Intrinsic backward emission of coherent Raman photons from homogeneous media - Nature

MainLight propagation in homogeneous media is strongly constrained by momentum conservation1. As a consequence, scattered photons cannot easily change direction unless the material provides the required spatial frequencies. In the linear regime, this means that light cannot simply ‘turn a corner’: without abrupt edges, gratings or disorder, backward-directed scattering remains strongly suppressed. This fundamental limitation shapes not only our intuition about how light interacts with matter, but also the design of optical instruments that seek to collect scattered photons efficiently, particularly in settings where only same-side access is available. Nonlinear optical interactions obey a more flexible form of momentum conservation2. By combining the wavevectors of multiple optical fields, they open scattering pathways that are strictly forbidden in the linear regime. Remarkably, this enables coherent photons to be redirected around a corner even in a perfectly homogeneous medium3.