Ultrasmall mode volumes in dielectric optical microcavities.

We theoretically demonstrate a mechanism for reduction of mode volume in high index contrast optical microcavities to below a cubic half wavelength. We show that by using dielectric discontinuities with subwavelength dimensions as a means of local field enhancement, the effective mode volume (V(eff)) becomes wavelength independent. Cavities with V(eff) on the order of 10(-2)(lambda/2n)(-3) can be achieved using such discontinuities, with a corresponding increase in the Purcell factor of nearly 2 orders of magnitude relative to previously demonstrated high index photonic crystal cavities.