Chee Wei Wong

Regenerative oscillation and four-wave mixing in graphene optoelectronics

We demonstrate the exceptionally-high third-order nonlinearity of integrated mono-layer graphene-silicon hybrid optoelectronics, enabling ultralow power resonant optical bistability, self-induced regenerative oscillations, and coherent four-wave mixing, all at few femto-joule cavity recirculating energies.

Near-field focusing and magnification through self-assembled nanoscale spherical lenses

It is well known that a lens-based far-field optical microscope cannot resolve two objects beyond Abbe’s diffraction limit. Recently, it has been demonstrated that this limit can be overcome by lensing effects driven by surface-plasmon excitation, and by fluorescence microscopy driven by molecular excitation. However, the resolution obtained using geometrical lens-based optics without such excitation schemes remains limited by Abbe’s law even when using the immersion technique, which enhances the resolution by increasing the refractive indices of immersion liquids. As for submicrometre-scale or nanoscale objects, standard geometrical optics fails for visible light because the interactions of such objects with light waves are described inevitably by near-field optics. Here we report near-field high resolution by nanoscale spherical lenses that are self-assembled by bottom-up integration of organic molecules. These nanolenses, in contrast to geometrical optics lenses, exhibit curvilinear trajectories of light, resulting in remarkably short near-field focal lengths. This in turn results in near-field magnification that is able to resolve features beyond the diffraction limit. Such spherical nanolenses provide new pathways for lens-based near-field focusing and high-resolution optical imaging at very low intensities, which are useful for bio-imaging, near-field lithography, optical memory storage, light harvesting, spectral signal enhancing, and optical nano-sensing.

High-speed microfabricated silicon turbomachinery and fluid film bearings

A single-crystal silicon micromachined air turbine supported on gas-lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater than 1 million revolutions per minute, with mechanical power levels approaching 5 W. The device is formed from a fusion bonded stack of five silicon wafers individually patterned on both sides using deep reactive ion etching (DRIE). It consists of a single stage radial inflow turbine on a 4.2-mm diameter rotor that is supported on externally pressurized hydrostatic journal and thrust bearings. This work presents the design, fabrication, and testing of the first microfabricated rotors to operate at circumferential tip speeds up to 300 m/s, on the order of conventional high performance turbomachinery. Successful operation of this device motivates the use of silicon micromachined high-speed rotating machinery for power microelectromechanical systems (MEMS) applications such as portable energy conversion, micropropulsion, and microfluidic pumping and cooling.

Temporal solitons and pulse compression in photonic crystal waveguides

We demonstrate soliton-effect pulse compression in mm-long photonic crystal waveguides resulting from strong anomalous dispersion and self-phase modulation. Compression from 3ps to 580fs, at low pulse energies(~10pJ), is measured via autocorrelation.

Observation of femtojoule optical bistability involving fano resonances in high-Q/Vm silicon photonic crystal nanocavities

The authors observe experimentally optical bistability enhanced through Fano interferences in high-Q localized silicon photonic crystal resonances (Q∼30000 and modal volume ∼0.98 cubic wavelengths). This phenomenon is analyzed through nonlinear coupled-mode formalism, including the interplay of χ(3) effects such as two-photon absorption and related free-carrier dynamics, and optical Kerr as well as thermal effects and linear losses. Experimental and theoretical results demonstrate Fano resonance based bistable states with switching thresholds of 185μW and 4.5fJ internally stored cavity energy (∼540fJ consumed energy) in silicon for scalable optical buffering and logic.

Observation of four-wave mixing in slow-light silicon photonic crystal waveguides

Four-wave mixing is observed in a silicon W1 photonic crystal waveguide. The dispersion dependence of the idler conversion efficiency is measured and shown to be enhanced at wavelengths exhibiting slow group velocities. A 12-dB increase in the conversion efficiency is observed. Concurrently, a decrease in the conversion bandwidth is observed due to the increase in group velocity dispersion in the slow-light regime. The experimentally observed conversion efficiencies agree with the numerically modeled results.

Strain-tunable silicon photonic band gap microcavities in optical waveguides

We report the design, device fabrication, and measurements of tunable silicon photonic band gap microcavities in optical waveguides, using direct application of piezoelectric-induced strain to the photonic crystal. We show, through first-order perturbation computations and experimental measurements, a 1.54 nm shift in cavity resonances at 1.56 μm wavelengths for an applied strain of 0.04%. The strain is applied through integrated piezoelectric microactuators. For operation at infrared wavelengths, we combine x-ray and electron-beam lithography with thin-film piezoelectric processing. This level of integration permits realizable silicon-based photonic chip devices, such as high-density optical filters, with active reconfiguration.

Demonstration of an Air-Slot Mode-Gap Confined Photonic Crystal Slab Nanocavity with Ultrasmall Mode Volumes

We demonstrate experimentally an air-slot mode-gap photonic crystal cavity with quality factor of 104 and modal volume of 0.02 cubic wavelengths, based on the design of an air-slot in a width-modulated line-defect in a photonic crystal slab. The origin of the high Q air-slot cavity mode is the mode-gap effect from the slotted PhCWG mode with negative dispersion. The high Q cavities with ultrasmall mode volume are important for applications such as cavity quantum electrodynamics, nonlinear optics, and optical sensing.

Temperature-Tuning of Near-Infrared Monodisperse Quantum Dot Solids at 1.5 µm for Controllable Förster Energy Transfer

We present the first time-resolved cryogenic observations of Forster energy transfer in large, monodisperse lead sulfide quantum dots with ground-state transitions near 1.5 microm (0.8 eV), in environments from 160 K to room temperature. The observed temperature-dependent dipole-dipole transfer rate occurs in the range of (30-50 ns) (-1), measured with our confocal single-photon counting setup at 1.5 microm wavelengths. By temperature-tuning the dots, 94% efficiency of resonant energy transfer can be achieved for donor dots. The resonant transfer rates match well with proposed theoretical models.

Femtogram doubly clamped nanomechanical resonators embedded in a high-Q two-dimensional photonic crystal nanocavity.

We present a nano-optomechanical system consisting of a 25-fg doubly-clamped nanomechanical resonator embedded in an engineered high-Q (Qo~10,000) defect nanocavity supported by two-dimensional photonic crystal. Optically transduced flexural nanomechanical motion is demonstrated at 1 GHz.