Oxide-film effect on infrared radiative properties of grating structures of aluminum

Aluminum tends to oxidize naturally, which hence changes the radiative properties of the metallic structure. The focus of this study is on the effect of oxide film on the infrared radiative properties of grating structures made of aluminum. Two kinds of grating-surface geometries (namely, square-wave grating and sawtooth grating) with alumina thin film are considered. The impact of the size of the oxide coating on the infrared radiative properties is predicted numerically by the finite difference time-domain method within an incident-wavelength range of 2 to 12 μm. The effect of the alumina film on the cavity resonance of square-wave gratings and the surface wave of sawtooth gratings is discussed. For the oxidized square-wave grating with a given film thickness, four different modes of cavity resonances are observed at different wavelengths. The resonance wavelengths will be changed as the oxidefilm thickness increases. Because of the absorption characteristics of alumina, one mode of resonance is only formed in alumina films, while the others are excited in both alumina films and cavity. The energy of the resonance is mainly absorbed by alumina films when the resonance wavelength within the range of 10.2 to 12 μm, and it is transmitted and absorbed by aluminum substrate within the range of 2 to 10:2 μm. For the oxidized sawtooth grating, no cavityresonance absorption peaks are observed; only one absorption peak exists, which is due to the surface waves absorbed by alumina films. The finding of this work may facilitate the design and applications of metallic gratings in cases in which the metal is easily oxidized.