Bulk Photovoltaic Effects in Helimagnets

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. Here, we investigate how helical spin polarization breaks P that is otherwise kept in its crystalline geometry. The magnetic helix widely exists in multiferroics and magnon excitation in collinear magnets. We apply symmetry analysis and a tight-binding model to show how the symmetry is reduced in helimagnetic platforms under various degrees of freedom, namely, different types of spin spiral configurations, spin winding angles, magnetic propagation vectors, and spin chirality. They offer effective and versatile manipulation of BPV generation in helimagnets. Our concept is further illustrated in the monolayer NiI2, a typical helimagnetic material. It could host observable direction-dependent BPV current, which serves as an opto-electric probe to track the subtle magnetic configurations and potential ferroic nature. Our work reveals the fundamental mechanism of the multiferroic order and the nonlinear optics process.