Microwave impedance readout of a hafnium microbridge detector

TL;DR

This paper introduces a microwave impedance-based electron thermometry method using a hafnium microbridge detector, enabling THz-range detection with microwave readout and multiplexing capabilities.

Contribution

It demonstrates a novel microwave impedance readout technique for hafnium micro-absorbers, advancing THz-range detectors with potential for multiplexed applications.

Findings

Device responds smoothly to microwave power at 50-350 mK.

Absorbed power fits hot electron gas model with n=5...6.

Estimated NEP of 10^-18 W/Hz^(-1/2) at 200 mK.

Abstract

We present proof-of-operation for a new method of electron thermometry using microwave impedance of a hafnium micro-absorber. The new method leads to an ultimate THz-range detector suitable for microwave readout and frequency division multiplexing. The sensing part of the device is a hot-electron-gas absorber responding to the incident radiation by variation of its impedance measured at probing frequency about 1.5 GHz. The absorber is a microbridge made from hafnium (Tc = 375 mK, RN = 30 Ohm) sized 2.5 um by 2.5 um by 50 nm and integrated with a planar 600-700 GHz antenna placed near the open end of a quarter-wave CPW resonator (Q-factor about 10^4). All elements of the circuit, except the microbridge, are made from 100-nm thick Nb, including the resonator, which is weakly coupled to a throughput line. The device was tested at 50-350 mK smoothly responding with its transmission coefficient S21 to applied microwave power at the resonance frequency. We have found that the power absorbed by the bridge fits to the model of hot electron gas, P=k(Te^n-Tph^n) (n = 5...6). The idle NEP down to about 10^-18 W/Hz^(-1/2) and the corresponding cross-over temperature for photon background about 5 K are estimated from the measured data. The saturation power of about 1 pW and possibility of moderate gain are anticipated for a practicable device operating at temperature 200 mK. Since the optimum readout frequency is found exactly at the resonance, the detector is insensitive to most phase instabilities at the probing frequency.