Presented here is the development and demonstration of a tunable cavity-enhanced terahertz (THz) frequency-domain optical Hall effect (OHE) technique. The cavity consists of at least one fixed and one tunable Fabry–Pérot resonator. The approach is suitable for the enhancement of the optical signatures produced by the OHE in semi-transparent conductive layer structures with plane parallel interfaces. Tuning one of the cavity parameters, such as the external cavity thickness, permits shifting of the frequencies of the constructive interference and provides substantial enhancement of the optical signatures produced by the OHE. A cavity-tuning optical stage and gas flow cell are used as examples of instruments that exploit tuning an external cavity to enhance polarization changes in a reflected THz beam. Permanent magnets are used to provide the necessary external magnetic field. Conveniently, the highly reflective surface of a permanent magnet can be used to create the tunable external cavity. The signal enhancement allows the extraction of the free charge carrier properties of thin films and can eliminate the need for expensive superconducting magnets. Furthermore, the thickness of the external cavity establishes an additional independent measurement condition, similar to, for example, the magnetic field strength, THz frequency, and angle of incidence. A high electron mobility transistor (HEMT) structure and epitaxial graphene are studied as examples. The tunable cavity-enhancement effect provides a maximum increase of more than one order of magnitude in the change of certain polarization components for both the HEMT structure and epitaxial graphene at particular frequencies and external cavity sizes.
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Note that for isotropic samples, or samples which do not cause conversion of polarization from parallel (p) to perpendicular (s) to the plane of incidence and vice versa, the off-block-diagonal elements of the Mueller matrix (M13, M23, M31, M32, M14, M24, M41, and M42) are zero throughout. The optical Hall effect causes anisotropy, and the off-block-diagonal elements differ from zero in strong external magnetic fields. Hence, these elements are often of particular interest for displaying changes in the sample’s optical properties upon the occurrence of the optical Hall effect. Any changes in these elements with the application of an external magnetic field are then directly related to the existence and to the properties of free charge carriers within the layer stack of a given multiple layer sample. See also Refs. 2 and 13.
