Polarization-preserving confocal microscope for optical experiments in a dilution refrigerator with high magnetic field

TL;DR

This paper introduces a compact, polarization-preserving confocal microscope designed for quantum optical experiments in high magnetic fields within dilution refrigerators, enabling precise optical control and measurement of electron spins.

Contribution

The authors developed a modular, fiber-based cryogenic confocal microscope compatible with high magnetic fields, optimized for polarization-sensitive quantum optical experiments in semiconductors.

Findings

Successful implementation inside a superconducting magnet bore.

Capability to measure photoluminescence, reflection, and transmission.

Demonstrated quantum optical experiment with donor-bound electrons in GaAs.

Abstract

We present the design and operation of a fiber-based cryogenic confocal microscope. It is designed as a compact cold-finger that fits inside the bore of a superconducting magnet, and which is a modular unit that can be easily swapped between use in a dilution refrigerator and other cryostats. We aimed at application in quantum optical experiments with electron spins in semiconductors and the design has been optimized for driving with, and detection of optical fields with well-defined polarizations. This was implemented with optical access via a polarization maintaining fiber together with Voigt geometry at the cold finger, which circumvents Faraday rotations in the optical components in high magnetic fields. Our unit is versatile for use in experiments that measure photoluminescence, reflection, or transmission, as we demonstrate with a quantum optical experiment with an ensemble of donor-bound electrons in a thin GaAs film.