Chip-scale, electrically-pumped terahertz (THz) quantum cascade lasers (QCLs) can be employed in scattering-type scanning near-field optical microscopy (s-SNOM) to image the response of organic and inorganic materials with nanometer spatial resolution and tomographic sensitivity, overcoming the diffraction limit. By exploiting the self-mixing mechanism, QCLs can work as both sources and detectors, being sensitive to the radiation that is re-injected in the laser cavity after interaction with the tip of the s-SNOM microscope. Interestingly, broadband THz QCL frequency combs (FCs) provide hyperspectral sensitivity to THz s-SNOM systems.
The developed technique can be used to perform fundamental investigations at the nanoscale, spanning from inspecting the carrier density distribution in two-dimensional materials, to monitoring the propagation of plasmon–polariton, and phonon–polariton modes with a ~10 nm spatial resolution and over a broad bandwidth. We applied this method to thin films of topological insulators grown by molecular beam epitaxy (MBE), revealing the presence of Dirac surface states by mapping the propagation of surface polaritons.
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