Midinfrared optical response and thermal emission from plasmonic lattices on Al films

We studied the midinfrared optical transmission and thermal light emission spectra of subwavelength hole arrays in the form of square lattice with 4 µm periodicity (plasmonic lattice) in aluminum films deposited on silicon substrates. The optical transmission of these films showed temperature independent resonance bands and antiresonance dips in the midinfrared spectral range, which could be explained by a model involving light coupling to surface plasmon polaritons (SPPs) on the two film interfaces. We fitted the transmission spectrum using a dielectric response function that is based on an effective plasma frequency determined by the individual holes and resonant modes associated with the reciprocal vectors in the lattice structure factor; subsequently, we also calculated the absorption spectrum. We found that the absorption spectrum shows bands opposite in phase compared to those in the transmission spectrum, where peaks are replaced by dips and vice versa. The thermal emissivity spectrum of the heated perforated films were measured at elevated temperatures and showed resonant bands similar to those in the transmission spectrum rather than the absorption spectrum, in apparent contradiction to Kirchhoff's law of radiation. We thus conclude that the perforated films act primarily as radiation filters, where thermal emission is suppressed for frequencies outside the resonant transmission bands. This optical filtering is characteristic of weak photonic crystals, where the photon density of states in the heated metal film is only weakly modified by the perforated interfaces, suggesting weak interaction between the SPP excitations on the metal/dielectric interfaces and the photonic states inside the metal film.