Continuously tunable middle-IR bandpass filters based on gradient metal-hole arrays for multispectral sensing and thermography

Abstract

Continuously tunable middle-infrared bandpass filters have been developed based on gradient metal-hole arrays with two distinct geometries. The rotation filter relies on an array of metal holes with gradually changing periods and hole sizes in the azimuthal direction, while the translation filter exploits a metal-hole array with a linear gradient. The filters are fabricated in a Ti film on a ZnSe substrate using electron-beam nanolithography. They are characterized experimentally using Fourier-transform infrared spectroscopy, and the observed results are compared with numerical predictions of the finite element method. The developed filters offer wide spectral tunability when operating with a focused beam. Particularly, the central wavelength of the transmission band is tunable in the [Formula: see text] range, for the rotation filter, and in the [Formula: see text] range for the translation one, as a linear function of the filter angular or linear displacement. The filters feature relatively broad bandwidths of [Formula: see text], while their spectral contrast and energy efficiency depend on the gradient type. The filter spectral response function shape and the extent of its spectra tunability can be further optimized by judicious design of the hole geometry and the metal-hole array gradient, respectively. The developed filters hold strong potential in the infrared multispectral sensing and imaging, thanks to their conceptual simplicity. Considering the linearity of Maxwell’s equations and availability of appropriate technologies for the fabrication of gradient arrays of sub-wavelength metal holes, the developed concept can be translated to other spectral ranges.