Single-Layer Dichroic Filters for Multi-Frequency Receivers at THz Frequencies

In this work, we report the design, construction, and characterization of two free-standing single-layer frequency-selective surface structures to be used as dichroic filters in the THz range. Their spectral responses are aimed to fulfill a stringent band-pass performance in the atmospheric window between $600\; \text{and } 725$  GHz. Specifically, the dichroics have been required to allow a transmission of electromagnetic radiation of at least 90%, achieve a rejection in the stop-band lower than $-25$  dB, and have cross-polarization levels below $-30$  dB. All these specifications were demanded to be satisfied at normal and nonnormal beam incidence. We have studied dichroic filters with hexagonal patterns of two different apertures, a well-known single-hole geometry and, in order to enhance the spectral performance, a novel aperture geometry that we call the flower type. Their transmission characteristics were measured using a Fourier transform spectrometer. The electromagnetic simulations and experimental results not only show a good agreement but they demonstrate that the flower-type geometry can greatly outperform its single-hole counterpart achieving all the desired requirements. In this way, we demonstrate the feasibility of implementing single-layer systems at (sub)-THz frequencies suitable for low-noise astronomical applications.