Terahertz frequency-domain 4 × 4 Mueller matrix ellipsometer instrument designed for high-frequency magnetic resonance measurements.

We report a Mueller matrix ellipsometer design using dual continuously rotating anisotropic meta wave plates, which determines the full set of Mueller matrix elements in the terahertz spectral range. The instrument operates in the frequency domain and employs a frequency tunable, solid state synthesizer based, continuous wave terahertz source with sub-MHz bandwidth. The implemented source permits operation within 82-125 and 170-250 GHz, without and with an additional frequency doubler, respectively. The rotating wave plates produce sufficient modulation of the Stokes vector components of terahertz light to use the wave plates in polarization state generator and polarization state detector devices. Fast terahertz light detection rate of a quasi-optical solid state detector permits acquisition with few microseconds temporal resolution and electronic sweeping control of the source frequency. We develop a fast frequency sweeping scheme while continuously rotating the terahertz wave plates. Subsequent sorting of measured data permits measurements of Mueller matrix elements at hundreds of different wavelengths. The Mueller matrix elements are obtained by forward numerical reduction of the measured data using an algorithm developed by Ruder et al. [Opt. Lett. 45, 3541-3544 (2020)]. The instrument is combined with a magnetocryostat capable of reaching magnetic field strengths of -8 T…8 T and sample temperatures from 4 K…400 K. Hence, the instrument is suitable to measure the full Mueller matrix of samples with magnetic resonances as demonstrated recently by Rindert et al. [Phys. Rev. B 110, 054413 (2024)]. We discuss design, calibration, and example applications.