• Terahertz
• near-field microscopy
• SNOM

THz radiation covers a portion of the electromagnetic spectrum that is very hard to investigate. Visible and radio frequencies are well studied and relatively easy to generate. The energy of photons with frequencies in the visible spectrum is of the same order of magnitude of atomic transitions. Exploiting the process of stimulated emission it is possible to create a laser that emits photons in the visible spectrum. Radio waves can be generated by alternating electrical current. In the region between visible and radio waves a common radiation sources are of Quantum Cascade Lasers (QCL) sources that actually require cryogenic temperatures. This makes the investigation of THz frequencies very difficult. This is unfortunate, since THz radiation has the potential for many applications. The energy of vibrational and rotational transitions of many atoms and molecules falls in this spectral range. This allows the use of THz radiation to identify atoms and molecules. Another possible application is in airport security where a non intrusive method of identification of illegal substances is required, or in industry to monitor production.


Using radiation in THz range in microscopy appears unfeasible at the first glance. Diffraction limit imposes that an optical microscope employing THz radiation would be unable to have a resolution lower then 0.61 λ/n, with λ being the THz wavelength and n being the index of refraction. This means that details smaller then 100 µm cannot be easily unveiled. This limit holds for microscopes that use far field radiation. In this work we demonstrate a novel THz near field optical microscopy technique in which a QCL source is simultaneously used as source and detector in the self-mixing configuration. We used a scattering-type Scanning Near Field Microscope (SNOM) by Neaspec GmbH.


Self-mixing interferometry is a measurement technique that makes use of the coherence properties of a laser beam and of the high sensitivity of interferometric detection. In this technique the laser beam is reflected from an object back inside the laser. The reflected light and the generated light interferes inside the laser cavity, causing changes in the optical mode and the laser voltage. Those changes are monitored and recorded by lock-in amplification.

In this work we describe the optical setup in details, together with the alignment procedures such as the use of the pilot laser for the crude alignment and the use of the self mixing signal itself for the finer alignment.


As application of this novel instrument we probed 2D nanomaterials. In particular, we investigated black phosphorus, tin selenide (SnSe) and tin diselenide (SnSe2). In order to be able to change the carriers density inside those materials, influencing their response to the THz radiation, we fabricated several field effect transistor devices (FETs) for each material. The fabrication process was performed inside the facilities of Laboratorio NEST - National Enterprise for nanoScience and nanoTechnology. The fabrication procedures such as substrate preparation, mechanical exfoliation, SEM image acquisition, transistor design, electronic beam lithography, reactive ion etching, evaporation, lift-off, atomic layer deposition and bonding are described. The setup for electrical characterization of the produced FETs is presented.


The final scans acquired by the self-mixing SNOM setup are presented and discussed. For devices of each material the topography maps, the THz scans and the electrical characteristics are provided.