High-performance broadband Faraday rotation spectroscopy of 2D materials and thin magnetic films

TL;DR

This paper introduces a high-speed, sensitive Faraday rotation spectroscopy technique capable of measuring 2D materials and thin magnetic films with broad spectral range and automatic background cancellation.

Contribution

The authors develop a CCD-based Faraday rotation spectroscopy method that significantly increases spectral acquisition speed while maintaining high sensitivity and broad spectral coverage.

Findings

Achieved spectral acquisition speeds many orders faster than existing methods.

Demonstrated sensitivity of 20 μrad over 525-800 nm range.

Successfully measured exciton g-factors and magnetic hysteresis in thin films.

Abstract

We present a Faraday rotation spectroscopy (FRS) technique for measurements on the micron scale. Spectral acquisition speeds of many orders of magnitude faster than state-of-the-art modulation spectroscopy setups are demonstrated. The experimental method is based on charge-coupled-device detection, avoiding speed-limiting components, such as polarization modulators with lock-in amplifiers. At the same time, FRS spectra are obtained with a sensitivity of 20 $\mu$rad (0.001$^\circ$) over a broad spectral range (525 nm - 800 nm), which is on par with state-of-the-art polarization-modulation techniques. The new measurement technique also automatically cancels unwanted Faraday rotation backgrounds. Using the setup, we perform Faraday rotation spectroscopy of excitons in a hBN-encapsulated atomically thin semiconductor WS$_2$ under magnetic fields of up to 1.4 T at room temperature and liquid helium temperature. We determine the A exciton g-factor of -4.4 $\pm$ 0.3 at room temperature, and -4.2 $\pm$ 0.2 at liquid helium temperature. In addition, we perform FRS and hysteresis loop measurements on a 20 nm thick film of an amorphous magnetic Tb$_{0.2}$Fe$_{0.8}$ alloy.