Development of MKIDs in the Optical and Near-Infrared Bands for SPIAKID

Abstract

SpectroPhotometric Imaging in Astronomy with Kinetic Inductance Detectors (SPI-AKID) aims at designing, building, and deploying on the sky a spectrophotometric imager based on microwave kinetic inductance detectors (MKIDs) in the optical and near-infrared bands. MKIDs show a fast response and the ability to resolve photon energy compared to the conventional Charge-coupled Devices (CCDs). In this paper, we present the design and simulation of the MKID arrays for SPIAKID. The detectors consist of four arrays with each array of 20,000 lumped-element pixels, and each array will be read with 10 readout lines. The meander material of the resonators is trilayer TiN/Ti/TiN to have better uniformity of the critical temperature across the array. We also present the measurement result for a test array with 30 × 30 pixels which is a subset of the designed 2000-pixel array to verify the design and fabrication. The current measured best energy-resolving power R = E∕ΔE is 2.4 at = 405 nm and the current medium R is around 1.7. We have also observed that the response of the TiN/Ti/TiN is much smaller than expected.

Investigation of quasi-particle relaxation in strongly disordered superconductor resonators

“…With the n 2 term influencing the recombination rate in the RT equation, a higher D qp translates to slower QP relaxation. In this case, η will increase as the temperature increases, consistent with the previous publications [3,13,14], where an increase of responsivity of the MKIDs has been observed when T bath rises. The fitted η shows large uncertainty as η is mainly related to the maximum of the pulse, which usually shows large uncertainty.…”

“…The TiN films are deposited on sapphire substrates by magnetron sputtering, which are then patterned into resonators using photolithography and reactive ion etching. The films' internal quality factor Q i is relatively low as they are fabricated on sapphire [13,14,39], which is probably due to the lattice mismatch on their interface. The MKIDs design of TiN 1K can be found in the appendix B The design of TiN 2K and TiN 4K can be found in our previous publication [13].…”

“…A direct measurement of the QP diffusion coefficient of TiN at low temperatures is needed to investigate the diffusion coefficient further. The low diffusion coefficient can be the reason for the response of the MKIDs made of disordered superconductors being small [13,14].…”

“…Second, their resonance frequency shift versus T bath is proportional to the optical power (P opt ) under steady illumination [3,11], while the resonance frequency of MKIDs made of superconductor described by BCS theory [12] is proportional to [9] √ P opt . Third, the response in the optical band is much smaller than expected [8,13,14]. Fourth, the density of states (DoS) of the QPs below the energy gap (∆) is not zero due to the energy gap being smeared by the disorder [15].…”