Triply Resonant Microwave to Optical Conversion in Erbium-170 Doped Yttrium Orthosilicate

TL;DR

This paper demonstrates microwave to optical upconversion in isotopically purified erbium-doped yttrium orthosilicate at millikelvin temperatures, achieving improved efficiency by removing hyperfine background absorption.

Contribution

It reports the first observation of increased efficiency at lower microwave powers in isotopically purified erbium-doped yttrium orthosilicate, with potential for further improvements.

Findings

Efficiency of 2×10^{-6} achieved at low microwave powers

Removal of erbium-167 hyperfine structure reduces background absorption

Efficiency increases as microwave power decreases at millikelvin temperatures

Abstract

We report microwave to optical upconversion in isotopically purified erbium-doped yttrium orthosilicate in a Fabry-P\'erot resonator at millikelvin temperatures. This follows on from investigations made at higher temperatures and with natural isotopic ratios for the erbium dopants. In these previous investigations the highest efficiency was seen only for moderately strong microwave powers. The removal of the unwanted erbium-167 which has hyperfine structure and provides unwanted background optical absorption, and the lower temperatures has removed this problem. We now see efficiencies still increasing as the microwave power is decreased when we reach the smallest input powers for which we could measure an output. Efficiencies of $2\times10^{-6}$ were observed and we discuss potential improvements, including better optical cavity frequency stability and better thermalisation of the erbium spins.