King Y. Fong

Optical frequency comb generation from aluminum nitride microring resonator

Aluminum nitride (AlN) is an appealing nonlinear optical material for on-chip wavelength conversion. Here we report optical frequency comb generation from high-quality-factor AlN microring resonators integrated on silicon substrates. By engineering the waveguide structure to achieve near-zero dispersion at telecommunication wavelengths and optimizing the phase matching for four-wave mixing, frequency combs are generated with a single-wavelength continuous-wave pump laser. Further, the Kerr coefficient (n₂) of AlN is extracted from our experimental results.

Tunable optical coupler controlled by optical gradient forces

We demonstrate optical gradient force-tunable directional couplers in free-standing silicon nitride slot waveguides. Utilizing device geometries optimized for strong optomechanical interactions allows us to control the optical transmission without the aid of a cavity. Static, wideband tuning as well as low-power optical modulation is achieved.

GHz optomechanical resonators with high mechanical Q factor in air

We demonstrate wheel-shaped silicon optomechanical resonators for resonant operation in ambient air. The high finesse of optical whispering gallery modes (loaded optical Q factor above 500,000) allows for efficient transduction of the wheel resonators mechanical radial contour modes of frequency up to 1.35 GHz with high mechanical Q factor around 4,000 in air.

Integrated high frequency aluminum nitride optomechanical resonators

Aluminum nitride (AlN) has been widely used in microeletromechanical resonators for its excellent electromechanical properties. Here, we demonstrate the use of AlN as an optomechanical material that simultaneously offers low optical and mechanical loss. Integrated AlN microring resonators in the shape of suspended rings exhibit high optical quality factor (Q) with loaded Q up to 125 000. Optomechanical transduction of the Brownian motion of a GHz contour mode yields a displacement sensitivity of 6.2 × 10−18 m/Hz1/2 in ambient air.

Cascaded optical transparency in multimode-cavity optomechanical systems

Electromagnetically induced transparency has great theoretical and experimental importance in many areas of physics, such as atomic physics, quantum optics and, more recent, cavity optomechanics. Optical delay is the most prominent feature of electromagnetically induced transparency, and in cavity optomechanics, the optical delay is limited by the mechanical dissipation rate of sideband-resolved mechanical modes. Here we demonstrate a cascaded optical transparency scheme by leveraging the parametric phonon-phonon coupling in a multimode optomechanical system, where a low damping mechanical mode in the unresolved-sideband regime is made to couple to an intermediate, high-frequency mechanical mode in the resolved-sideband regime of an optical cavity. Extended optical delay and higher transmission as well as optical advancing are demonstrated. These results provide a route to realize ultra-long optical delay, indicating a significant step towards integrated classical and quantum information storage devices.

Modeling of the optical force between propagating lightwaves in parallel 3D waveguides.

We present a rigorous analysis of the optical gradient force between coupled single-mode waveguides in three dimensions. Using eigenmode expansion we determine the optical mode patterns in the coupled system. In contrast to previous work, the sign and amplitude of the optical force is found to vary along the waveguide with a characteristic beating length. Our results establish design principles for optomechanically tunable directional couplers.

Electrical tuning and switching of an optical frequency comb generated in aluminum nitride microring resonators

Aluminum nitride (AlN) has been shown to possess both strong Kerr nonlinearity and electro-optic Pockels effect. By combining these two effects, here we demonstrate on-chip reversible on/off switching of the optical frequency comb generated by an AlN microring resonator. We optimize the design of gating electrodes and the underneath resonator structure to effectively apply an electric field without increasing the optical loss. The switching of the comb is monitored by measuring one of the frequency comb peaks while varying the electric field. The controlled comb electro-optic response is investigated for direct comparison with the transient thermal effect.

Nano-Optomechanical Resonators in Microfluidics.

Operation of nanomechanical devices in liquid has been challenging due to the strong viscous damping that greatly impedes the mechanical motion. Here we demonstrate an optomechanical microwheel resonator integrated in microfluidic system that supports low-loss optical resonances at near-visible wavelength with quality factor up to 1.5 million, which allows the observation of the thermal Brownian motion of the mechanical mode in both air and water environment with high signal-to-background ratio. A numerical model is developed to calculate the hydrodynamic effect on the device due to the surrounding water, which agrees well with the experimental results. With its very high resonance frequency (170 MHz) and small loaded mass (75 pg), the present device has an estimated mass sensitivity at the attogram level in water.

Microwave-assisted coherent and nonlinear control in cavity piezo-optomechanical systems

We present a cavity piezo-optomechanical system where microwave and optical degrees of freedom are coupled through an ultra-high frequency mechanical resonator. By utilizing the coherence among the three interacting modes, we demonstrate optical amplification, coherent absorption and a more general asymmetric Fano resonance. The strong piezoelectric drive further allows access to the large-amplitude-induced optomechanical nonlinearity, with which optical transparency at higher harmonics through multi-phonon scattering is demonstrated.

Backaction limits on self-sustained optomechanical oscillations

The maximum amplitude of mechanical oscillators coupled to optical cavities are studied both analytically and numerically. The optical backaction on the resonator enables self-sustained oscillations whose limit cycle is set by the dynamic range of the cavity. The maximum attainable amplitude and the phonon generation quantum efficiency of the backaction process are studied for both unresolved and resolved cavities. Quantum efficiencies far exceeding one are found in the resolved sideband regime where the amplitude is low. On the other hand the maximum amplitude is found in the unresolved system. Finally, the role of mechanical nonlinearities is addressed.