Non-locality and collective emission in disordered lasing resonators

Random lasing is observed in optically active resonators in the presence of disorder. As the optical cavities involved are open, the modes are coupled, and energy may pour from one state to another provided that they are spatially overlapping. Although the electromagnetic modes are spatially localized, our system may be actively switched to a collective state, presenting a novel form of non-locality revealed by a high degree of spectral correlation between the light emissions collected at distant positions. In a nutshell, light may be stored in a disordered nonlinear structure in different fashions that strongly differ in their spatial properties. This effect is experimentally demonstrated and theoretically explained in titania clusters embedded in a dye, and it provides clear evidence of a transition to a multimodal collective emission involving the entire spatial extent of the disordered system. Our results can be used to develop a novel type of miniaturized, actively controlled all-optical chip. Nonlocal behaviour can occur in a random laser, according to scientists in Europe. Marco Leonetti and co-workers studied the light emitted from a disordered resonator comprising clusters of titanium dioxide, around 1 μm in size, in a dye doped solution. They found that the resonator lased when optically pumped, and that using a spatial light modulator to change the shape of the pump spot switched the emission between two distinct regimes. In one regime, the emission spectrum included a set of sharp spectral peaks spatially originantig from different regions of the laser, whereas in the other the spectrum incorporated only a single peak originated from the whole system. The researchers believe that the latter regime occurs in the case of strong interaction and thus is a sign of nonlocal behaviour, with a coherent oscillation over the entire system.