We introduce a microfluidic refractive index sensor based on a directional coupler architecture using solid-core photonic crystal fibers. The sensor achieves very high sensitivity by coupling the core mode to a mode in the adjacent fluid-filled waveguide that is beyond modal cutoff, and with strong field overlap. We demonstrate the device through the selective infiltration of a single hole with fluid along a microstructured optical fiber. A detection limit of 4.6x10(-7) refractive index units has been derived from measurements with a sensitivity of 30,100 nm per refractive index unit, which is the highest for a fiber device to date.

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We demonstrate the spectral and spatial reconfigurability of photonic crystal double-heterostructure cavities in silicon by microfluidic infiltration of selected air holes. The lengths of the microfluidic cavities are changed by adjusting the region of infiltrated holes in steps of several microns. We systematically investigate the spectral signature of these cavities, showing high Q-factor resonances for a broad range of cavity lengths. The fluid can be removed by immersing the device in toluene, offering complete reconfigurability. Our cavity writing technique allows for tolerances in the infiltration process and provides flexibility as it can be employed at any time after photonic crystal fabrication.

We demonstrate postprocessed and reconfigurable photonic crystal double-heterostructure cavities via selective fluid infiltration. We experimentally investigate the microfluidic cavities via evanescent probing from a tapered fiber at telecommunication wavelengths. Fabry-Pérot fringes associated with modes of the induced cavity are in good agreement with the theory. We also demonstrate a cavity with quality factor Q=4300. Our defect-writing technique does not require nanometer-scale alterations in lattice geometry and may be undertaken at any time after photonic crystal waveguide fabrication.