I don't see any technical reason you couldn't implement a relu in optics. There is a whole area, nonlinear optics (I think you may have intended that when you you said 'simple media?' Well, let me see. https://arxiv.org/abs/2201.03787
From two years ago: https://opg.optica.org/ol/fulltext.cfm?uri=ol-45-17-4819&id=... but again, this isn't simple media.