Ultrahigh-$Q$ Nanocavities in Two-Dimensional Photonic Crystal Slabs

In this paper, we discuss methods to suppress the radiation loss of ultrasmall cavities, of the size of the optical wavelength, in two-dimensional photonic crystal slabs. An important design concept to suppress radiation loss is introduced: The envelope of the cavity mode field should have no abrupt changes and should ideally follow a Gaussian function. Cubic wavelength order cavities, with experimental Q factors of 100 000 and nearly 1 000 000 are obtained by tailoring the envelope functions using air-hole shifts and multistep heterostructures, respectively. In addition, the experimental Q factors of the latest cavities are shown to be determined by the imperfections in the fabricated structures and not by the cavity design. The differences between the experimental and the theoretical Q factors are investigated in order to demonstrate how higher Q factors could be realized in the future