Strong optical confinement between nonperiodic flat dielectric gratings

We present a novel design of optical microcavity where the optical energy resides primarily in free space and therefore is readily accessible to foreign objects such as atoms, molecules, mechanical resonators, etc. We describe the physics of these resonators and propose a design method based on stochastic optimization. Cavity designs with diffraction-limited mode volumes and quality factors in the range of 10(4)-10(6) are presented. With a purely planar geometry, the cavity can be easily integrated on-chip by using conventional micro- and nanofabrication processes.