Cross-Polarization Reduction in Kinetic Inductance Detectors Based on Quasi-Lumped Resonators

TL;DR

This work investigates cross-polarization in meandered LEKIDs and tests whether the interdigitated capacitor contributes to polarization leakage. By comparing a conventional LEKID (LER) with a capacitor to a quasi-lumped resonator (qLER) that omits the capacitor, the study shows that parasitic currents in the capacitor degrade polarization purity, while the qLER design reduces cross-polarization through geometric reconfiguration. Cryogenic optical measurements and EM simulations reveal that the qLER maintains similar absorption performance but improves the polarization discrimination from an average XPD of approximately −2.8 dB to about −5.1 dB. Although an improvement, further design refinements are needed to reach polarization-sensitive instrument requirements, with proposed avenues including structural modifications and hybrid resonator approaches to push XPD toward the −20 dB target.

Abstract

Kinetic Inductance Detectors (KIDs) have emerged as a leading technology for millimeter- and submillimeter-wave astronomy due to their high sensitivity, natural multiplexing capabilities and scalable fabrication. In polarization-sensitive applications-such as Cosmic Microwave Background (CMB) studies-cross-polarization, or unintended response to the orthogonal polarization, poses a significant limitation to measurement fidelity. This work investigates the origin of cross-polarization in meandered Lumped Element KIDs (LEKIDs), with particular emphasis on the role of parasitic currents in the interdigitated capacitor. A comparative study between conventional LEKIDs and a quasi-lumped resonator design is presented, demonstrating that removing the capacitive element may improve cross-polarization discrimination, confirming the capacitor's contribution to polarization leakage.

Figures (7)

• Figure 1: (a) Design used for the LER-type detectors. The inset shows a zoomed region of the interdigitated capacitor. (b) Design used for the qLER-type detectors. The inset shows a zoomed region of the ‘compressed’ part of the inductor. A black scale bar indicating 1 mm is included.
• Figure 2: Simulated current distribution for a (a) LER design and (b) qLER design. In both simulations the current flows uniformly across the active region of the detector. Color scale represents the magnitude of the superconducting current normalized by its maximum. Color white indicates where the value of the current is strictly zero.
• Figure 3: Color plot showing the magnitude of the simulated currents in a zoomed region of the interdigitated capacitor. Color scale represents the magnitude of the current, which has been normalized to the maximum value in the inductor. The continuous line in the circular inset indicates the polarization direction aligned with the active region, while the dashed line indicates the cross-polarization direction, which is aligned with the interdigitated fingers in the capacitor.
• Figure 4: (a) Layout of the two types of design. Green color represents the lumped detectors and blue color the quasi-lumped ones. Both types of detectors are interleaved in the chip and are placed with their longer meander sections perpendicular between them. (b) Optical image of the prototype mounted in the measurement holder prior to the cryogenic characterization.
• Figure 5: (a) Amplitude of transmission as a function of frequency, showing resonances corresponding to both designs. LER resonances are highlighted in green at lower frequencies, while qLER resonances are indicated in blue. (b) Internal and external quality factors ($Q_i$, $Q_c$) for each detector family, using the same color scheme as in panel (a). $Q_i$ and $Q_c$ are represented with close and open symbols respectively.