High-speed electro-optic modulator integrated with graphene-boron nitride heterostructure and photonic crystal nanocavity.

Nanoscale and power-efficient electro-optic (EO) modulators are essential components for optical interconnects that are beginning to replace electrical wiring for intra- and interchip communications.1-4 Silicon-based EO modulators show sufficient figures of merits regarding device footprint, speed, power consumption, and modulation depth.5-11 However, the weak electro-optic effect of silicon still sets a technical bottleneck for these devices, motivating the development of modulators based on new materials. Graphene, a two-dimensional carbon allotrope, has emerged as an alternative active material for optoelectronic applications owing to its exceptional optical and electronic properties.12-14 Here, we demonstrate a high-speed graphene electro-optic modulator based on a graphene-boron nitride (BN) heterostructure integrated with a silicon photonic crystal nanocavity. Strongly enhanced light-matter interaction of graphene in a submicron cavity enables efficient electrical tuning of the cavity reflection. We observe a modulation depth of 3.2 dB and a cutoff frequency of 1.2 GHz.

High-contrast electrooptic modulation of a photonic crystal nanocavity by electrical gating of graphene.

We demonstrate high-contrast electro-optic modulation of a photonic crystal nanocavity integrated with an electrically gated monolayer graphene. A silicon air-slot nanocavity provides strong overlap between the resonant optical field and graphene. Tuning the Fermi energy of the graphene layer to 0.85 eV enables strong control of its optical conductivity at telecom wavelengths, which allows modulation of cavity reflection in excess of 10 dB for a swing voltage of only 1.5 V. The cavity resonance at 1570 nm is found to undergo a shift in wavelength of nearly 2 nm, together with a 3-fold increase in quality factor. These observations enable a cavity-enhanced determination of graphene's complex optical sheet conductivity at different doping levels. Our simple device demonstrates the feasibility of high-contrast, low-power, and frequency-selective electro-optic modulators in graphene-integrated silicon photonic integrated circuits.

Reduction of scattering loss of silicon slot waveguides by RCA smoothing.

Because of stronger optical confinement density, silicon slot waveguides tend to have higher scattering loss than normal ridge waveguides with same sidewall roughness. A wet chemical process is found to be highly effective in reducing the surface roughness and scattering loss. A reduction in scattering loss by 10.2 dB/cm for TE and 8.5 dB/cm for TM polarizations has been achieved.

Efficient fiber coupler for vertical silicon slot waveguides.

A mode size converter for efficient fiber coupling to silicon slot waveguides was proposed and demonstrated. It consists of two inverted lateral tapers that extend from the two strips of the silicon slot waveguide, and an overlaid low index waveguide with expanded mode size. Parameters including taper length and taper tip width were optimized with computer simulations. Samples were fabricated with a combined electron beam and photolithography process on a silicon-on-insulator wafer. The measured coupling loss to a standard single mode optical fiber was reduced by 8 dB for TE mode and 5.2 dB for TM mode with the converter.

Chemiresistive response of silicon nanowires to trace vapor of nitro explosives.

Silicon nanowires are observed to behave as chemically modulated resistors and exhibit sensitive and fast electrical responses to vapors of common nitro explosives and their degradation by-products. The nanowires were prepared with a top-down nano-fabrication process on a silicon-on-insulator wafer. The surface of the silicon nanowires was modified by plasma treatments. Both hydrogen and oxygen plasma treatments can significantly improve the responses, and oxygen plasma changes the majority carrier from p- to n-type on the surface of silicon nanowire thin films. The sensitivity is found to increase when the cross-section of the nanowires decreases.