Extending the Optical Absorption in a Lumped Element Meander Structure to Far-Infrared Wavelengths

Superconducting radiation detectors typically exhibit detection and single-photon sensitivity limited to the mid-infrared wavelength range. Extending their detection capabilities into the far-infrared range (>10 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i>m) requires careful selection of substrate materials and detector geometries. The overall detection efficiency of such detectors is inherently linked to their absorption and coupling efficiencies. In this study, the resonator geometry and its absorption efficiency were estimated using electromagnetic simulations in CST Microwave Studio simulation software for a lumped-element meander structure. Simulations were performed over the 12–50 μm wavelength range, which corresponds to the Infrared Free-Electron Laser (IR-FEL) at RRCAT, Indore. The absorption in the meander inductor was significantly influenced by the substrate material and thickness, as well as the impedance matching between the detector structure and the incident photon medium. The results indicate that SiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and diamond substrates are well-suited for developing lumped element kinetic inductance detectors (LEKID) in the above wavelength range. Optimised meander geometries on diamond substrates demonstrated absorption efficiencies of up to 95% for narrow bandwidths and greater than 50% for wide bandwidths. Based on the simulations, a 30-pixel LEKID structure was fabricated by electron-beam lithography on a 500 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i>m SiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>-coated Si substrate. The resonator was made from a 20 nm-thick Ti<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</sub>V<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> alloy thin film. Optical characterization was also performed through simultaneous transmission and reflection measurements of the 30-pixel LEKID. Simulation studies demonstrate that the designed LEKID structure is well-suited for detecting far-infrared wavelengths above 10 μm, offering high absorption efficiency.