Mechanically tunable nonlinear optics from a van der Waals interface in twisted hexagonal boron nitride heterostructures

In the bulk limit, the second harmonic generation (SHG) response from hexagonal boron nitride (BN) includes electric quadrupole and dipole contributions. Both the total layer number and layer parity play a critical role in determining the overall nonlinear optical response. Here we investigate micromechanical devices consisting of a rotatable crystal of bulk BN placed on top of another crystal of BN, where the relative rotational alignment of the two crystal lattices can be dynamically tuned via an atomic force microscope (AFM) in contact mode. We find significant angle dependence of the unpolarized SHG response where the SHG signal is strongly suppressed or enhanced depending on the stacking order (AA’ or AA, respectively) at the homojunction between the top and bottom BN. At intermediate rotation angles of the top BN crystal between 0 and 60 degrees, we observe continuous modulation of both the SHG intensity and polarization. The non-symmetry-selective third harmonic generation (THG) signal remains unchanged over all rotation angles, suggesting that the observed modulation in nonlinear optical response is induced by mechanically tuning the symmetry at the van der Waals interface between the crystals.