KCS

Atom-thick two-dimensional (2D) transition metal dichalcogenides (TMDs) with strong excitonic transitions in the NIR and visible range are excellent media for optical second-harmonic generation (SHG). SHG is not only a powerful structural method owing to its sensitivity to crystallographic symmetry, but also a coherent probe because of its instantaneous but non-dissipative response. In this talk,[1] I will present our recent findings on SHG interference occurring in 2D TMD heterostructures. All 2D crystal samples were prepared by mechanical exfoliation and characterized with a polarized SHG micro-spectroscopy setup powered by a tunable femtosecond Ti-sapphire laser. Polarization-resolved SHG polar plots of MoS2 homo-bilayers exhibited a six-petal pattern with six angular nodes like those of MoS2 monolayers. Remarkably, the SHG behavior of MoS2/WS2 was much different from that of MoS2 bilayers in that the former lacked angular nodes. This anomalous angular background became larger for more staggered hetero-bilayers, but decreased for increased fundamental wavelength. All the results were nicely explained by an SHG interference model considering material-dependent phase delay in their nonlinear response, which was corroborated with phase-resolved interferometric SHG measurements. I will also speak about our efforts on application of SHG spectroscopy in characterizing physicochemical changes in 2D crystals.[2,3] *******Reference******* [1] W. Kim and S. Ryu,* “Optical Second-Harmonic Interference in Atom-Thick Crystals”, To be submitted [2] Y. Ryu, W. Kim, S. Koo, H. Kang, K. Watanabe, T. Taniguchi, and S. Ryu,* “Interface-Confined Doubly Anisotropic Oxidation of Two-Dimensional MoS2”, Nano Lett. 17, 7267 (2017) [2] T. Y. Ko, A. Jeong, W. Kim, J. Lee, Y. Kim, J. E. Lee, G. H. Ryu, K. Park, D. Kim, Z. Lee, M. H. Lee, C. Lee and S. Ryu,* “On-Stack Two-Dimensional Conversion of MoS2 into MoO3”, 2D Materials 4, 014003 (2017)