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The optical resonances of nanostructures have shown great potential to enhance and control nonlinear optical processes, which are intrinsically weak in ultrathin volumes. There is presently a strong drive towards reconfiguring the resonant response, enabling versatile or broadband functionalities. Among several proposed physical mechanisms (mechanical, thermal,…) all-optical approaches stand out for their fast switching and contactless operation. Pump–probe experiments have demonstrated various forms of intensity-based tuning, exploiting a transient alteration of the material properties. Phase, conversely, has received little attention as a potential control tool. We recently developed a two-pump scheme mixing a pulse at ω with its frequency-doubled replica. The resulting sum-frequency (ω+2ω) and third-harmonic emissions (ω+ω+ω) are coherent and degenerate at 3ω. Because of their opposite parity, their interference is enabled by a symmetry breaking—through directional filtering or by the nanostructure geometry. We reported recently (ArXiv:2307.01794) a 90% intensity modulation and directional routing by an AlGaAs metasurface controlled via the relative phase between the two pumps.
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