Chiral modes near exceptional points in symmetry broken H1 photonic\n crystal cavities

Abstract

The H1 photonic crystal cavity supports two degenerate dipole modes of\northogonal linear polarization which could give rise to circularly polarized\nfields when driven with a $\\pi$/$2$ phase difference. However, fabrication\nerrors tend to break the symmetry of the cavity which lifts the degeneracy of\nthe modes, rendering the cavity unsuitable for supporting circular\npolarization. We demonstrate numerically, a scheme that induces chirality in\nthe cavity modes, thereby achieving a cavity that supports intrinsic circular\npolarization. By selectively modifying two air holes around the cavity, the\ndipole modes could interact via asymmetric coherent backscattering which is a\nnon-Hermitian process. With suitable air hole parameters, the cavity modes\napproach the exceptional point, coalescing in frequencies and linewidths as\nwell as giving rise to significant circular polarization close to unity. The\nhandedness of the chirality can be selected depending on the choice of the\nmodified air holes. Our results highlight the prospect of using the H1 photonic\ncrystal cavity for chiral-light matter coupling in applications such as\nvalleytronics, spin-photon interfaces and the generation of single photons with\nwell-defined spins.\n