High-efficiency broadband mid-infrared absorption in asymmetrically matched metallic meanders: Development of Ti40V60 alloy-based lumped element kinetic inductance detectors for mid-far IR

Fast, highly sensitive, and broadband detectors operating in the mid-to-far-infrared (MIR–FIR) spectral region are essential for applications ranging from astrophysics to time-resolved spectroscopy using laboratory-based sources and beamlines at Infrared Free-Electron Laser (IR-FEL) facilities. Conventional Lumped Element Kinetic Inductance Detectors (LEKIDs) achieve high optical efficiency using resonant quarter-wavelength (λ/4) backshort cavities. However, this cavity-based approach is inherently narrowband and becomes optomechanically challenging at the mid-to-far-infrared (MIR–FIR) wavelengths. Here, we present a completely backshort free LEKID absorber architecture that achieves broadband absorption exceeding 50%. The meander absorbers were fabricated out of superconducting Ti–V alloys and characterized using the IR radiation from the IR-FEL at the Raja Ramanna Centre for Advanced Technology (RRCAT), India. By illuminating through the silicon substrate and matching the sheet resistance of the meander to the wave impedance of the silicon substrate, front-side reflection is strongly suppressed. Simultaneously, the sub-wavelength periodicity of the dense meander inhibits propagating transmission into free space via evanescent-wave confinement. This combined mechanism drives efficient Ohmic dissipation within the metallic meander, yielding an experimental absorption efficiency ranging from 50% to 90% across the 12.5–30 μm wavelength range. The cavity-free design greatly simplifies the focal plane array fabrication while providing the broadband response required for the next-generation detectors. To assess the suitability of the Ti40V60 alloy as an active superconducting detector material, a test LEKID resonator was also designed, fabricated, and experimentally characterized. An RF resonance at a frequency close to the simulated response was observed at temperatures below superconducting transition, thereby confirming the suitability of Ti–V alloys for LEKID development.