Terahertz near-field nanoscopy based on detectorless laser feedback interferometry under different feedback regimes

Near-field imaging techniques, at terahertz frequencies (1-10 THz), conventionally rely on bulky laser sources and detectors. Here, we employ a semiconductor heterostructure laser as a THz source and, simultaneously, as a phase-sensitive detector, exploiting optical feedback interferometry combined with scattering near-field nanoscopy. We analyze the amplitude and phase sensitivity of the proposed technique as a function of the laser driving current and of the feedback attenuation, discussing the operational conditions ideal to optimize the nano-imaging contrast and the phase sensitivity. As a targeted nanomaterial, we exploit a thin (39 nm) flake of Bi2Te2.2Se0.8, a topological insulator having infrared active optical phonon modes. The self-mixing interference fringes are analyzed within the Lang-Kobayashi formalism to rationalize the observed variations as a function of Acket's parameter C in the full range of weak feedback (C < 1).