Fabry-P\'{e}rot open resonant cavities for measuring the dielectric parameters of mm-wave optical materials

TL;DR

This paper presents the design and application of Fabry-Pérot open resonant cavities for precise measurement of dielectric and scattering properties of millimeter-wave optical materials at cryogenic temperatures, aiding cosmology experiments.

Contribution

It introduces a hemispherical resonator compatible with cryogenic conditions and discusses improvements for better measurement resolution of optical material properties.

Findings

Effective measurement of dielectric losses at cryogenic temperatures.

Enhanced cavity finesse for improved mode resolution.

Application to materials used in BICEP experiments.

Abstract

As millimeter-wave cosmology experiments refine their optical chains, precisely characterizing their optical materials under cryogenic conditions becomes increasingly important. For instance, as the aperture sizes and bandwidths of millimeter-wave receivers increase, the design of antireflection coatings becomes progressively more constrained by an accurate measure of material optical properties in order to achieve forecasted performance. Likewise, understanding dielectric and scattering losses is relevant to photon noise modeling in presently-deploying receivers such as BICEP Array and especially to future experiments such as CMB-S4. Additionally, the design of refractive elements such as lenses necessitates an accurate measure of the refractive index. High quality factor Fabry-P\'{e}rot open resonant cavities provide an elegant means for measuring these optical properties. Employing a hemispherical resonator that is compatible with a quick-turnaround 4 Kelvin cryostat, we can measure the dielectric and scattering losses of low-loss materials at both ambient and cryogenic temperatures. We review the design, characterization, and metrological applications of quasioptical cavities commissioned for measuring the dielectric materials in the BICEP3 (95 GHz) and BICEP Array mid-frequency (150 GHz) optics. We also discuss the efforts to improve the finesse of said cavities, for better resolution of degenerate higher order modes, which can provide stronger constraints on cavity parameters and sample material thickness.