Thermal emission from metallic films perforated with subwavelength hole arrays

This report presents an overview of our study on the optical transmission and thermal light emission properties of subwavelength hole arrays fabricated in a square lattice with 4 um periodicity. The structures were fabricated in thin aluminum (Al) films on silicon (Si) substrates using conventional photolithography. The spectra were obtained using a Fourier transform infrared spectrometer with a port for an external cryostat configured for thermal emission measurements. The perforated films showed extraordinary transmission bands in the mid-infrared spectral range, which could be well explained as due to light coupling to surface plasmon-polaritons on the two film interfaces. We fitted the transmission spectra and calculated the absorption spectra of these structures using a model for the dielectric response that utilizes an effective plasma frequency determined by the individual holes, as well as several resonant modes associated with the reciprocal vectors in the lattice structure factor. We found that the thermal emission spectrum from the perforated films followed the transmission spectrum characteristics, rather than the obtained absorption spectrum; in apparent contrast to Kirchhoff s law of radiation. We conclude that the perforated films behave as radiation filters, where the thermal emission radiation is suppressed in the frequency range outside the transmission resonant bands in the spectrum.