Generalized phase-extraction of amplitude and phase contrast in coherent THz-s-SNOM based on laser feedback interferometry

We report the use of a generalized phase-extraction algorithm to extract amplitude and phase data in THz scattering-type scanning near-field optical microscopy (s-SNOM) employing laser feedback interferometry in a THz quantum cascade laser. Our approach allows for only a small number of measurements of the self-mixing voltage, thus greatly improving acquisition rates. We investigate the accuracy of this method experimentally and demonstrate its use for fully coherent imaging of the out-of-plane field supported by an individual micro-scale resonator. Furthermore we report a dual-modulation approach that allows direct electronic measurement of the complex scattered field without recourse to fringe fitting.