Harold Chong

A compact two-dimensional grating coupler used as a polarization splitter

We demonstrate a novel polarization splitter based on a two-dimensional grating etched in a silicon-on-insulator waveguide. The device couples orthogonal modes from a single-mode optical fiber into identical modes of two planar ridge waveguides. The extinction ratio is better than 18 dB in the wavelength range of 1530-1560 nm and the coupling efficiency is approximately 20%. The device is very compact and couples light only to transverse-electric modes of the planar waveguides. Therefore, it may be used in a polarization diversity configuration to implement a polarization insensitive photonic integrated circuit based on photonic crystal waveguides.

Comprehensive FDTD modelling of photonic crystal waveguide components

Planar photonic crystal waveguide structures have been modelled using the finite-difference-time-domain method and perfectly matched layers have been employed as boundary conditions. Comprehensive numerical calculations have been performed and compared to experimentally obtained transmission spectra for various photonic crystal waveguides. It is found that within the experimental fabrication tolerances the calculations correctly predict the measured transmission levels and other major transmission features.

2D Photonic crystal thermo-optic switch based on AlGaAs/GaAs epitaxial structure

The realisation of a thermo-optically controlled symmetric Mach- Zehnder interferometer switch based on an AlGaAs/GaAs epitaxial waveguide structure operating at wavelengths in the region of lambda = 1550 nm is reported. The device is based on a very compact two-dimensional photonic crystal channel waveguide structure. The measured and simulated transmission spectra for the devices are in good agreement. The Pi-phase shift switching power for the device is as low as 42 mW.

Tuning of photonic crystal waveguide microcavity by thermooptic effect

A tunable photonic crystal microcavity with in-filling holes has been experimentally realized by using the thermooptic effect in silicon-on-insulator waveguide at /spl lambda/=1530 nm. The device was fabricated by reactive ion etching through the silicon core layer for high vertical confinement. A thin film heater has been integrated onto the microcavity. A small change in temperature (and consequent change in refractive index) was sufficient to cause an observable resonance shift. A resonance shift of 5 nm was achieved when the heater was switched on to a current of 1.2 mA (with switching power of about 9 mW).

Silicon photonic devices and platforms for the mid-infrared

Due to its excellent electronic and photonic properties, silicon is a good candidate for mid-infrared optoelectronic devices and systems that can be used in a host of applications. In this paper we review some of the results reported recently, and we also present several new results on mid-infrared photonic devices including Mach-Zehnder interferometers, multimode interference splitters and multiplexers based on silicon-on-insulator, polysilicon, suspended silicon, and slot waveguide platforms.

Silicon waveguides and devices for the mid-infrared

We report on the design, fabrication, and characterization of silicon-on-insulator rib and strip waveguides at wavelengths longer than 3.7µm. Propagation losses of 1.5±0.2 dB/cm at 3.73µm and 1.8±0.2 dB/cm at 3.8µm have been measured for rib waveguides, whilst submicron strip waveguides exhibited propagation losses of 4.6±1.1 dB/cm at the wavelength of 3.74µm. A 1×2 multimode interference (MMI) splitter and racetrack resonators based on submicron strip waveguides are also examined. Optical losses of 3.6±0.2 dB/MMI and a racetrack resonator Q-value of 8.2k are obtained at 3.74µm.

Efficient propagation of TM polarized light in photonic crystal components exhibiting band gaps for TE polarized light

We have investigated the properties of TM polarized light in planar photonic crystal waveguide structures, which exhibit photonic band gaps for TE polarized light. Straight and bent photonic crystal waveguides and couplers have been fabricated in silicon-on-insulator material and modelled using a 3D finite-difference-time-domain method. The simulated spectra are in excellent agreement with the experimental results, which show a propagation loss as low as 2.5+/-4 dB/mm around 1525 nm and bend losses at 2.9+/-0.2 dB for TM polarized light. We demonstrate a high coupling for TM polarized light in a simple photonic crystal coupler with a size of ~ 20 m x 20 m. These promising features may open for the realization of ultra-compact photonic crystal components, which are easily integrated in optical communication networks.

Metal-free plasma-enhanced chemical vapor deposition of large area nanocrystalline graphene

This paper reports on large area, metal-free deposition of nanocrystalline graphene (NCG) directly onto wet thermally oxidized 150 mm silicon substrates using parallel-plate plasma-enhanced chemical vapor deposition. Thickness non-uniformities as low as 13% are achieved over the whole substrate. The cluster size of the as-obtained films is determined from Raman spectra and lies between 1.74 and 2.67 nm. The film uniformity was further confirmed by Raman mapping. The sheet resistance of 3.73 and charge carrier mobility μ of are measured. We show that the NCG films can be readily patterned by reactive ion etching. NCG is also successfully deposited onto quartz and sapphire substrates and showed % optical transparency in the visible light spectrum.

Transparent conducting oxides for active hybrid metamaterial devices

We present here a study of the combined nonlinear response of plasmonic antenna—transparent conducting oxide hybrids for activation of metamaterial devices. Nanoantenna layers consisting of randomly positioned gold nanodisk dimers are fabricated using hole-mask lithography. The nanoantenna layers are covered with a 20 nm thin layer of transparent conducting oxide (TCO). We investigate the response of atomic layer deposited aluminum-doped zinc oxide (AZO) next to indium–tin oxide (ITO) produced using sputter coating. We show that our results are in agreement with the hypothesis of fast electron-mediated cooling, facilitated by the Ohmic interface between the gold nanodisks and the TCO substrate, which appears a universal mechanism for providing a new hybrid functionality to active metamaterial devices.

Electrical Characteristics of Top-Down ZnO Nanowire Transistors Using Remote Plasma ALD

Top-down fabrication is used to produce ZnO nanowires by remote plasma atomic layer deposition over a SiO₂ pillar and anisotropic dry etching. Nanowire field-effect transistors (FETs), with channel lengths in the range of 1.3-18.6 μm, are then fabricated using these 80 nm × 40 nm nanowires. Measured electrical results show n-type enhancement behavior and a breakdown voltage ≥75 V at all channel lengths. This is the first report of high-voltage operation for ZnO nanowire FETs. Reproducible well-behaved electrical characteristics are obtained, and the drain current scales with 1/<i>L</i>, as expected for long-channel FETs. A respectable <i>I</i>_ON/<i>I</i>_OFF ratio of 2×10⁶ is obtained.