Ultrahigh-resolution nonlinear optical imaging of the armchair orientation in 2D transition metal dichalcogenides

Abstract

We used nonlinear laser scanning optical microscopy to study atomically thin transition metal dichalcogenides (TMDs) and revealed, with unprecedented resolution, the orientational distribution of armchair directions and their degree of organization in the two-dimensional (2D) crystal lattice. In particular, we carried out polarization-resolved second-harmonic generation (PSHG) imaging for monolayer WS2 and obtained, with high-precision, the orientation of the main crystallographic axis (armchair orientation) for each individual 120 × 120 nm2 pixel of the 2D crystal area. Such nanoscale resolution was realized by fitting the experimental PSHG images, obtained with sub-micron precision, to a new generalized theoretical model that accounts for the nonlinear optical properties of TMDs. This enabled us to distinguish between different crystallographic domains, locate boundaries and reveal fine structure. As a consequence, we can calculate the mean orientational average of armchair angle distributions in specific regions of interest and define the corresponding standard deviation as a figure-of-merit for the 2D crystal quality.

Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation

“…The present work further supports the unique capabilities of PSHG as a method for determining the crystalline integrity of 2D TMDs 23 . First, the crystal characterization can be performed rapidly in an all-optical manner via a single measurement process.…”

“…, where x, y, z denote the crystalline coordinates, with x being the mirror symmetry axis (the armchair direction), and y the axis along which the mirror symmetry is broken (the zigzag direction). The finite second-order optical susceptibility, along with the atomic thickness of 2D TMDs which ensures phase matching, suggest strong optical SHG, which, indeed, has been observed and studied [23][24][25][26][27][28][29][30][31] . For the case of TMDs with D 3h point symmetry, including the monolayers, the SHG equation can be written in matrix form as Methodology for measuring 2D TMD crystal orientation using PSHG The adopted experimental configuration, presented in Fig.…”

“…Recently, the potential for nonlinear optical processes in TMDs has been attracting significant scientific interest. Several groups have explored methodologies to acquire information about the TMD crystals by analysing their second-harmonic generation (SHG) signal [23][24][25][26][27][28][29][30][31] , which is very sensitive to crystal symmetry. The capability of mapping the armchair orientation distribution over large areas of 2D materials with high resolution, could provide a unique tool towards the evaluation of their crystal quality.…”

Polarization‐dependent second‐harmonic generation for collagen‐based differentiation of breast cancer samples

“…The extinction ratio, using cross polarization measurements at the sample plane, was measured to be higher than 25:1 for all linear polarization orientations, which is in agreement with a study that employed a similar setup. 37…”

The research about high-dynamic and low-gray target image differential capture technology based on laser active detection

“…The results of calculation and simulation were verified by equal proportion external field experiments. It was proved that this kind of high-dynamic and low-gray target image differential capture technology based on laser active detection has very good effectiveness [13][14][15][16][17].…”