Authors: Fabrizio Antenucci, Giovanni Lerario, Blanca Silva Fernandéz, Luisa De Marco, Milena De Giorgi, Dario Ballarini, Daniele Sanvitto, and Luca Leuzzi

Physical Review Letters, 126, 173901 – Published 27 April 2021
DOI:  https://doi.org/10.1103/PhysRevLett.126.173901

Abstract: In ultrafast multimode lasers, mode locking is implemented by means of saturable absorbers or modulators, allowing for very short pulses. This occurs because of nonlinear interactions of modes with well equispaced frequencies. Though theory predicts that, in the absence of any device, mode locking would occur in random lasers, this has never been demonstrated so far. Through the analysis of multimode correlations we provide clear evidence for nonlinear mode coupling in random lasers. The behavior of multiresonance intensity correlations is tested against the nonlinear frequency matching condition equivalent to the one underlying phase locking in ordered ultrafast lasers. Nontrivially large correlations are clearly observed for spatially overlapping resonances that sensitively depend on the frequency matching condition to be satisfied, eventually demonstrating the occurrence of nonlinear mode-locked mode coupling. This is the first example, to our knowledge, of an experimental realization of self-starting mode locking in random lasers, allowing for many new developments in the design and use of nanostructured devices.

Diagram of the experimental setup for identifying resonances of random laser modes in space and energy