Single-Layer Dichroic Filters for Multifrequency Receivers at THz Frequencies

In this work, we report the design, construction,
and characterization of two free-standing single-layer frequencyselective 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 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 flowertype 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.