  A two-dimensional photonic crystal semiconductor microcavity with a quality
factor Q ~ 40,000 and a modal volume Veff ~ 0.9 cubic wavelengths is
demonstrated. A micron-scale optical fiber taper is used as a means to probe
both the spectral and spatial properties of the cavity modes, allowing not only
measurement of modal loss, but also the ability to ascertain the in-plane
localization of the cavity modes. This simultaneous demonstration of high-Q and
ultra-small Veff in an optical microcavity is of potential interest in quantum
optics, nonlinear optics, and optoelectronics. In particular, the measured Q
and Veff values could enable strong coupling to both atomic and quantum dot
systems in cavity quantum electrodynamics.
