Photonic crystal slab cavity simultaneously optimized for ultra-high Q/V and vertical radiation coupling

We present a design for a two-dimensional photonic crystal slab cavity in which the electric field localization is due to an extra hole in the lattice, as opposed to the more standard procedure of removal of holes. This leads to a tighter field confinement and a mode volume that is several times smaller than that of conventionally used designs. Through small modifications of the holes around the cavity, we optimize the theoretical quality factor (Q) to an ultra-high value of 20.9 million, and furthermore illustrate the possibility for high coupling efficiency to free-space modes in the vertical direction, while keeping a high Q of 3.7 million.