Twist Angle mapping in layered WS2 by Polarization-Resolved Second Harmonic Generation

Abstract

Stacked atomically thin transition metal dichalcogenides (TMDs) exhibit fundamentally new physical properties compared to those of the individual layers. The twist angle between the layers plays a crucial role in tuning these properties. Having a tool that provides high-resolution, large area mapping of the twist angle, would be of great importance in the characterization of such 2D structures. Here we use polarization-resolved second harmonic generation (P-SHG) imaging microscopy to rapidly map the twist angle in large areas of overlapping WS2 stacked layers. The robustness of our methodology lies in the combination of both intensity and polarization measurements of SHG in the overlapping region. This allows the accurate measurement and consequent pixel-by-pixel mapping of the twist angle in this area. For the specific case of 30° twist angle, P-SHG enables imaging of individual layers.

Guide to optical spectroscopy of layered semiconductors

“…For SHG on heterostructures/multilayers, interference effects impacting the SHG intensity from different layers (such as hBN and different TMD materials) need to be considered and can be useful to distinguish between 0° and 60° relative crystallographic orientations 192 . New techniques can exploit this effect to map with high spatial resolution (~400 nm) the armchair orientation in twisted bilayers 194 , as well as in large monolayer areas (>10 4 μm 2 ), and evaluate their crystal quality, as dislocations and grain boundaries can affect the armchair orientation, for example by changing the lattice vector 195 . Also, the presence of uniaxial strain can be quantified by measuring the SHG intensity along different polarization directions 196 .…”

In-Depth Analysis of Egg-Tempera Paint Layers by Multiphoton Excitation Fluorescence Microscopy

“…MPEF imaging was performed using the device developed at the Institute of Electronic Structure and Laser-Foundation for Research and Technology (IESL-FORTH), consisting of a modified inverted Zeiss Axio Observer Z1 microscope and a pair of galvanometric scanning mirrors from Cambridge Technology for raster-scanning. A description of the apparatus can be found in [33,34]. The excitation source was a Yb:KGW Pharos SP laser from Light Conversion, centered at 1028 nm and delivering pulses of 90 fs at a repetition rate of 76 MHz.…”

Probing valley population imbalance in transition metal dichalcogenides via temperature-dependent second harmonic generation imaging

“…Recently, SHG spectroscopy has been highly appreciated as a powerful tool to study two-dimensional (2D) TMDs [20][21][22][23][24][25][26][27] . Owing to the vanishing thickness of these thin crystals, phase-matching conditions are readily satisfied and thus the second-order nonlinear susceptibility, χ (2) , is large 28 .…”

“…In addition, both SHG intensity and polarization have been found to reveal information about the main crystallographic axis 20,21,25,26 , grain boundaries 29,30 , stacking sequence, twist angle 24,27 , number of layers, and crystal homogeneity 26 . Indeed, the ability to control the polarization state of the SHG signal enables the extraction of additional information from a single measurement, since the second-order response is determined by a third rank susceptibility tensor 31 ; therefore, measurements at higher order of response enable access to a larger number of independent quantities of a system 32,33 .…”