Papers - Ota, Yasutomo

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

Zhang C., Lu G., Pholsen N., Ota Y., Iwamoto S.

Optical Materials Express 15 ( 10 ) 2447 - 2459 2025.10

Research paper (scientific journal), Joint Work, Accepted

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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 the 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.

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Quantum-dot single-photon source based on a two-dimensional photonic crystal nanocavity transfer-printed on a Si wire waveguide

Fujita A., Pholsen N., Ji S., Kamei T., Okano M., Kakuda M., Iwamoto S., Arakawa Y., Ota Y.

Journal of Applied Physics 138 ( 10 )  2025.09

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00218979

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We report a quantum-dot single-photon source (QD SPS) with a two-dimensional photonic crystal (2D PhC) nanocavity hybrid-integrated on a low-loss hydrogenated amorphous silicon waveguide via transfer printing. The large bandgap effect of the 2D PhC supports efficient coupling of QD radiation into the cavity. We suppressed unwanted cavity loss due to polarization conversion in the on-glass 2D PhC by loading an additional glass thin film on top of the structure, leading to low-loss evanescent coupling from the cavity to the underlying waveguide. We observed single-photon generation from a QD in the cavity and its propagation in the waveguide by low-temperature photoluminescence measurements.

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Space-Time Optical Hopfion Crystals

Lin W., Mata-Cervera N., Ota Y., Shen Y., Iwamoto S.

Physical Review Letters 135 ( 8 )  2025.08

Research paper (scientific journal), Joint Work, Accepted

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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. Practical methodologies using bichromatic structured light beams or dipole arrays to assemble 1D and higher dimensional hopfion lattices are proposed, and a technique for tailoring topological orders is elucidated. Optical hopfion crystals constitute a new platform for high-dimensional topological information transfer.

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Flatband localization in 1D moiré bilayer photonic crystals with staggered potential

Trushin S.M., Ito T., Ishii Y., Iwamoto S., Ota Y.

Optics Letters 50 ( 7 ) 2405 - 2408 2025.04

Research paper (scientific journal), Joint Work, Accepted,  ISSN  01469592

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Moiré photonic crystals exhibit a variety of exciting phenomena, including flatband localization. In bilayer moiré photonic crystals, activating flatband localization often requires precise tuning of various structural parameters, which may limit its experimental realization and applications. In this work, we discuss one-dimensional moiré bilayer photonic crystals with staggered potential introduced into each layer. Moiré patterns were formed by slightly differentiating the lattice periods of the top and bottom grating layers, which were double-periodically modulated for introducing the staggered potential. We found that the staggered potential helps in inducing flatband localization in a wider parameter space. We identified the magnitude of the staggered potential as a handy tuning parameter for the manipulation of photonic moiré bands and their wave functions. Our finding here will accelerate the nano-/microcavity formation through moiré physics.

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Slow light waveguides based on bound states in the continuum

Tanimura Y., Ishii Y., Takata K., Uemura T., Notomi M., Iwamoto S., Ota Y.

Optics Letters 50 ( 6 ) 2013 - 2016 2025.03

Research paper (scientific journal), Joint Work, Accepted,  ISSN  01469592

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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.

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Transfer-printed nanophotonic waveguide on an erbium doped crystal towards hybrid photonic quantum devices

Okajima T., Ohta R., Sato T., Tachizaki Y., Xu X., Okamoto H., Ota Y.

Japanese Journal of Applied Physics Part 1 Regular Papers and Short Notes and Review Papers 64 ( 3 )  2025.03

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00214922

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We report the design, fabrication and optical characterization of silicon nanophotonic waveguides on an erbium doped crystal. The silicon waveguide allows for efficient optical addressing of erbium ions embedded near the crystal surface. We employed the transfer printing method to assemble the heterogeneous photonic structure in a simple pick-and-place manner. We observed coupling of the emission from erbium ions into the waveguide in the telecommunication wavelength band. The developed methodology opens the way to functional hybrid quantum devices mediating between photons, electrons and phonons.

