Millimeter-Wavelength Lens-Absorber-Coupled Ti/Al Kinetic Inductance Detectors

This work presents Ti/Al bi-layer Microwave Kinetic Inductance Detectors (MKIDs) based on lens-coupled spiral absorbers as the quasi-optical coupling mechanism for millimeter-wavelength radiation detection. From simulations, the lens-coupled absorbers provide a 70% lens aperture efficiency in both polarizations over an octave band with a spiral array absorber and over 10% relative bandwidth with a single spiral. We have fabricated and measured two devices with bare Ti/Al MKIDs: a 3x3 cm chip with 9 pixels to characterize the optical response at 85 GHz of the two variations of the absorber; and a large format demonstrator with 253 spiral-array pixels showing potential towards a large format millimeter-wavelength camera. We estimate a sensitivity of 1 $\mathrm{m}\mathrm{K}\cdot {\mathrm{H}\mathrm{z}}^{-0.5}$ and a 95% detector yield.