Second-harmonic generation of 2D-hBN

Step 0: Theoretical framework

Non-linear response calculations are similar to the linear response case, with the only difference that we excite the system with a sinusoidal external field. A perturbation with a specific frequency allows to expand the polarization in the form [math]\displaystyle{ \bf{P}(t) = \sum_{n=-\infty}^{+\infty} \bf{p}_n e^{-i\omega_n t} }[/math] where the coefficient [math]\displaystyle{ \bf{p}_1,...,\bf{p}_n }[/math] are related to [math]\displaystyle{ \chi^{(1)},...,\chi^{(n)} }[/math]. In practice we use the scheme described in Ref. and show below to extract the non-linear coefficients: