Design of a Narrowband Filter for Implementing the Undersamping Method in Terahertz Time-Domain Spectrometers

Read more Abstract
In this paper, we propose a design of a narrow-band quasi-optical filter with a central frequency ν = 806 GHz (λ = 372 μm) suitable for the implementation of the undersampling technique in terahertz time-domain spectroscopy. The frequency was chosen to monitor the absorption line of the molecular gas CO in the local transparency window of the atmosphere. The filter is designed as the Fabry-Perot etalon based on a polypropylene film with frequencyselective surfaces (FFS) on both sides consisting of square slots in a sprayed aluminum layer. Through numerical simulation of the transmission of the proposed structure, we defined that the optimum ratio of the width of the metal bridge a to the pitch g of the FSS lies in the vicinity of a/g = 0.5. For g less than half of the operating wavelength λ, the FWHM of the filter is less than 4%, which is sufficient for the implementation of the undersampling method in terahertz time-domain spectroscopy.

Keywords
terahertz time-domain spectroscopy, frequency-selective surfaces, quasi-optical filters, undersampling
Funding
The reported study was funded by RFBR according to the research project № 20-32-90137. The authors acknowledge the NSU Shared Equipment Center “VTAN” for the equipment provided for the project
УДК 535-14

Design of a Narrowband Filter for Implementing the Undersamping Method in Terahertz Time-Domain Spectrometers