  Imaging applications in the terahertz (THz) frequency range are severely
restricted by diffraction. Near-field scanning probe microscopy is commonly
employed to enable mapping of the THz electromagnetic fields with
sub-wavelength spatial resolution, allowing intriguing scientific phenomena to
be explored such as charge carrier dynamics in nanostructures and THz
plasmon-polaritons in novel 2D materials and devices. High-resolution THz
imaging, so far, has been relying predominantly on THz detection techniques
that require either an ultrafast laser or a cryogenically-cooled THz detector.
Here, we demonstrate coherent near-field imaging in the THz frequency range
using a room-temperature nanodetector embedded in the aperture of a near-field
probe, and an interferometric optical setup driven by a THz quantum cascade
laser (QCL). By performing phase-sensitive imaging of strongly confined THz
fields created by plasmonic focusing we demonstrate the potential of our novel
architecture for high-sensitivity coherent THz imaging with sub-wavelength
spatial resolution.