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Electrically tunable InAs/GaAs QD in a nanocavity with a transfer-printed ITO electrode

Pholsen N., Fujita A., Kakuda M., Arakawa Y., Ikeda K., Kobayashi N., Ota Y., Iwamoto S.

Optical Materials Express 15 ( 2 ) 290 - 298 2025.02

Research paper (scientific journal), Joint Work, Accepted

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We demonstrate the electrical tuning of InAs/GaAs quantum dots (QDs) embedded in a photonic-crystal (PhC) cavity with transparent indium tin oxide (ITO) electrodes hybridintegrated by transfer printing. The design includes spacer layers between the cavity and the electrodes to reduce absorption loss while still maintaining a significant bias field. The bottom electrode, PhC cavity, and top electrode were fabricated independently and integrated by transfer printing, and an electrical connection was made by inkjet printing, both implemented locally without affecting the entire chip. We observed the electrical tuning of QD emissions into the cavity mode and Purcell enhancement. This work paves the way for utilizing multiple hybrid-integrated QDs on the same chip.

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Telecom-Band Quantum Dots Compatible with Silicon Photonics for Photonic Quantum Applications

Katsumi R., Ota Y., Benyoucef M.

Advanced Quantum Technologies 8 ( 2 )  2025.02

Research paper (scientific journal), Joint Work, Accepted

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Silicon photonics is promising for quantum photonics applications owing to its large-scale and high-performance circuitry enabled by complementary-metal-oxide-semiconductor fabrication processes. However, there is a lack of bright single-photon sources (SPSs) capable of deterministic operation on Si platforms, which largely limits their applications. To this end, on-Si integration of high-performance solid-state quantum emitters, such as semiconductor quantum dots (QDs), is greatly desired. In particular, it is preferable to integrate SPSs emitting at telecom wavelengths for fully leveraging the power of silicon photonics, including efficient chip-to-fiber coupling. In this review, recent progress and challenges in the integration of telecom QD SPSs onto silicon photonic platforms are discussed.

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Nanocavity-based quantum-dot single-photon source on a SiN waveguide integrated by transfer printing

Pholsen N., Fujita A., Okano M., Bart N., Ludwig A., Wieck A.D., Ota Y., Iwamoto S.

Optics Express 33 ( 1 ) 252 - 262 2025.01

Research paper (scientific journal), Joint Work, Accepted

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Silicon nitride (SiN) integrated photonics is a highly promising platform for photonic quantum information processing. However, the efficient generation of single photons remains a significant challenge. Epitaxial InAs/GaAs quantum dots (QDs) embedded in wavelength-scale nanocavities offer a promising solution as single-photon sources (SPSs), but their integration with SiN has not yet been demonstrated. In this work, we employed a photonic crystal (PhC) nanobeam cavity to realize a QD SPS on a SiN waveguide. We found that the second-order mode of a period-modulated PhC nanobeam cavity is located close to the waveguide mode in the momentum space and could present a way to efficiently couple photons from the QD SPS into the waveguide. We fabricated the SiN circuitry and QD SPSs separately and integrated the SPS onto the waveguide by transfer printing. We observed Purcell enhancement by the cavity and confirmed the coupling of single photons into the waveguide.

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Magneto nanophotonics platform based on monocrystalline yttrium iron garnets

Ota Y., Gao S., Taniguchi K., Yambe T., Tanimura Y., Iwamoto S.

International Conference on Metamaterials Photonic Crystals and Plasmonics    741 - 742 2025

Research paper (international conference proceedings), Joint Work, Accepted

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Monocrystalline yttrium iron garnets (YIGs) are an outstanding material for magneto-optics. However, their application to nanophotonics has been limited due to the difficulty of fine nanopatterning. Here, we report a platform of magneto-nanophotonics based on YIG realized by the development of high-quality YIG membranes and their nanopatterning using dry etching.

Micro-transfer Printed Membrane Laser and Electroabsorption Modulator on Si waveguide Integrated with Polymer Spot-size Converter

Maeda Y., Hiraki T., Aihara T., Homma H., Fujii T., Takeda K., Sato T., Ota Y., Iwamoto S., Arakawa Y., Matsuo S.

