Two-dimensional materials offer a versatile platform to study high-harmonic generation (HHG), encompassing as limiting cases bulk-like and atomic-like harmonic generation [Tancogne-Dejean and Rubio, Science Advance textbf{4}, eaao5207 (2018)]. Understanding the high-harmonic response of few-layer semiconducting systems is important, and might open up possible technological applications. Using extensive first-principle calculations within a time-dependent density functional theory framework, we show how the in-plane and out-of-plane nonlinear non-perturbative response of two-dimensional materials evolve from the monolayer to the bulk. We illustrate this phenomenon for the case of multilayer hexagonal BN layered systems. Whereas the in-plane HHG is found not to be strongly altered by the stacking of the layers, we found that the out-of-plane response is strongly affected by the number of layers considered. This is explained by the interplay between the induced electric field by electron-electron interactions and the interlayer delocalization of the wave-functions contributing most to the HHG signal. The gliding of a bilayer is also found to affect the high-harmonic emission. Our results will have important ramifications for the experimental study of monolayer and few-layer two-dimensional materials beyond the case of hexagonal BN studied here as the result we found arew generic and applicable to all 2D semiconducting multilayer systems.
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