THz Filters Made by Laser Ablation of Stainless Steel and Kapton Film

THz band-pass filters were fabricated by femtosecond-laser ablation of 25- [Formula: see text] m-thick micro-foils of stainless steel and Kapton film, which were subsequently metal coated with a ∼70 nm film, closely matching the skin depth at the used THz spectral window. Their spectral performance was tested in transmission and reflection modes at the Australian Synchrotron’s THz beamline. A 25- [Formula: see text] m-thick Kapton film performed as a Fabry–Pérot etalon with a free spectral range ([Formula: see text]) of 119 cm [Formula: see text] , high finesse [Formula: see text] , and was tuneable over ∼ [Formula: see text] m (at ∼5 THz band) with [Formula: see text] tilt. The structure of the THz beam focal region as extracted by the first mirror (slit) showed a complex dependence of polarisation, wavelength and position across the beam. This is important for polarisation-sensitive measurements (in both transmission and reflection) and requires normalisation at each orientation of linear polarisation.