Phase-sensitive hyperspectral near-field nanoscope based on self-induced phase locking of terahertz frequency combs

We present a self-mixing intermode-beatnote spectroscopy system, based on a THz quantum cascade laser operating as a frequency comb (FC). The emitted radiation is back-scattered from the tip of a scanning near-field optical-microscope (s-SNOM), which allows to exploit the sensitivity of FC phase-coherence to optical feedback to manipulate the frequency, amplitude and linewidth of the intermode FC beatnote. By stabilizing the phase-locked regimes, we demonstrate, for the first time, a FC based hyper-spectral, THz s-SNOM nanoscope providing coherent detection of multiple phase-locked modes and mapping of the THz near-field response of materials in the 2.29-3.6 THz range, with noise-equivalent-powers of $\sim 400pW/\sqrt{\mathrm{H}}\mathrm{z}$.