Nanoscale terahertz scanning probe microscopy

Terahertz radiation has become an important diagnostic tool in the development of new technologies. However, the diffraction limit prevents terahertz radiation (lambda approximate to 0.01-3 mm) from being focused to the nanometre length scale of modern devices. In response to this challenge, terahertz scanning probe microscopy techniques based on coupling terahertz radiation to subwavelength probes such as sharp tips have been developed. These probes enhance and confine the light, improving the spatial resolution of terahertz experiments by up to six orders of magnitude. In this Review, we survey terahertz scanning probe microscopy techniques that achieve spatial resolution on the scale of micrometres to angstroms, with particular emphasis on their overarching approaches and underlying probing mechanisms. Finally, we forecast the next steps in the field. Recent progress in terahertz scanning probe microscopy is reviewed with an emphasis on techniques that access length scales below 100 nm relevant to material science. An outlook on the future of nanoscale terahertz scanning probe microscopy is also provided.