Femtogram dispersive L3-nanobeam optomechanical cavities: design and experimental comparison.

We present the design and experimental comparison of femtogram L3-nanobeam photonic crystal cavities for optomechanical studies. Two symmetric nanobeams are created by placing three air slots in a silicon photonic crystal slab where three holes are removed. The nanobeams' mechanical frequencies are higher than 600 MHz with ultrasmall effective modal masses at approximately 20 femtograms. The optical quality factor (Q) is optimized up to 53,000. The optical and mechanical modes are dispersively coupled with a vacuum optomechanical coupling rate g(0)/2π exceeding 200 kHz. The anchor-loss-limited mechanical Q of the differential beam mode is evaluated to be greater than 10,000 for structures with ideally symmetric beams. The influence of variations on the air slot width and position is also investigated. The devices can be used as ultrasensitive sensors of mass, force, and displacement.