Published papers

We report synthetic frequency dimension Su-Schrieffer-Heeger model and its band structure observation using coupled ring cavities on an integrated photonic chip. Intra-cell and inter-cell couplings between hybridized modes are controlled using radio frequency electro-optic modulation.

We report synthetic frequency dimension Su-Schrieffer-Heeger model and its band structure observation using coupled ring cavities on an integrated photonic chip. Intra-cell and inter-cell couplings between hybridized modes are controlled using radio frequency electrooptic modulation. optic modulation.

We use electro-optically modulated ring cavities, integrated on thin-film lithium niobate, to model frequency dimension tight-binding lattices with versatile connectivity. Inter-mode coupling range, strength, and hopping phase are controlled via the modulating radio frequency signals.

We use electro-optically modulated ring cavities, integrated on thin-film lithium niobate, to model frequency dimension tight-binding lattices with versatile connectivity. Inter-mode coupling range, strength, and hopping phase are controlled via the modulating radio frequency signals.

The Epsilon near-zero (ENZ) effect has been suggested as a method to enhance the magneto-optical (MO) effect; however, the enhancement of the MO effect in the ENZ materials has not been experimentally verified. Here, we measured the MO properties of ENZ materials and analyzed the correlation between the diagonal permittivity and MO effect. The Faraday effect is enhanced in the wavelengths where the real part of diagonal permittivity is almost zero and the Kerr effect is enhanced in the wavelengths where the diagonal permittivity is almost one. Our findings would lead to various potential applications, including on-chip isolators and one-way topological waveguides.

We created a modulatorequipped thinfilm lithium niobate integrated ring resonator simulating a synthetic frequency dimension tightbinding model. It produces frequency states spaced over a 600 GHz bandwidth. Reconfigurable coupling enables simulation of disparate twodimensional interactions.

We analyzed a bilayer photonic structure consisting of two topologically distinct square lattices with π flux by using a tight-binding model. In the weak interlayer coupling regime, a topological corner state exists at each corner of the finite bilayer lattice, while it disappears when the coupling strength exceeds a critical value, indicating the occurrence of a topological phase transition induced by the interlayer coupling. This phenomenon could be applicable to control light localization in bilayer photonics systems.

An InP-based membrane laser is transfer-printed onto a thin sapphire on a Si substrate. Thanks to the low refractive index and high thermal conductivity of the sapphire, the fabricated laser has a 3-dB bandwidth of 26 GHz and performs 53.125-Gbit/s operations at 50 degrees Celsius.

We demonstrate the transfer-printing integration of silicon waveguides on Er:YSO crystals. The versatile integration technique, together with strong optical interactions with Er ions through the confined photons, facilitates the development of efficient spin-photon converters.

Magneto-optical (MO) devices play a key role in introducing nonreciprocity in optical systems, but tend to be bulky due to weak MO effects available in the optical regime. In this talk, we discuss our recent efforts to design of all-dielectric MO metasurfaces toward ultrathin MO devices. We found that large Faraday rotations and high light transmittance can be realized in MO metasurfaces that are much thinner than the operating wavelength.

A membrane laser with a buried sapphire layer on Si has 3-dB bandwidths of 40-25 GHz at 25-80°C. The laser demonstrates 80-Gbit/s PAM4 operations up to 80°C at an energy cost of 0.77 pJ/bit.

A membrane laser with a buried sapphire layer on Si has 3-dB bandwidths of 40-25 GHz at 25-80̊C. The laser demonstrates 80-Gbit/s PAM4 operations up to 80̊C at an energy cost of 0.77 pJ/bit.

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 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 NIR 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 ICs.

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 SiO2 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.

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.

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.

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.

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.

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.