  The bulk photovoltaic (BPV) effect that converts light into electric current
is highly sensitive to the system symmetry and its electronic Bloch
wavefunction. To create a sizable net electric current, it is necessary to
break the centrosymmetry P in its host material. While prior studies mainly
focus on P-broken nonmagnetic (time reversal T-reserved) and collinear
antiferromagnetic PT systems, here we adopt magnetic group theory to scrutinize
how helical spin polarization breaks P that is otherwise kept in its
crystalline geometry. Such spin helix widely exists in intrinsic multiferroics
and magnon excitation in collinear magnets. We demonstrate that magnetic
symmetries are determined by spin spiral configurations, helical propagation
wavevectors, magnetic chirality, and spin winding angles at the home site,
offering effective and versatile manipulation of BPV generation in helimagnets.
This is illustrated by tight-binding model calculations. Furthermore, we apply
our theory to monolayer NiI2, a realistic showcase for helimagnetic material.
Our first-principles calculations show that it could host observable
direction-dependent BPV current, which could serve as a photoelectric probe to
track the subtle magnetic configurations and potential multiferroic nature.
This work reveals the fundamental mechanism of the interplay between spin
spiral order and nonlinear optics process.