Optical Fiber Communication Conference in Proceedings Optical Fiber Communication Conference Ofc 2025 and Optical Fiber Communication Conference Ofc Postdeadline Papers 2025  2025

Research paper (international conference proceedings), Joint Work, Accepted

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We demonstrate micro-transfer printing for integrating multiple III-V devices with different bandgaps on Si platforms. Membrane DFB lasers and EA modulators are fabricated on Si waveguides with polymer-based spot-size converters, modulating at 128 Gbit/s-NRZ signal.

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Micro-transfer Printed Membrane Laser and Electroabsorption Modulator on Si waveguide Integrated with Polymer Spot-size Converter

Maeda Y., Hiraki T., Aihara T., Homma H., Fujii T., Takeda K., Sato T., Ota Y., Iwamoto S., Arakawa Y., Matsuo S.

2025 Optical Fiber Communications Conference and Exhibition Ofc 2025 Proceedings  2025

Research paper (international conference proceedings), Joint Work, Accepted

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We demonstrate micro-transfer printing for integrating multiple III-V devices with different bandgaps on Si platforms. Membrane DFB lasers and EA modulators are fabricated on Si waveguides with polymer-based spot-size converters, modulating at 128Gbit/s-NRZ signal.

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Reconfigurable synthetic dimension frequency lattices in an integrated lithium niobate ring cavity

Dinh H.X., Balčytis A., Ozawa T., Ota Y., Ren G., Baba T., Iwamoto S., Mitchell A., Nguyen T.G.

Communications Physics 7 ( 1 )  2024.12

Research paper (scientific journal), Joint Work, Accepted

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Harnessing non-spatial properties of photons as if they represent an additional independent coordinate underpins the emerging synthetic dimension approach. It enables probing of higher-dimensional physical models within low-dimensional devices, such as on a planar chip where this method is relatively nascent. We demonstrate an integrated thin-film lithium niobate ring resonator that, under dynamic modulation, simulates a tight-binding model with its discrete frequency modes representing lattice sites. Inter-mode coupling, and the simulated lattice geometry, can be reconfigured by controlling the modulating signals. Up to a quasi-3D lattice connectivity with controllable gauge potentials has been achieved by simultaneous synchronized nearest-, second- and third-nearest-neighbor coupling, and verified by acquiring synthetic band structures. Development of synthetic frequency dimension devices in the thin-film lithium niobate photonic integration platform is a key step in increasing the complexity of topological models achievable on a chip, combining efficient electro-optic mode coupling with non-linear effects for long-range mode interactions.

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On-chip optical skyrmionic beam generators

Lin W., Ota Y., Arakawa Y., Iwamoto S.

Optica 11 ( 11 ) 1588 - 1594 2024.11

Research paper (scientific journal), Joint Work, Accepted,  ISSN  23342536

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Optical skyrmion beams, which encompass a two-dimensional topology in their spatial structures, are promising candidates for ultra-dense optical communications and advanced matter manipulation. Generating such light beams using a chip-based approach will broaden their applications and promote the advancement of untapped fundamental science. Herein, 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 behaviors that closely align with theoretical predictions and numerical simulations. The realization of on-chip optical skyrmion beam generators ushers a new dawn in optical and material sciences.

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Topological Laser in Anomalous Quadrupole Topological Phases

Lu G., Ota Y., Iwamoto S.

Laser and Photonics Reviews 18 ( 11 )  2024.11

Research paper (scientific journal), Joint Work, Accepted,  ISSN  18638880

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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 the 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 experimental demonstration of a topological laser in an anomalous quadrupole topological phase, an instance of higher-order phases. To facilitate this, a topological nanocavity with a quality factor near 6000 is engineered through a twisting operation. The topological nature of this system is validated by the calculation of nested Wannier centers and the emergency condition of corner states. The experimental observations reveal the manifestation of corner states and the achievement of single-mode pulsed laser, driven by an optical gain from multiple quantum wells at telecommunication wavelengths and a temperature of 4 K. A lasing threshold of 23 µW and a cold quality factor of 1500 are deduced through rate equation. This work gives a new potential in the application of topological principles to advance nanophotonic technologies.

