Design and performance of hafnium optical and near-IR kinetic inductance detectors

Abstract

We report on the design and performance of Microwave Kinetic Inductance Detectors (MKIDs) sensitive to single photons in the optical to near-infrared range using hafnium as the sensor material. Our test device had a superconducting transition temperature of 395 mK and a room temperature normal state resistivity of 97 µΩ cm with an RRR = 1.6. Resonators on the device displayed internal quality factors of around 200 000. Similar to the analysis of MKIDs made from other highly resistive superconductors, we find that modeling the temperature response of the detector requires an extra broadening parameter in the superconducting density of states. Finally, we show that this material and design is compatible with a full-array fabrication process which resulted in pixels with decay times of about 40 µs and resolving powers of ∼9 at 800 nm.Optical and near-IR (OIR) MKIDs are superconducting sensors capable of measuring the arrival time and energy of optical to near-infrared photons. 1 They are less sensitive to false counts and radiation damage 2 than semiconductor devices operating in the same wavelength range and can achieve higher readout speeds. Moreover, each MKID is a high quality factor resonator which allows for natural frequency domain multiplexing and distinguishes the technology from other superconducting detectors. These advantages make arrays of OIR MKIDs useful as astrophysics cameras focusing on timedomain astronomy 3-5 and high contrast imaging. 6-8 To date, commissioned instruments have used either nonstoichiometric titanium nitride or platinum silicide alloys as the photon-sensitive material in the resonators and have achieved resolving powers, R = E/∆E, of up to 8 at 800 nm. 9 This resolving power has been shown to be limited equally by stationary noise, generated by two-level systems (TLS) in the device and amplifiers in the readout chain, as well as an intrinsic variance in the photon signal pulse height, likely caused by phonon escape from the superconductor to the substrate during the initial photon energy down-conversion. 10

Phonon-Trapping-Enhanced Energy Resolution in Superconducting Single-Photon Detectors

“…It is striking that most substrate-based MKIDs with a high enough R SN are all limited to R ≈ 10 − 20 3,[12][13][14]16,17 . Here we conclusively show that in Al MKIDs on substrate R is limited by hot phonon loss.…”

“…For PtSi, with T c = 0.9 K, R = 5.8 − 8.1 at wavelengths of 1310-808 nm (a best δE = 0.16 eV) [13]. For Hf with T c = 0.4 K, R = 6.8 − 9.4 (best δE = 0.14 eV) for the same wavelengths [14]. In devices optimized for photon-number resolution using TiN/Ti/TiN trilayers R = 3.6 at 1550 nm [15].…”

Characterization of sputtered hafnium thin films for high quality factor microwave kinetic inductance detectors

“…The error bars represents the standard deviation over the wafer. The data point from N. Zobrist et al [10] is also shown for comparison even though a different sputter system was used. The superconducting transition temperature (Bottom Left) and calculated kinetic inductance using equation 1 (Bottom Right) for the same films are also given.…”

Improving the dynamic range of single photon counting kinetic inductance detectors

“…A photon's energy, then, can be extracted from the overall size of the signal. These types of MKIDs are called single photon counting, and examples of them include lumped element optical to near-IR detectors, [2][3][4][5][6] X-ray thermal kinetic inductance detectors, [7][8][9][10] and position dependent strip detectors. 11,12 The properties of an MKID are encoded in the forward scattering matrix element, S 21 , of the resonator.…”