Narrow-Band Water-Based Absorber With High Return Loss for Terahertz Spectroscopy

A simple, compact, high-performance absorbing load (absorber) for narrow-band THz spectroscopy is described. Current commercially available THz absorbers offering more than 20 dB return loss are bulky, made of proprietary materials, and intrinsically broadband. This 240 GHz absorber consists of a precisely machined thin layer of PMMA (plexiglas) placed over a small volume of water. By diluting the water with glycerol, the effective refractive index of the back medium may be adjusted finely to maximize return loss. The resonant frequency of the absorber is tuned with the PMMA thickness. Reflectometry measurements of the absorber with a vector network analyzer (VNA)-based terahertz spectrometer show a peak return loss of over 47 dB at 240 GHz and an overall peak return loss between 230 and 250 GHz of over 60 dB. Preliminary calculations suggest that such absorbers can be designed to be effective over much of the sub-THz band. In addition, the absorber is highly sensitive to the refractive index of the back medium, suggesting this type of absorber can be used to track minute changes in the dielectric properties of aqueous solutions.