Optical Second-Harmonic Interference in Two-Dimensional Heterostructures

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, I will present our recent findings on SHG interference occurring in 2D TMD heterostructures. All 2D crystal samples were prepared by mechanical exfoliation and studied 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.