Indirect Measurements of Coherent Thermal Emission from a Truncated Photonic Crystal Structure

Abstract

Recently, coherent thermal emission has been demonstrated using a multilayer structure consisting of a one-dimensional photonic crystal coated on a silver layer, that is, a photonic-crystal-on-Ag structure. The key to enabling coherent emission is to excite a surface wave at the photonic crystal-Ag interface in the stop band of the photonic crystal. In this paper, we describe a detailed theoretical and experimental investigation of coherent thermal emission from the fabricated photonic-crystal-on-Ag structure in the near-infrared region. A Fourier-transform infrared spectrometer, equipped with a specular reflectance accessory, was used to measure the spectral reflectance at incidence angles of 10, 30, and 45 deg. In addition, a laser scatterometer was used to measure the angle-resolved reflectance at the wavelength of 891 nm. The emissivity obtained from the measured reflectance exhibits temporal and spatial coherence. The surface-wave dispersion relation at the photonic crystal-Ag interface was calculated using the supercell method with modified matrix equations that guarantee the convergence of solutions, especially when a metallic layer is involved. The resonance condition obtained from measurements closely matches the dispersion relation. Along with the dispersion curves, calculated field magnitude at the photonic crystal-Ag interface also confirms the existence of surface waves where the field is highly localized.