Light emission from a point-dipole source embedded in a metal-capped chiral sculptured thin film

Abstract

At optical wavelengths, a chiral sculptured thin film (CSTF) may be viewed as a unidirectionally nonhomogeneous continuum that locally possesses orthorhombic symmetry and globally is structurally chiral. The circular Bragg phenomenon exhibited by CSTFs (i.e., a structurally right/left-handed CSTF of sufficient thickness almost completely reflects right/left-circularly polarized light which is normally incident, but left/right-circularly polarized light is reflected very little, within a specific wavelength regime) has been exploited in circular polarization and spectral-hole filters, among other CSTF applications. The multiscale porosity of CSTFs, combined with their polarization-dependent electromagnetic properties, makes them highly promising platforms for optical sensing and light source applications. After developing a theory based on a spectral Green function for light emission from a point-dipole source embedded in a metal-capped CSTF, we found that the intensity and polarization of the emitted light are strongly influenced by the structural handedness of the CSTF as well as the placement and orientation of the source dipole. The emission patterns across both pupils of the dipole-containing CSTF can be explained in terms of the circular Bragg phenomenon exhibited by CSTFs when illuminated by normally as well as obliquely incident plane waves. Much less radiation is emitted through the metal-capped CSTF surface as compared with the non-metal-capped surface. The emission characteristics augur well for the future of CSTFs as optical biosensors as well as light emitters with controlled circular polarization and bandwidth.