A long wavelength infrared narrow-band reflection filter based on an asymmetric hexagonal structure

Abstract

An asymmetric hexagon structure is designed on the micrometer scale to fabricate a narrow-band reflection filter in the long wavelength infrared band. By rotating the hexagon structure 20 degrees, a single narrow-band resonance can be achieved at 7.99μm with high reflectivity of 92% and Q value of 200 simultaneously. The spectral resolution is 40 nm, and the average reflectivity exceeds 90%. By changing the size of the unit cell through a scaling factor k, it can filter across 8-12μm (25 THz–37.5 THz). The dephasing time of the proposed filter is 1.7 ps. We also studied the filtering properties of the proposed filter with significant figure of merit and insertion losses. The average figure of merit can reach 300, and the insignificant insertion loss is only 0.4 dB. The subwavelength filter structure consists of a periodic array of dielectric substrates made of BaF2 and hexagon structure made of Ge with low intrinsic loss, which requires very few layers compared to traditional filters with multilayer films. In addition, the proposed structure is independent to the angles of the incident light. Moreover, it possesses a good tolerance of rounding issue in micro-fabrication process. The position of the center wavelength of the spectra has hardly shift when the sharp vertices become round. The structure can be applied in biosensor and hyperspectral imaging systems.