  Broken symmetries induce strong nonlinear optical responses in materials and
at interfaces. Twist angle can give complete control over the presence or lack
of inversion symmetry at a crystal interface, and is thus an appealing knob for
tuning nonlinear optical systems. In contrast to conventional nonlinear
crystals with rigid lattices, the weak interlayer coupling in van der Waals
(vdW) heterostructures allows for arbitrary selection of twist angle, making
nanomechanical manipulation of fundamental interfacial symmetry possible within
a single device. Here we report highly tunable second harmonic generation (SHG)
from nanomechanically rotatable stacks of bulk hexagonal boron nitride (BN)
crystals, and introduce the term twistoptics to describe studies of optical
properties in dynamically twistable vdW systems. We observe SHG intensity
modulated by a factor of more than 50, polarization patterns determined by
moir\'e interface symmetry, and enhanced conversion efficiency for bulk
crystals by stacking multiple pieces of BN joined by symmetry-broken
interfaces. Our study provides a foundation for compact twistoptics
architectures aimed at efficient, scalable, and tunable frequency-conversion,
and demonstrates SHG as a robust probe of buried vdW interfaces.
