Published papers

The unique optical properties of 2D layered materials are attractive for achieving increased functionality in integrated photonics. Owing to the van der Waals nature, these materials are ideal for integrating with nanoscale photonic structures. Here a carefully designed air-mode silicon photonic crystal nanobeam cavity for efficient control through 2D materials is reported. By systematically investigating various types and thicknesses of 2D materials, the authors are able to show that enhanced responsivity allows for giant shifts of the resonant wavelength. With atomically precise thickness over a macroscopic area, few-layer flakes give rise to quantization of the mode shifts. The dielectric constant of the flakes is extracted and found to be independent of the layer number down to a monolayer. Flexible reconfiguration of a cavity is demonstrated by stacking and removing ultrathin flakes. With an unconventional cavity design, these results open up new possibilities for photonic devices integrated with 2D materials.

We numerically investigate the deterministic generation of a perfect soliton crystal (PSC) in an optical microresonator functionalized with a saturable absorber (SA). The SAallows the direct formation of a PSC from an initial, periodic Turing roll. It prevents passage through a chaotic state, which induces a stochastic nature with regard to the number of generated dissipativeKerr solitons.We showthat PSCs form deterministically, and the number is controlled by adjusting the input power and SA parameter. Our work provides a simple approach for obtaining a stable PSC that offers an ultrahigh repetition rate and a high comb output power.

This study explores sol-gel methods for fabricating erbium-doped silica microtoroid resonators, addressing the limitations of conventional doping techniques and enhancing device scalability. We develop a reproducible sol-gel process that yields defect-free films for photonic applications and detail common defects and troubleshooting strategies. Two fabrication methods are compared: traditional film deposition on substrates and the direct coating of prefabricated resonators. The latter enables the fabrication of larger resonator diameters (up to 450 pm) without buckling while achieving a high-g factor and a low lasing threshold of 350 pW. These erbium-doped resonators exhibit multi-mode laser oscillations at 1550 nm, revealing the sol-gel method’s potential for realizing scalable, gain-doped photonic devices.

We present a 300 GHz photonic filter based on a soliton microcomb, addressing the increasing demand for high-frequency, low-phase-noise signals in THz-band wireless communications. Utilizing a uni-traveling-carrier photodiode (UTC-PD), our configuration enables the generation of 300 GHz waves through beat signals from adjacent longitudinal modes of the microcomb. By combining the UTC-PD with a photonic filter approach, we demonstrate that RF photonic filters with high suppression ratios are achieved in the 300 GHz band. Additionally, by mitigating the effects of higher-order dispersion, we achieve a stopband rejection exceeding 30 dB. These results underscore the potential of our photonic filter for advanced THz applications and provide a promising pathway for low-noise, high-bandwidth wireless communication systems.

We developed a systematic measurement method that simultaneously captures pump detuning, the optical spectrum, RF beat noise, and the linewidth of a microresonator frequency comb. This comprehensive measurement approach enables us to investigate and visualize the phase transition between two distinct states within the modulation instability (MI) comb: a low-noise state (MI comb phase 1) and a high-noise state (MI comb phase 2). While phase 2 is typically recognized as the MI comb, our findings suggest that phase 1 represents a transition from the Turing pattern to the MI comb, maintaining a low-noise profile despite exhibiting the spectral characteristics of an MI comb.

The interaction between surface acoustic waves (SAWs) and magnons is an interesting phenomenon that can be used to control spins in spintronic devices, and thus the visualization of the related spatiotemporal dynamics can be useful. In this study, we used dual-frequency-comb-based asynchronous optical sampling to record the spatiotemporal evolution of the surface vibration associated with an optically excited SAW and that of the spin precession associated with the magnon under the same excitation condition at effectively 160 frames/ns. Isotropic and anisotropic propagation were observed for phonons and magnons, respectively, demonstrating the capability of measuring such dynamics with high resolution.

We present first-time demonstration of short-reach and low-latency optical communication within a real network, employing a microresonator frequency comb as a light source. The modulated signal is transmitted through a 9-km single-mode optical fiber installed in a metropolitan network. This demonstration paves the way for realizing low-latency massively parallel optical communication, which is the key to beyond-5G and 6G network. For a proof-of-concept experiment, we employ an MgF2 crystalline microresonator with a 20-GHz free-spectral range that could be used for dense wavelength division multiplexing communication. We generated a stable soliton comb and modulated it with simple 10-Gbps intensity modulation and direct detection to achieve a small excess delay of 3.1 µs.

We present a novel method for generating spin currents using the gyromagnetic effect, a phenomenon discovered over a century ago. This effect, crucial for understanding the origins of magnetism, enables the coupling between various macroscopic rotational motions and electron spins. While higher rotational speeds intensify the effect, conventional mechanical rotations, typically, below 10,000 RPM, produce negligible results comparable to geomagnetic fluctuations, limiting applied research. Our studies demonstrate that spin current generation comparable to that of rare metals can be achieved through atomic rotations induced by GHz-range surface acoustic waves and the rotational motion of conduction electrons in metallic thin films with nanoscale gradient modulation of electrical conductivity. These effects, termed the acoustic gyromagnetic effect and the current-vorticity gyromagnetic effect, are significant in different contexts. The acoustic gyromagnetic effect is notable in high-conductivity materials like aluminum and copper, which are more abundant than conventional spintronics materials with strong spin–orbit interactions (SOIs). Conversely, the current-vorticity gyromagnetic effect requires a large conductivity gradient to produce current vorticity efficiently. This is achieved by using composition gradient structures from highly conductive metals to poorly conductive oxides or semiconductors. Consequently, unlike traditional strong-SOI materials, we can create highly efficient spin current generators with low energy dissipation due to reduced Joule loss.

