Fabrication and optical characterization of a 2D metal periodic grating structure for cold filter application

Cold filters, which simultaneously reflect infrared light and transmit visible light, prevent overheating in charge-coupled device cameras, microscopes, and other heat-sensitive equipment. This study proposes a cold filter based on a two dimensional (2D) metal periodic grating structure. Conventional dielectric multilayer films with abrupt filtering characteristics are undesirably affected by incident angle, temperature, and polarization. To solve these problems, a 2D metal periodic grating structure, which does not depend on the polarization, was applied. The grating structure comprises an Au layer and an electron beam resist layer, and was fabricated by electron beam lithography. The optical characteristics of this structure in the visible light region were measured by a spectrometer, and the optical properties were related to structural parameters of the double-layer, 2D grating structure. In particular, the reflectance over the entire visible light spectrum decreased at periods of 800 nm and 1 μm. The wavelengths of minimum and maximum reflectance were shifted by changing the spacing between the upper and lower metal layers from 270 to 370 nm. Simulation results suggested that the interference between the upper and lower layers and the surface plasmon resonance between the metal and resist layers occur simultaneously. Therefore, in the visible light region, the reflectance and transmission spectra were controlled by altering the structure of the 2D metal periodic grating.