THz near-field nanoscopy

The terahertz (THz) gap is the region of the electromagnetic spectrum covering the frequency range of 0.3 to 10 THz. Standing in between microwaves and infrared spectral range, it offers a plethora of applications in electronics and photonics, currently under rapid development. The long-free-space wavelength of THz radiation represents one of the major challenges for the realization of optical components for imaging.
Different near-field probing schemes have been developed to overcome the diffraction limit. Among these, apertureless scattering type Scanning Near-field Optical Microscopy (s-SNOM) achieves wavelength independent spatial resolution that can reach tens of nanometers. The field concentration into sub-diffraction volumes allows to investigate the THz response of nanosized samples and to explore regimes of strong light-matter interaction dominated by polaritonic excitations, such as plasmon- and phonon- polaritons.
In this colloquium, the multimodal THz near-field microscope developed at CNR-NANO will be presented as a compact implementation built on THz quantum cascade lasers, which serve simultaneously as powerful THz sources and as phase-sensitive detectors. The exploited photodetection process is the self-mixing effect occurring upon reinjection of a portion of the output field into the laser cavity after interaction. Most recent results will be discussed including two cases of study. First, the identification of the photodetection mechanisms in semiconductor InAs nanowires at room temperature by spatially resolving THz photocurrents with <35 nm resolution. Second, the study of THz surface polaritons in three-dimensional topological insulator based on bismuth’s compounds to elucidate the interplay of surface and bulk states underlying their near-field THz response. Finally, we will mention perspectives for setup development offered by novel random THz-QCL sources with reduced spatial coherence for speckle-free imaging.

Fabio Taddei (9038) - fabio.taddei@nano.cnr.it
Stefan Heun (9472) - stefan.heun@nano.cnr.it