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Weyl semimetals have emerged as a promising quantum material system todiscover novel electrical and optical phenomena, due to their combination ofnontrivial quantum geometry and strong symmetry breaking. One crucial class ofsuch novel transport phenomena is the diode effect, which is of great interestfor both fundamental physics and modern technologies. In the electrical regime,giant electrical diode effect (the nonreciprocal transport) has been observedin Weyl systems. In the optical regime, novel optical diode effects have beentheoretically considered but never probed experimentally. Here, we report theobservation of the nonlinear optical diode effect (NODE) in the magnetic Weylsemimetal CeAlSi, where the magnetic state of CeAlSi introduces a pronounceddirectionality in the nonlinear optical second-harmonic generation (SHG). Byphysically reversing the beam path, we show that the measured SHG intensity canchange by at least a factor of six between forward and backward propagationover a wide bandwidth exceeding 250 meV. Supported by density-functional theorycalculations, we establish the linearly dispersive bands emerging from Weylnodes as the origin of the extreme bandwidth. Intriguingly, the NODEdirectionality is directly controlled by the direction of magnetization. Byutilizing the electronically conductive semimetallic nature of CeAlSi, wedemonstrate current-induced magnetization switching and thus electrical controlof the NODE in a mesoscopic spintronic device structure with current densitiesas small as 5 kA/cm$^2$. Our results advance ongoing research to identify novelnonlinear optical/transport phenomena in magnetic topological materials. TheNODE also provides a way to measure the phase of nonlinear opticalsusceptibilities and further opens new pathways for the unidirectionalmanipulation of light such as electrically controlled optical isolators.

Nonlinear optical diode effect in a magnetic Weyl semimetal