Layer-dependent third-harmonic generation in multi-layer SnSe2

We report spatially resolved measurement of third-harmonic generation (THG) emission from a Tin diselenide (SnSe₂) multi-layer flake at a fundamental excitation wavelength of 1550 nm using a nonlinear optical microscopy system and study its thickness dependence. We also estimate the magnitude of the real part of the electronic nonlinearity susceptibility (χ⁽³⁾ coefficient) by analyzing the thickness-dependence and found to be approximately 1.6×10^-19 m²/V², which is around 1500 times higher than that of the glass when measured with the same settings. We find excellent agreement between the measured THG thickness dependence and the analytical model considering absorption of harmonic emission in SnSe₂ medium, phase mismatch and the multipath interference due to the underlying oxide/Si substrate. We also measure the second harmonic generation from same flake and find this to be maximum for thickness in the range of 10-12nm.