light waveguides are based on hole-based PhCs, offering novel on-chip applications of topological waveguides.

physics.optics↗

arXiv2023

Efficient light couplers to topological slow light waveguides in valley photonic crystals

We numerically and experimentally demonstrate efficient light couplers between topological slow light waveguides in valley photonic crystals (VPhCs) and wire waveguides. By numerical simulations, we obtained a high coupling efficiency of -0.84 dB/coupler on average in the slow light regime of a group index ng = 10 - 30. Experimentally, we fabricated the couplers in a Si slab and measured the transmitted power of the devices. We realized a high coupling efficiency of approximately -1.2 dB/coupler in the slow light region of ng = 10 - 30, which is close to the result from the numerical simulations. These demonstrations will lay the groundwork for low-loss photonic integrated circuits using topological slow light waveguides.

physics.optics↗

arXiv2023

High-Q two-dimensional photonic crystal nanocavity on glass with an upper glass thin film

We numerically analyze two-dimensional photonic crystal (PhC) nanocavities on glass with a thin glass film on top of the structure. We investigated a multi-step heterostructure GaAs PhC nanocavity located on glass. We found that covering the structure even with a very-thin glass film efficiently suppresses unwanted polarization mode conversion occurring due to the asymmetric refractive index environment around the PhC. We also uncovered that the glass-covered structure can exhibit a higher Q factor than that observed in the structure symmetrically cladded with thick glass. We point out that the mode mismatch between the PhC nanocavity and modes in the upper glass film largely contributed to the observed Q-factor enhancement. These observations were further analyzed through the comparison among different types of on-glass PhC nanocavites covered with thin glass films. We also discuss that the in-plane structure of the upper glass film is important for additionally enhancing Q factor of the nanocavity.

physics.optics↗

arXiv2023

Nonadiabatic nonlinear non-Hermitian quantized pumping

We analyze a quantized pumping in a nonlinear non-Hermitian photonic system with nonadiabatic driving. The photonic system is made of a waveguide array, where the distances between adjacent waveguides are modulated. It is described by the Su-Schrieffer-Heeger model together with a saturated nonlinear gain term and a linear loss term. A topological interface state between the topological and trivial phases is stabilized by the combination of a saturated nonlinear gain term and a linear loss term. We study the pumping of the topological interface state. We define the transfer-speed ratio $ω/Ω$ by the ratio of the pumping speed $% ω$ of the center of mass of the wave packet to the driving speed $ Ω$ of the topological interface. It is quantized as $ω/Ω=1$ in the adiabatic limit. It remains to be quantized for slow driving even in the nonadiabatic regime, which is a nonadiabatic quantized pump. On the other hand, there is almost no pump for fast driving. We find a transition in pumping as a function of the driving speed.

cond-mat.mes-hall↗

arXiv2023

Valley photonic crystal waveguides fabricated with CMOS-compatible process

Valley photonic crystal (VPhC) waveguides have attracted much attention because of their ability to enable robust light propagation against sharp bends. However, their demonstration using a complementary metal-oxide-semiconductor (CMOS)-compatible process suitable for mass production has not yet been reported at the telecom wavelengths. Here, by tailoring the photomask to suppress the optical proximity effect, VPhC patterns comprising equilateral triangular holes were successfully fabricated using photolithography. We optically characterized the fabricated VPhC devices using microscopic optics with near-infrared imaging. For comparison, we also fabricated and characterized line-defect W1 PhC waveguides, in which the transmission intensities decreased at some regions within the operating bandwidth when sharp turns were introduced into the waveguide. In contrast, the developed VPhC waveguides can robustly propagate light around the C-band telecommunication wavelengths, even in the presence of sharp bends. Our results highlight the potential of VPhC waveguides as an interconnection technology in silicon topological photonic integrated circuits.

physics.optics↗

arXiv2022

Supersymmetric non-Hermitian topological interface laser

We investigate laser emission at the interface of a topological and trivial phases with loss and gain. The system is described by a Su-Schrieffer-Heeger model with site-dependent hopping parameters. We study numerically and analytically the interface states. The ground state is described by the Jackiw-Rebbi mode with a pure imaginary energy, reflecting the non-Hermiticity of the system. It is strictly localized only at the A sites. We also find a series of analytic solutions of excited states based on SUSY quantum mechanics, where the A and B sites of the bipartite lattice form SUSY partners. We then study the system containing loss and gain with saturation. The Jackiw-Rebbi mode is extended to a nonlinear theory, where B sites are also excited. The relative phases between A and B sites are fixed, and hence it will serve as a large area coherent laser.

cond-mat.mes-hall↗