“…Optical microcavities [1], which confine light in small volumes, offer applications for low-threshold microlasers [2,3], optical add/drop filters [4 -7], nonlinear optics [2], single-photon sources [8,9], biosensing [10], and so forth. Recent studies have shown that microcavities in 2D photonic-crystal (PhC) slabs (for example, see [11][12][13]) could provide both a high quality factor (Q) and ultra-small, wavelength-scale mode volume (V), which is critical for these applications. For example, a cavity with Q ϭ 4.5 ϫ 10 4 and V ϭ 7.0 ϫ 10 Ϫ14 cm 3 has been demonstrated experimentally [11], by carefully tuning the position of surrounding air holes.…”