Diffractive Infrared Filters Fabricated by Electron-beam Lithography

With the increasing number of optical systems for use in the near- and middle-infrared spectral regions, the demand for optical elements that exhibit predetermined spectral and polarizing characteristics has increased also. Although continuous thin films are capable of producing a wide variety of optical characteristics, the use of metal meshes adds another dimension to spectral and polarization selectivity. Metal meshes are essentially amplitude diffraction gratings whose periodicity is less than the wavelength of the incident radiation. As such, only a single propagating mode exists, all others being evanescent. Diffractive structures of this type have been shown to produce spectral and polarizing properties that depend on the geometry of the mesh. Microlithographic techniques were used to fabricate diffractive patterns of aluminum and gold, with minimum feature sizes less than 0.25 p,m for use as mesh filters with near and middle-infrared.