Optimized Cross-Polarized LEKIDs for W-Band Using Sawtooth Inductors

Lumped-element kinetic inductance detectors (LEKIDs) based on sawtooth inductors for $W$ -band are presented in this article. A careful analysis is carried out for the cross-polarization in the inductor geometry, which brings out the absorption of the nondesired $E$ -field component of an incident wave plane. The proposed inductor geometry with sawtooth sections demonstrates improved cross-polarization. The analytical results are verified by comparison with 3-D electromagnetic (EM) simulations. As the first proof of concept, $W$ -band optical response is demonstrated through quasioptical characterization at room temperature of an aluminum LEKID array. Moreover, a LEKID array based on bilayer superconducting titanium/aluminum (Ti/Al) thin film is developed for evaluating the performance at millikelvin temperatures. Darkness characterization confirms the high-quality factor of the fabricated detectors and the low-frequency design reliability. In addition, cryogenic optical experiments are performed for spectroscopic and detector sensitivity characterization. The proposed geometry opens the possibility of developing large-format polarimetric cameras based on on-chip LEKID structures for future astronomical experiments.