Continuous-wave cryogenic optical absorption spectrometer for sub-THz frequencies

TL;DR

This paper introduces a continuous-wave cryogenic optical spectrometer operating between 50 and 1000 GHz, capable of highly sensitive measurements of sample absorption, suitable for various magnetic resonance experiments at low temperatures.

Contribution

The paper presents a novel cryogenic optical spectrometer design that achieves high sensitivity and broad frequency coverage for sub-THz measurements, including compatibility with high magnetic fields.

Findings

Successfully measured ferromagnetic resonance in YTiO₃

Detected electron spin resonance in a reference compound

Observed antiferromagnetic resonance in a van der Waals magnetic material

Abstract

We present the design of a continuous-wave, highly sensitive optical spectrometer for millimeter-wave frequencies between 50 and 1000 GHz. The spectrometer uses photomixing of near-infrared light to generate radiation in a wide frequency range, and the absorbed optical power is determined directly through measurements of the sample temperature. This enables relative sensitivities of up to $10^6$ for the sample absorption coefficient below liquid-helium temperatures, suitable for measurements on highly reflective samples. The instrument is also compatible with high magnetic fields. In order to validate its performance, we measure the ferromagnetic resonance in the Mott insulator YTiO$_3$, the electron spin resonance in a standard free-radical reference compound, and the antiferromagnetic resonance in a van der Waals magnetic material.