Bolometric technique for high-resolution broadband microwave spectroscopy of ultra-low-loss samples

TL;DR

This paper introduces a new low-temperature bolometric technique for high-resolution broadband microwave spectroscopy of ultra-low-loss samples, enabling precise measurements of microwave surface resistance across frequencies.

Contribution

The paper presents a novel non-resonant bolometric method with in-situ calibration for accurate microwave surface resistance measurements of small, low-absorption samples at cryogenic temperatures.

Findings

Minimum detectable power at 1.3 K is 1.5 pW.

Surface resistance sensitivity is approximately 1 micro-ohm.

Method allows temperature-dependent absorption measurements.

Abstract

A novel low temperature bolometric method has been devised and implemented for high-precision measurements of the microwave surface resistance of small single-crystal platelet samples having very low absorption, as a continuous function of frequency. The key to the success of this non-resonant method is the in-situ use of a normal metal reference sample that calibrates the absolute rf field strength. The sample temperature can be controlled independently of the 1.2 K liquid helium bath, allowing for measurements of the temperature evolution of the absorption. However, the instrument's sensitivity decreases at higher temperatures, placing a limit on the useful temperature range. Using this method, the minimum detectable power at 1.3 K is 1.5 pW, corresponding to a surface resistance sensitivity of $\approx$1 $\mu\Omega$ for a typical 1 mm$\times$1 mm platelet sample.