High-Q silicon two-dimensional photonic crystal slot nanocavities with asymmetric low-index claddings

Photonic crystal nanocavities with high quality (Q) factors find extensive application in silicon (Si)-integrated photonics owing to their highly selective wavelength filtering, optical buffering, and enhanced nonlinear optical effects in the telecommunication band. High-Q Si photonic nanocavities with asymmetric claddings offer mechanical stability, high functionalities from heterogeneous materials, and vertical integration of optoelectronic devices. However, achieving a high Q factor in an asymmetric structure remains challenging because of the TE–TM coupling loss in the Si slab. To suppress the TE–TM coupling, we designed a high-Q two-dimensional (2D) Si photonic crystal slot cavity by significantly reducing the electric field components in the slab, leveraging a large dielectric discontinuity between Si and the low-index slot. We fabricated 2D Si photonic crystal slot nanocavities with asymmetric claddings consisting of a lower cladding of thermal oxide (nlc = nBOX = 1.45) and an upper cladding of infiltrated spin-on glass (nuc = nSOG = 1.3). The Q factor of this slot cavity is as high as 6.32 × 105, which is the highest Q value ever recorded among nanocavities with asymmetric claddings. Our results are useful for heterogeneous integration of Si photonic crystal nanocavities with various functionalities such as active and nonlinear optical materials, which are unattainable in conventional Si photonics.