Hybrid all-optical switching devices that combine silicon nanocavities and 2D semiconductor materials are proposed and demonstrated. By exploiting the refractive index modulation caused by photo-induced carriers in the 2D material instead of the silicon substrate, the switching speed limitation imposed by the carrier lifetime of silicon is overcome while maintaining a low switching energy. Air-mode photonic crystal nanobeam cavities capable of efficient interaction with 2D materials are fabricated, and molybdenum ditelluride, a 2D material with rapid carrier recombination, is transferred onto the cavities. The molybdenum ditelluride flake is excited by an optical pump pulse to shift the resonant wavelength of the cavity for switching operation. All-optical switching operations are achieved on the time scale of tens of picoseconds while requiring low switching energies of a few hundred femtojoules.

Dissipative Kerr soliton microcombs have inspired various intriguing applications such as spectroscopy, ranging, telecommunication, and high purity microwave generation. Mechanically actuated soliton microcombs provide enhanced controllability and flexibility for Kerr solitons, thus enabling technological progress to be made on such practical applications. Here, we present architectures for coherent dual-comb techniques and ultralow-noise microwave generation by exploiting the mechanical actuation of ultrahigh-Q crystalline microresonators. By unifying a pump laser, we demonstrate highly coherent dual-soliton combs using distinct resonators with slightly different repetition rates. We also report significant phase-noise reduction achieved by directly generating Kerr solitons from a sub-Hz linewidth ultrastable laser. This study paves the way for further advancements in a wide variety of applications based on Kerr soliton microcombs.

We study the impact of inherent mode coupling between clockwise (CW) and counter-clockwise (CCW) modes on Kerr comb generation in a small whispering-gallery mode microcavity. Our numerical analysis using a coupled Lugiato-Lefever equation reveals the range of the coupling strength in which a soliton pulse can be obtained in the CW direction. It also showed that CCW comb power depends on the coupling strength between the CW and CCW modes. In addition to the simulation, we conducted an experiment to confirm that the power ratio between the CW and CCW comb modes depends on the coupling strength, and the experimental results agree well with the simulation results. This study helps us to understand the relationship between CW and CCW mode coupling and Kerr comb generation, and the e ect on soliton formation.

We demonstrated the deterministic generation of blue light emission (438 nm) via the third-harmonic process from an infrared pump by carefully engineering the dispersion of a high-quality-factor whispering gallery mode microcavity. We present two different approaches to obtaining broad bandwidth light. One is based on a clustered comb and the other employs a dispersive wave, and a broad Kerr comb spanning a half-octave is obtained. This allowed frequency conversion over a broad bandwidth ranging from 438 to 612 nm. This approach will enable the development of micro-scale light sources and frequency converters for future optical processing.

We theoretically and experimentally investigated the transition between modulation instability and Raman gain in a small silica microcavity with a large free-spectral range, which reveals that we can selectively switch from a four-wave mixing dominant state to a stimulated Raman scattering dominant state. Both the theoretical analysis and the experiment show that a Raman-dominant region is present between transitions of Kerr combs with different free-spectral range spacings. We can obtain a stable Kerr comb and a stable Raman state selectively by changing the driving power, coupling between the cavity and the waveguide, and laser detuning. Such a controllable transition is achieved thanks to the presence of gain competition between modulation instability and Raman gain in silica whispering gallery mode microcavities.

Comb generation in different mode families via a stimulated Raman scattering (SRS) process is studied using a silica toroid microcavity. The broad gain bandwidth of SRS in silica allows us to excite longitudinal modes at long wavelengths belonging to mode families that are either the same as or different from the pump mode. We found through experiment and numerical analysis, that an SRS comb in a different mode family with a high quality factor (Q) is excited when we pump in a low-Q mode. No transverse mode interaction occurs when we excite in a high-Q mode resulting the generation of a single comb family. We studied the condition of the transverse mode interaction while varying the mode overlap and Q of the Raman mode. Our experimental results are in good agreement with the analysis and this enables us to control the generation of one- and two-mode combs.

We generate broad bandwidth visible light ranging from 498 to 611 nm via thirdharmonic generation in a silica toroid microcavity. The silica toroid microcavity is fed with a continuous-wave input at a telecom wavelength, where third-harmonic generation follows the generation of an infrared Kerr comb via cascaded four-wave-mixing and stimulated Raman scattering effects. Thanks to these cascaded effects (four-wave mixing, stimulated Raman scattering, and third-harmonic generation) in an ultrahigh-Q microcavity, a broad bandwidth visible light is obtained. The visible light couples with the whispering gallery mode of the cavity by demonstrating the evanescent coupling of the generated visible light with a tapered fiber based on an add-drop configuration.