High-Speed Imaging of Second-Harmonic Generation in MoS2 Bilayer under Femtosecond Laser Ablation

Abstract

We report an in situ characterization of transition-metal dichalcogenide (TMD) monolayers and twisted bilayers using a high-speed second-harmonic generation (SHG) imaging technique. High-frequency laser modulation and galvano scanning in the SHG imaging enabled a rapid identification of the crystallinity in the TMD, including the orientation and homogeneity with a speed of 1 frame/s. For a twisted bilayer MoS2, we studied the SHG peak intensity and angles as a function of the twist angle under a strong interlayer coupling. In addition, rapid SHG imaging can be used to visualize laser-induced ablation of monolayer and bilayer MoS2 in situ under illumination by a strong femtosecond laser. Importantly, we observed a characteristic threshold behavior; the ablation process occurred for a very short time duration once the preheating condition was reached. We investigated the laser thinning of the bilayer MoS2 with different twist angles. When the twist angle was 0°, the SHG decreased by approximately one-fourth of the initial intensity when one layer was removed. Conversely, when the twist angle was approximately 60° (the SHG intensity was suppressed), the SHG increased abruptly close to that of the nearby monolayer when one layer was removed. Precise layer-by-layer control was possible because of the unique threshold behavior of the laser-induced ablation.

Anisotropic Second-Harmonic Generation Induced by Reduction of In-Plane Symmetry in 2D Materials with Strain Engineering

“…In particular, second-harmonic generation (SHG) as an essential second-order nonlinear optical effect is widely used to characterize the nonlinear optical properties of 2D materials and in turn for high-efficient on-chip frequency conversion. − Therefore, tuning the SHG properties of 2D materials is desirable for the optimization of the nonlinear optical performance of optoelectronic devices. Currently, several ways have been developed to engineer the nonlinear optical properties of 2D materials such as heterostructure design, ,− electrical control, − temperature control, , defect engineering, wavelength control, − stacking order control, ,,− 2D material/metasurface hybridization, − 2D materials on an epsilon-near-zero substrate, structural engineering by nanoscroll, nanotubes, integration with nanowires or quantum dots, as well as strain engineering. ,,,,,− In these methods, strain engineering can directly change the lattice constant of 2D materials, control the SHG intensity, and tune the SHG pattern. ,, There are two kinds of strain engineering methods. One is the biaxial strain method, and the other is the uniaxial strain method. ,,, Compared with biaxial strain, uniaxial strain is an intrinsic anisotropic method for the sensitive in-plane symmetry engineering for 2D materials, which could introduce large flexibility and...…”

Second harmonic generation from aluminum plasmonic nanocavities: from scanning to imaging

“…We think that, especially at this polarization, the localized EM field, leads to other multiphoton processes as well, among them photoluminescence. 13…”

“…Non-linear optical processes, including second harmonic generation (SHG), are highly sensitive to surface modifications, such as asymmetry, defects, dipole orientation, changes in the local chemical environment, or the refractive index, 1–7 and they are used for the readout of surface properties and modification in many fields. 8–15…”

Global imaging for polarization resolved second harmonic generation of WS2 monolayers

“…Polarization-resolved second harmonic generation imaging (PRSHGI) has become a powerful and fundamental technique for analyzing 2D materials due to its atomic-scale sensitivity, versatility, and simplicity [23]. The PRSHGI experimental setup in the literature produces a pixel-by-pixel mapping SH image where the laser beam could raster scan the sample, or the sample could be mounted on a fully motorized scanning stage [24][25][26][27][28][29][30]. The sample is pumped with a linearly polarized and highly focused laser beam then the SH signal is analyzed by a linear polarizer rotated to select the SH polarization component parallel to the polarization of the exciting beam [24][25][26][27][28][29][30].…”

“…The PRSHGI experimental setup in the literature produces a pixel-by-pixel mapping SH image where the laser beam could raster scan the sample, or the sample could be mounted on a fully motorized scanning stage [24][25][26][27][28][29][30]. The sample is pumped with a linearly polarized and highly focused laser beam then the SH signal is analyzed by a linear polarizer rotated to select the SH polarization component parallel to the polarization of the exciting beam [24][25][26][27][28][29][30]. In this work, a simple and fast PRSHGI technique is introduced where a global one-shot SH image for WS 2 monolayer is produced.…”