Wataru Yoshiki

CMOS compatible high-Q photonic crystal nanocavity fabricated with photolithography on silicon photonic platform

Progress on the fabrication of ultrahigh-Q photonic-crystal nanocavities (PhC-NCs) has revealed the prospect for new applications including silicon Raman lasers that require a strong confinement of light. Among various PhC-NCs, the highest Q has been recorded with silicon. On the other hand, microcavity is one of the basic building blocks in silicon photonics. However, the fusion between PhC-NCs and silicon photonics has yet to be exploited, since PhC-NCs are usually fabricated with electron-beam lithography and require an air-bridge structure. Here we show that a 2D-PhC-NC fabricated with deep-UV photolithography on a silica-clad silicon-on-insulator (SOI) structure will exhibit a high-Q of 2.2 × 105 with a mode-volume of ~1.7(λ/n)3. This is the highest Q demonstrated with photolithography. We also show that this device exhibits an efficient thermal diffusion and enables high-speed switching. The demonstration of the photolithographic fabrication of high-Q silica-clad PhC-NCs will open possibility for mass-manufacturing and boost the fusion between silicon photonics and CMOS devices.

All-optical switching using Kerr effect in a silica toroid microcavity

We demonstrate experimentally an all-optical switching operation using the Kerr effect in a silica toroid microcavity. Thanks to the small mode volume and high quality factor of the silica toroid microcavity, we achieved on-chip optical Kerr switching with an input power of 2 mW. This value is the smallest among all previously reported on-chip optical Kerr switches. We also show that this value can be reduced to a few tens of μW by employing a mode with a Q factor of > 2 × 10⁷.

Octagonal silica toroidal microcavity for controlled optical coupling

We fabricated polygonal silica toroidal microcavities to achieve stable mechanical coupling with an evanescent coupler such as a tapered fiber. The octagonal cavity was fabricated by using a combination of isotropic etching, anisotropic etching, and laser reflow. It offers both high and low coupling efficiencies with the cavity mode even when the coupler is in contact with the cavity, which makes the device more practically because of easy alignment. A numerical simulation showed that an octagonal silica toroidal microcavity had an optical quality factor of 8.8 × 106.

Analysis of bistable memory in silica toroid microcavity

We modeled nonlinear behavior in a silica toroid microcavity using coupling mode theory and the finite element method, and obtained Kerr bistability that did not suffer from the thermo-optic effect by optimizing the fiber-cavity coupling.

Impact of the photorefractive and pyroelectric-electro-optic effect in lithium niobate on whispering-gallery modes

Whispering-gallery resonators made of undoped and MgO-doped congruently grown lithium niobate are used to study electro-optic refractive index changes. Hereby, we focus on the volume photovoltaic and the pyroelectric effect, both providing an electric field driving the electro-optic effect. Our findings indicate that the light-induced photorefractive effect, combining the photovoltaic and electro-optic effect, is present only in the non-MgO-doped lithium niobate for exposure with light having wavelengths of up to 850 nm. This leads to strong resonance frequency shifts of the whispering-gallery modes. No photorefractive effect was observed in the MgO-doped material. One has to be aware that surface charges induced by the pyroelectric effect result in a similar phenomenon and are present in both materials.

Hysteresis behavior of Kerr frequency comb generation in a high-quality-factor whispering-gallery-mode microcavity

A numerical and experimental study of Kerr frequency comb generation in a silica toroid microcavity is presented. We use a generalized mean-field Lugiato–Lefever equation and solve it with the split-step Fourier method. We observe that a stable mode-locked regime can be accessed when we reduce the input power after strong pumping due to the bistable nature of the nonlinear cavity system used. The experimental results agree well with the results of the numerical analysis, where we obtain a low-noise Kerr comb spectrum by gradually reducing the pumping input after strong pumping. This finding complements the results obtained by a previous wavelength scanning method and clarifies the procedure for achieving mode-locked states in such high-Q microcavity systems.

Observation of energy oscillation between strongly-coupled counter-propagating ultra-high Q whispering gallery modes

We report the first experimental observation of an energy oscillation between two coupled ultra-high Q whispering gallery modes in the time domain. Two counter-propagating whispering gallery modes in a silica toroid microcavity were employed for this purpose. The combination of a large coupling coefficient between the two modes and an ultra-high Q factor, which creates a large Γ value of > 10, results in a clear energy oscillation. Our measurement is based on a drop-port measurement technique, which enables us to observe the light energy in the two modes directly. The oscillation period measured in the time domain precisely matched that inferred from mode splitting in the frequency domain, and the measured results showed excellent agreement with results calculated with the developed numerical model.

All-optical tunable buffering with coupled ultra-high Q whispering gallery mode microcavities

All-optical tunable buffering was recently achieved on a chip by using dynamically tuned coupled mode induced transparency, which is an optical analogue of electromagnetically induced transparency. However, the small Q s of about 105 used in those systems were limiting the maximum buffering time to a few hundred ps. Although employing an ultra-high Q whispering gallery mode (WGM) microcavity can significantly improve the maximum buffering time, the dynamic tuning of the WGM has remained challenging because thermo-optic and pressure tunings, which are widely used for WGM microcavities, have a very slow response. Here we demonstrate all-optical tunable buffering utilizing coupled ultra-high Q WGM cavities and the Kerr effect. The Kerr effect can change the refractive index instantaneously, and this allowed us to tune the WGM cavity very quickly. In addition, from among the various WGM cavities we employed a silica toroid microcavity for our experiments because it has an ultra-high Q factor (>2 × 107) and a small mode volume, and can be fabricated on a chip. Use of the Kerr effect and the silica toroid microcavity enabled us to observe an on-chip all-optical tunable buffering operation and achieve a maximum buffering time of 20 ns.

Effect on Kerr comb generation in a clockwise and counter-clockwise mode coupled microcavity

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 effect on soliton formation.

Kerr-induced controllable adiabatic frequency conversion in an ultrahigh Q silica toroid microcavity

In this Letter, we report, based on our knowledge, the first demonstration of Kerr-induced adiabatic frequency conversion in a silica toroid microcavity. Taking advantage of the instantaneous response of the Kerr effect, we achieved adiabatic frequency conversion with a controllable amount of frequency shift and time width. In addition, thanks to the combination of the Kerr effect and the ultrahigh Q (>107) of the silica toroid microcavity, we also observed multiple frequency conversion within a photon lifetime. Furthermore, use of the Kerr effect allowed us to investigate the influence of the relative phase between the original and converted light.