Nonlinear optical imaging of monolayered MoS2 using vector beams

Abstract

There is an increasing worldwide interest in 2D materials, which exhibit extremely low thicknesses, down to a single atomic layer. 2D materials have shown to be promising components for a variety of nanophotonics devices utilizing nonlinear optical (NLO) effects. Especially interesting in the context of NLO devices are the transition metal dichalcogenides (TMDs) which in their 2D form offer a variety of interesting and exploitable properties, such as band gap tuning, and controlling the crystal symmetry of the materials. NLO microscopy has been shown to be promising in characterizing various types of different nanoparticles, as well as monolayered TMDs. This characterization technique utilizes NLO effects, such as second-harmonic generation (SHG), or third-harmonic generation (THG). The strict selection rules and the strong directional dependence of these effects allow NLO techniques to be highly useful in probing the crystal symmetry of the sample. By also utilising unconventional structured light beams, such as cylindrical vector beams (CVB), the electric field distribution in the focal volume of the microscope objective can be used and tailored to probe the orientational dependence of the NLO signal efficiently. The strong out-of-plane polarization created by the radially-polarized (RP) beam could potentially be used to probe the so-called dark excitons of such materials. In this thesis, we used NLO microscopy to study monolayered TMD crystals. First, we used a linearly-polarized (LP) fundamental wavelength to map the SHG and THG responses of the monolayered TMD. The directions of the in-plane resonances of the monolayered TMDs, as well as their wavelength dependence, are already known. We validated the measured NLO signals, and verified the wavelength dependent NLO response of the crystal. We conducted the validation and the wavelength dependent measurements using SHG, and THG microscopy. After that, we used polarization resolved NLO microscopy to map the direction of the SHG signal in relation to the input beam. After the experiments using LP beam, we used CVBs to probe the vectorial response of the monolayer in the focal volume of our microscope objective. As opposed to the strong out-of-plane component created by the RP beam, purely transverse electric field distribution can be created by using the azimuthally-polarized (AP) beam. The monolayered TMDs were imaged using the RP beam, AP beam, and the LP beam. The NLO responses of RP and AP beams were found to be comparable, despite the dissimilar polarization structures. After that, we performed polarization resolved scans using the RP beam, and the SHG signal was found be isotropically polarized. Altogether, these two findings suggest that the longitudinal fields are playing a role in the NLO response of TMD crystals. To our knowledge, this is the first demonstration on the use of CVBs for the characterization of microscopic SHG and THG responses from TMDs.