Coherence of Raman light arises from disorder

Abstract

Light propagation in random materials is a topic of great interest for the scientific community, not only for the possible relevant applications in the fields of photonics and renewable energies but even more since it allows to unveil new fascinating phenomena related to wave physics. Among these physical events, the most robust and always surviving any ensemble average is the coherent backscattering of light (CBS). It is a very subtle interference effect in disordered scattering media (such as semiconductor powders or micro-particle suspensions like milk or fog), in which wave coherence is preserved even after a very large number of random scattering events, eventually manifesting as a maximum of interference in the exact backscattering direction. CBS is related to the well-defined wave character and to the preservation of the optical information, for this reason it has been so far experimentally observed and theoretically studied only for elastic scattering, while the occurrence of inelastic scattering is known to reduce the degree of coherence in the diffusion process, affecting the visibility of the effect. Fazio et al. (2017) have demonstrated that this experimental evidence surprisingly survives also for the inelastic light scattering, such as the spontaneous Raman process, as long as the optical information of the propagating wave is retained. In this kind of inelastic scattering events, light loses a small part of its energy by slightly changing wavelength. Its phase coherence, however, is preserved for a very short time, thus making interference between Raman scattered waves still possible. The observed maximum of interference in the exact backscattering direction is therefore a signature of the coherent nature of individual Raman scattering processes. To date, indications on the coherence properties of Raman scattering have been reported only by looking at the nanoscopic scale, through complex near-field experiments making use of very sharp tips or through ultra-fast time resolved techniques. This time, however, we did not rely on complex experiments or advanced techniques. Conversely, the combination of an accurate experimental procedure and the unique structural properties of a silicon-based material were the only simple ingredients for the observation of a new unexpected physical phenomenon. In particular, a dense forest of ultrathin silicon wires arranged in a disordered fashion, where light waves bounce back and forth countless times before coming out, was the medium that allowed us to reveal this new effect, which opens the way for new and important discoveries.