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We investigate photonic crystal waveguides that are formed by holes of reduced diameter within a hexagonal lattice of cylindrical airholes in thin freestanding silicon slabs. The waveguides operate in both an even-symmetry bandgap and a partial gap of odd-symmetry modes that form a complete two-dimensional bandgap under the light line. The operating frequency is tuned by the small-hole diameter to fit within the range of both bandgaps and to match a free-space wavelength of 1550 nm. Their narrow bandwidth and low group velocity of light propagation renders the waveguides useful as filters or sensing elements. Because of the strong dependence of the waveguide mode characteristics on structural changes, the highest-precision lithographic fabrication techniques must be applied.

In this paper we present a laser sensor for highly spatially resolved flow imaging without using a camera. The sensor is an extension of the principle of laser Doppler anemometry (LDA). Instead of a parallel fringe system, diverging and converging fringes are employed. This method facilitates the determination of the tracer particle position within the measurement volume and leads to an increased spatial and velocity resolution compared to conventional LDA. Using a total number of four fringe systems the flow is resolved in two spatial dimensions and the orthogonal velocity component. Since no camera is used, the resolution of the sensor is not influenced by pixel size effects. A spatial resolution of 4 microm in the x direction and 16 microm in the y direction and a relative velocity resolution of 1x10(-3) have been demonstrated up to now. As a first application we present the velocity measurement of an injection nozzle flow. The sensor is also highly suitable for applications in nano- and microfluidics, e.g., for the measurement of flow rates.