Far-field evanescent wave propagation using coupled subwavelength gratings for a MEMS sensor.

A technique was developed to couple near-field evanescent waves into observable diffraction orders in the far-field region. This investigation was of two gratings that have a 1.0 microm grating period in glass and 1.1 microm in silicon and are individually subwavelength, but when coupled together yield an 11.0 microm effective grating period. This effective grating period is not subwavelength to a 1.550 microm infrared incident source and exhibits higher diffraction orders. Optimum evanescent wave coupling efficiency was simulated by varying the grating thickness and the grating separation between the subwavelength gratings. This proposed evanescent wave coupling concept is being investigated for use in a bulk silicon MEMS accelerometer.