The PI proposes a development of a new type of optical biosensor that utilizes a photonic crystal (PC) surface as one end facet of a diode-pumped external cavity laser that is capable of both high resolution label-free detection and fluorescence enhancement. The PC is fabricated inexpensively using plastic materials over large surface areas by nanoreplica molding for incorporation into multiwell microplates, and is designed to operate as a resonant reflection filter. For label-free biosensing, adsorption of biomaterial onto the PC surface results in a shift in its resonant wavelength, producing tuning of the external cavity laser operating wavelength. For enhanced fluorescence, rapid buildup of optical standing waves upon the PC surface at the laser emission wavelength will be designed to coincide with the optical absorption wavelength of fluorescent dye molecules used as biomolecule tags, resulting in amplified fluorophore excitation compared to an ordinary glass surface. The external cavity configuration provides single mode, continuous wave, widely tunable, narrowband laser output in which the stimulated emission process, with gain provided by a semiconductor diode, generates an active optical biosensor, in contrast to the wide variety of previous generations of passive reflectors/transmitters. The approach described in this proposal represents an unprecedented combination of high sensitivity + high resolution label-free detection. The project will expand upon recent preliminary demonstration of the external cavity laser biosensor to implement PC sensor surfaces and detection instrumentation capable of multi-mode operation for highly accurate referencing, sub-pm wavelength tuning, and reduction in detection limits for both label-free and fluorescence-tagged biomolecular assays.