Frequency and magnetic field scanning THz and high-field electron paramagnetic resonance generalized spectroscopic ellipsometry

We present a new technique to detect the optical signatures of electron paramagnetic resonances (EPR) in materials at terahertz (THz) frequencies and high magnetic fields, using polarized light propagation and principles of generalized spectroscopic ellipsometry. Spectroscopic ellipsometry is the preeminent technique to measure accurate optical properties in thin films. Here, this method is extended to the THz range. In contrast to traditional EPR, measurements are performed without modulation and dispense with the need for resonance cavities. Instead, all 15 elements of the normalized Mueller matrix are determined, which contain all information about the full polarization, frequency, and field response of the spin transitions. Measurements between reversal of the magnetic field direction reveal unambiguously the reversal of the handedness of the spin transitions. We present spin characterization for substitutional nitrogen in 4H SiC, substitutional iron in GaN, and substitutional iron in monoclinic-structure Ga2O3. A new dimension for studying the complex response of high spin systems such as Fe3+, for example, is now available with measurements of the frequency-dependence of the magnetic spin susceptibilities. We discuss approaches to model analysis of the magnetic susceptibility tensor which provides quantitative access to amplitude, broadening, and resonance transition parameters, which include handedness and spatial orientation of spin transitions in frequency and magnetic field domains. We revisit the Bloch equations and their implications for the analogue of the Lyddane-Sachs-Teller relationship for dielectric phonon resonances, and we provide sum and superconvergence rules in combination with Onsager's principles and Kramers-Kronig integral conditions. As a result, integral laws are derived which predict the DC magnetic susceptibility and we demonstrate validity from combined DC magnetic susceptibility measurements and THz EPR ellipsometry analyses.