  Diode effects are of great interest for both fundamental physics and modern
technologies. Electrical diode effects (nonreciprocal transport) have been
observed in Weyl systems. Optical diode effects arising from the Weyl fermions
have been theoretically considered but not probed experimentally. Here, we
report the observation of a nonlinear optical diode effect (NODE) in the
magnetic Weyl semimetal CeAlSi, where the magnetization introduces a pronounced
directionality in the nonlinear optical second-harmonic generation (SHG). We
show demonstrate a six-fold change of the measured SHG intensity between
opposite propagation directions over a bandwidth exceeding 250 meV. Supported
by density-functional theory, we establish the linearly dispersive bands
emerging from Weyl nodes as the origin of this broadband effect. We further
demonstrate current-induced magnetization switching and thus electrical control
of the NODE. Our results advance ongoing research to identify novel nonlinear
optical/transport phenomena in magnetic topological materials and further opens
new pathways for the unidirectional manipulation of light.
