  Chiral magnets, which break both spatial inversion and time reversal
symmetries, carry a potential for quadratic optical responses. Despite the
possibility of enhanced and controlled responses through the magnetic degree of
freedom, the systematic understanding remains yet to be developed. We here
study nonlinear optical responses in a prototypical chiral magnetic state with
a one-dimensional conical order by using the second-order response theory. We
show that the photovoltaic effect and the second harmonic generation are
induced by asymmetric modulation of the electronic band structure under the
conical magnetic order, and the coefficients, including the sign, change
drastically depending on the frequency of incident lights, the external
magnetic field, and the strength of spin-charge coupling. We find that both
effects can be enormously large compared to those in the conventional
nonmagnetic materials. Our results would pave the way for next-generation
optical electronic devices, such as unconventional solar cells and optical
sensors, based on chiral magnets.
