Nonlinear Optical Imaging, Precise Layer Thinning, and Phase Engineering in MoTe₂ with Femtosecond Laser

The control of layer thickness and phase structure in two-dimensional transition metal dichalcogenides (2D TMDCs) like MoTe₂ has recently gained much attention due to their broad applications in nanoelectronics and nanophotonics. Continuous-wave laser-based thermal treatment has been demonstrated to realize layer thinning and phase engineering in MoTe₂, but requires long heating time and is largely influenced by the thermal dissipation of the substrate. The ultrafast laser produces a different response but is yet to be explored. In this work, we report the nonlinear optical interactions between MoTe₂ crystals and femtosecond (fs) laser, where we have realized the nonlinear optical characterization, precise layer thinning, and phase transition in MoTe₂ using a single fs laser platform. By using the fs laser with a low fluence as an excitation light source, we observe the strong nonlinear optical signals of second-harmonic generation and four-wave mixing in MoTe₂, which can be used to identify the odd–even layers and layer numbers, respectively. With increasing the laser fluence to the ablation threshold (<i>F</i>_th), we achieve layer-by-layer removal of MoTe₂, while 2H-to-1T′ phase transition occurs with a higher laser fluence (2<i>F</i>_th to 3<i>F</i>_th). Moreover, we obtain highly ordered subwavelength nanoripples on both the thick and few-layer MoTe₂ with a controlled fluence, which can be attributed to the fs laser-induced reorganization of the molten plasma. Our study provides a simple and efficient ultrafast laser-based approach capable of characterizing the structures and modifying the physical properties of 2D TMDCs.