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

Zhang C., Ota Y., Iwamoto S.

Optical Materials Express 14 ( 7 ) 1756 - 1766 2024.07

Research paper (scientific journal), Joint Work, Accepted

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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.

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Nonadiabatic nonlinear non-Hermitian quantized pumping

Ezawa M., Ishida N., Ota Y., Iwamoto S.

Physical Review Research 6 ( 3 )  2024.07

Research paper (scientific journal), Joint Work, Accepted,  ISSN  26431564

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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 the 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 topologically as ω/ω=1 in the adiabatic limit. It remains to be quantized dynamically unless the driving is not too fast even in the nonadiabatic regime. On the other hand, the wave packet collapses and there is no quantized pumping when the driving is too fast. In addition, the stability against disorder is more enhanced by stronger nonlinearity.

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Design of GaAs microcavity on SiN waveguide for efficient single-photon generation by resonant excitation

Pholsen N., Ota Y., Iwamoto S.

Materials for Quantum Technology 4 ( 2 )  2024.06

Research paper (scientific journal), Joint Work, Accepted

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Silicon nitride (SiN) photonic circuits are attracting significant interest as a platform for photonic quantum information processing. Integration of deterministic single photon sources (SPSs) is required for large-scale single-photon-based quantum applications. InAs/GaAs quantum dots (QDs) have been demonstrated to be state-of-the-art deterministic SPSs under resonant excitation. However, InAs/GaAs QD SPSs integrated on chip often suffer from unwanted crosstalk from scattering of resonant excitation laser. Moreover, the mismatch in refractive indices of SiN and GaAs impedes efficient coupling of single photons into the photonic circuit. In this work, we design a GaAs photonic crystal (PhC) nanobeam cavity with an embedded QD on top of a SiN waveguide in SiO<inf>2</inf> that can suppress crosstalk from resonant excitation and realize high coupling efficiency at the same time. The crosstalk is reduced by employing a carefully designed nanobeam cavity that removes complex structures around the excitation spot. The high coupling efficiency is achieved with a weak hybridized mirror formed by proximity of GaAs PhC nanobeam and SiN waveguide that makes the cavity and helps transferring photons into the waveguide. This enables more than 90% coupling efficiency. The designed device is expected to be a bright source of indistinguishable photons.

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Micro-Transfer-Printed InP-Based Membrane Photonic Devices on Thin-Film Lithium Niobate Platform

Maeda Y., Nishi H., Diamantopoulos N.P., Fujii T., Aihara T., Yamaoka S., Hiraki T., Takeda K., Segawa T., Ota Y., Iwamoto S., Arakawa Y., Matsuo S.

Journal of Lightwave Technology 42 ( 11 ) 4023 - 4030 2024.06

Research paper (scientific journal), Joint Work, Accepted,  ISSN  07338724

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We demonstrate a heterogeneous integration of electrically-pumped membrane lasers and semiconductor optical amplifiers (SOAs) on a thin-film lithium niobate (TFLN) platform by a micro-transfer-printing (μTP) method. Thanks to the low coupling loss (∼1.0 dB) between the InP-based membrane photonic devices and the TFLN waveguides, mW-class output power laser and low-power-consumption SOA on a TFLN platform are achieved. A 128-Gbit/s non-return-to-zero (NRZ) signal modulation is also demonstrated with a TFLN Mach-Zehnder modulator and an integrated membrane laser. These results indicate that the heterogeneous integration of membrane devices using μTP is promising for manufacturing TFLN photonic integrated circuits for optical communications and other applications.

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

Kawata R., Fujita A., Pholsen N., Iwamoto S., Ota Y.

Optics Letters 49 ( 9 ) 2345 - 2348 2024.05

Research paper (scientific journal), Joint Work, Accepted,  ISSN  01469592

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We numerically analyze two-dimensional photonic crystal (PhC) nanocavities on glass with a thin glass film on top of the structure. We investigated a multistep 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 nanocavities covered with thin glass films. We also discuss that the in-plane structure of the upper glass film is important for additionally enhancing the Q factor of the nanocavity.

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