Spectrally-resolved Polarization-resolved Multiphoton Luminescence and Second Harmonic Imaging in multi-layer 2D Materials

Low-dimensional materials, especially two-dimensional (2D) materials, can have significantly enhanced nonlinear optical coefficients. In particular, the second-order nonlinear polarizability Χ⁽²⁾, an important materials parameter for the generation of entangled photon pairs via parametric downconversion, can be notably enhanced in monolayer materials. The relationship between this nonlinear susceptibility and multi-layer stacking, orientation and material defects can be revealed by spectroscopic, polarization-resolved imaging. A spectrally-resolved multi-photon luminescence (MPL) and second harmonic generation (SHG) imaging system, based on a closed loop piezoelectric stage, a transmission grating and an EMCCD is used to examine the nonlinear optical properties of low-dimensional materials. From the spectrally-resolved multiphoton emission, we form ratiometric images of select spectral bands, reducing sensitivity to variations in intensity caused by scattering or attenuation of the excitation beam. We visualize the spectrally-resolved MPL and SHG in multi-layer two-dimensional (2D) materials WSe₂ and In₂Se_3. In WSe₂, we observe the correlation between the symmetry-induced suppression of SHG for even-numbered layers, where inversion symmetry is established, and the presence of edge and localized defect states on the finite-sized 2D flakes, revealed by variations in MPL. We observe an order-of magnitude enhancement in Χ ⁽²⁾ for 3R vs. 2H stacked In₂Se₃. The polarization-resolved SHG in the single and multi-layer 2D material identifies the symmetry and orientation of the layers. The application of the methods to determine Χ ⁽²⁾ in other low-dimensional materials is also described.