Observations and Maps of Second Harmonic Generation on 2D Chalcogenide MoS₂ Monolayers and Single Crystalline Thin Films

Non-linear optical responses in 2D transition metal dichalcogenides (TMD) have attracted increasing attention due to unique layered structures and the ability to generate a series of non-linear harmonic waves, such as second harmonic generation (SHG). As an exotic class of atomically thin semiconductors, TMDs have emerged as a new generation of electronic and optoelectronic devices. MoS₂ monolayers and single crystalline thin films were deposited on Si substrates by a high-temperature tube furnace-based physical transport CVD system. MoS₂ monolayers showed SHG signals conducted by a homebuilt optical microscope system with filters to block the fundamental and higher frequency lasers. It was found that powerful SHG signals can be generated from single crystalline thin films compared to monolayers, indicating that the intensity of the non-linear optical responses is proportional to the number of layers. We have successfully mapped SHG profiles on monolayers and thin films through reflection mode by an optical detector, and the maps are analogous to their optical microscopic images. The mapping technique utilizes the intensity contrast of reflected lasers between the SHG area and the substrate region, which is different from the general method used by others. Powerful SHG signals are generated from thick single crystalline thin films, potentially enabling immediate applications in optoelectronic devices and SHG light generators.