Far-infrared bandpass grid filters

The spectral performance of bandpass grid filters centered at terahertz frequencies is presented. The filters are composed of a periodic array of cross-shaped apertures in an evaporated metal film on a thin dielectric substrate. A commercial Fourier-transform spectrometer is used to measure spectral transmittance. Results are presented for filters using gold films of different thickness. The peak transmittance is correlated with the DC surface resistance R(s) of the film, which is probed using an insitu test structure on the fabricated filters. This is true even though the gold films all have a thickness several times the classical skin depth at the resonant frequency. Measurements of R(s) and peak transmittance at 77 K show an equivalent correlation, indicating that some of the loss is due to the finite conductivity of the gold film. The dielectric substrate causes a downward shift in the center frequency of the filter proportional to the dielectric constant and thickness of the substrate. The center frequency shifts rapidly with increasing dielectric thickness and reaches an asymptotic value reduced from the freestanding case by the square root of the mean dielectric constant. Measurements are compared to a finite-difference time domain (FDTD) model, which accurately predicts the center frequency, but does not account for losses. The performance of these filters, along with their simple fabrication, makes them useful frequency-selective elements at far-infrared wavelengths.