Single Ti–Al bi-layer LEKID resonator design (top). The resonator is composed of a long inductive meander line and a capacitive element that is also used for tuning the resonant frequency. The effective sheet impedance of the meander line (seen by GHz–THz photons) is adjusted by the geometrical filling factor for a given the film resistivity. To optimise 100 GHz photon absorption, the pixels are back-illuminated through the silicon substrate. In addition, we use a back-short cavity situated at an optimised distance of 750 μm from the pixels (bottom).