Detector-less near-field imaging from comb-emitting THz quantum cascade lasers

Cnr NANO, NEST and Scuola Normale Superiore
Fabio Taddei, Stefan Heun

Optical frequency combs (FCs) are laser sources whose spectrum consists of a set of evenly-spaced lines which share a fixed phase relation with each other. Since their demonstration in the late 90s, they have revolutionised the fields of optical synthesis, metrology and quantum science, with applications in several distinctive fields including molecular spectroscopy, telecommunications, and imaging. The generation of FC in the terahertz (THz) range requires, however, the use of sizable and complex setups since the physics behind the operation of sources and optical components is substantially different at longer wavelengths. Quantum cascade lasers (QCLs) have emerged over the last few years as the most promising source of FCs in the mid-IR and THz spectral ranges [1]. Owing to the large third-order nonlinearity of their gain media, these sources display spontaneous FC operation by self-phase locking of the cavity modes through intracavity four-wave mixing (FWM).

Scattering-type scanning near-field optical microscopes (s-SNOM) operating in the THz range are used to perform imaging overcoming the poor spatial resolution (300µm at 1THz in vacuum) of radiation at long wavelengths. Compared with traditional broad bandwidth sources, such as time-domain spectroscopy systems, THz QCLs possess much higher average power (mW compared to µW) but so far only single-frequency QCLs have been successfully applied to s-SNOM systems, thus granting measurements with greater signal-to-noise ratio but sacrificing the spectral information [2].
In this work, I will discuss the fundamentals of QCLs sources and their use as frequency combs. I will then offer an overview over the current landscape of THz-SNOM systems, primarily focusing onto the application of QCL sources to THz nanoscopy. Finally, I will show our recent successes in applying THz QCL generated FCs to our s-SNOM, discussing our findings and providing the perspectives this technique offers to the field of THz nanoimaging.


[1] J. Faist et al., ‘Quantum Cascade Laser Frequency Combs’, Nanophotonics, vol. 5, no. 2, Jan. 2016, doi: 10.1515/nanoph-2016-0015.
[2] E. A. A. Pogna, M. Asgari, V. Zannier, L. Sorba, L. Viti, and M. S. Vitiello, ‘Unveiling the detection dynamics of semiconductor nanowire photodetectors by terahertz near-field nanoscopy’, Light Sci. Appl., vol. 9, no. 1, p. 189, Nov. 2020, doi: 10.1038/s41377-020-00425-1.


See the SLIDES of the presentation.