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Air-slot photonic nanocavities have many relevant advantages for hybrid integration of external light emitters/absorbers in Si-based optoelectronic devices. Still an experimental major drawback is the difficulty of performing near-field polarization mapping of the photonic modes localized in air-regions. Here, we demonstrate that near-field resonant transmission can achieve high-fidelity vectorial imaging of the electric-field distribution of a nanocavity mode in the challenging case of a silicon heterostructure photonic-crystal cavity realized in an air-slot waveguide where the mode is localized both in air and dielectric regions. We discern and control the in-plane independent polarization components of the electric field of the photonic mode by exploiting the Fano imaging technique. The use of metallic coated aperture near-field probes allows to map the two polarizations with a deep sub-wavelength imaging ( $\lambda $ /15) resolution.

The evolution of silicon optical modulators is recalled, from the first effect demonstrations to the characterization of high-performance devices integrated in optical waveguides. Among possibilities to achieve optical modulation in silicon-based materials, the carrier depletion effect has demonstrated good capacities. Carrier depletion in Si and SiGe/Si structures has been theoretically and experimentally investigated. Large phase modulation efficiency, low optical loss, and large cutoff frequency are obtained by considering simultaneously optical and electrical structure performances. Integrated Mach-Zehnder interferometers and resonators are compared to convert phase modulation into intensity modulation. Finally, recent results on high-speed and low-loss silicon optical modulator using an asymmetric Mach-Zehnder interferometer are presented. It is based on a p-doped slit embedded in the intrinsic region of a lateral pin diode integrated in a silicon-on-insulator waveguide. This design allows a good overlap between the optical mode and carrier density variations. An insertion loss of 5 dB has been measured with a -3 dB bandwidth of 15 GHz.

Three-dimensional (3-D)-tapered structures are designed to carry out efficient coupling from a single-mode optical fiber to a submicrometer square waveguide. The main advantages of such a structure are wavelength and polarization insensitivities and compactness. 3-D taper realization in silicon-on-insulator micro-waveguide is achieved using two electronic lithography steps, the first one being grayscale lithography. Experimental characterizations clearly evidence optical mode expansion fitted to the optical fiber mode and a coupling loss of about 7 dB for a 170-nm square cross section waveguide. Low polarization and wavelength sensitivities are also demonstrated using such a structure

This paper reports modeling and simulation of low-loss coupling between polarization-insensitive silicon-on-insulator (SOI) microwaveguides and single-mode optical fiber, using a two-dimensional taper. The proposed structure is a reduction of both sizes of a square-strip waveguide from a 350/spl times/350-nm/sup 2/ cross section to about 155/spl times/155-nm/sup 2/ cross section. At the output facet, the guided mode is spread out and has a non-Gaussian profile, but low-loss coupling can be achieved by optimizing the overlap integral between the submicrometer waveguide mode and the optical fiber one. The calculated coupling losses are lower than 0.2 dB, the alignment tolerance is larger than 4 /spl mu/m at /spl plusmn/1 dB for the coupling losses, and the total length is about 10 /spl mu/m.

A comb slot photonic crystal waveguide geometry allowing flexible dispersion engineering of wide slot photonic crystal waveguides is proposed. Additionally, a figure of merit combining confinement within the slot and high group index is introduced to quantify the electromagnetic density within slow wave slot waveguides, and is applied to the proposed waveguide geometries. Group indices exceeding 130 and 90 over bandwidths of 1.5 and 2 nm are achieved for slots wider than 300 nm. The figure of merit exhibits sevenfold and fortyfold enhancements in comparison with classical slot waveguides with slot widths around 50 and 100 nm, respectively.