Random lasers driven by engineered pumping

Summary form only given. Random lasers (RL) have attracted much attention from the research community but they have been largely regarded as an academic topic with little or no technological interest. Mostly this is due to the fact that, being a system based on disorder, they are hard to subject to any form of control on their performance. Their spectrum is defined by the scattering material which comprises innumerable modes of random shapes and energies filling the gain band of the lasing material. Early advances in trying to gain control over RLs include the tuning of the broad band emission achieved by engineering the scatterers [1]. This permits to channel the gain to a preselected nanometre-broad line and is observed to work in the intensity feedback regime where no single modes are apparent.By engineering the pumping, a large degree of additional control can be achieved. Our technique uses a spatial light modulator (SLM) to create rays of amplified spontaneous emission (ASE) in the dye bath surrounding and imbibing the TiO2 nanoparticles clusters constituting the scattering part of our colloidal self-assembled random laser. These rays can be easily manipulated and provide a great degree of control over the functioning of the RL[2]. In particular it allows to drive the RL between two lasing mechanisms where feedback is provided either by mode resonance (Fig.1a) or by intensity (Fig.1b). This is based on the interaction between participating modes and allows their synchronisation by mode-locking [3]. We have observed that this interaction leads to a spreading of the energy and expansion of the modes (Fig. 1 right) whereby highly localized, distinct lasing modes end up covering the cluster when many modes are excited [4]. By further engineering the pumping it is possible to isolate single modes selected out of the spiky spectrum generated and optimise their intensity suppressing that of the competing modes.When many single-cluster RLs form an ensemble two length- scales are involved: that related to scattering within the lasing cluster (which is sub-micrometre) and that related to inter-cluster amplification by ASE in the dye bath. By controlling the chemistry of the solution where the clusters assemble it is possible profit from the interplay between both length scales without changing the net amount of scattering material [5].