Random laser tailored by directional stimulated emission

Abstract

A disordered structure embedding an active gain material and able to lase is called random laser (RL). The RL spectrum may appear either like a set of sharp resonances or like a smooth line superimposed to the fluorescence. A recent letter [1] accounts for this duality with the onset of a mode locked regime in which increasing the number of activated modes results in an increased inter mode correlation and a pulse shortening ascribed to a synchronization phenomenon. An extended discussion of our experimental approach together with an original study of the spatial properties of the RL is reported here.PACS numbers: 42.55. Zz, 42.60.Fc The possibility of obtaining lasing into a scattering medium has been predicted in the sixties by Letokov [2] and experimentally realized for bulk lasing material and scattering particles dispersed in liquid dye in the last decades [3][4][5]. These pioneering experiments on random lasers (RL) have been characterized by a line narrowed frequency spectrum (from tens to some nanometers) on the top of the active medium fluorescence spectrum. This phenomenon, denoted as Intensity Feedback Random Lasers (IFRL), may be explained in the framework of diffusion approximation [6] in which all the properties of the photons propagation are defined by the transport mean free path ℓ. This approach, which neglects the wavelike nature of light, is particulary useful in different scattering regimes where it may be used to predict particular properties of RL, like fluctuations [7], or the appearance of spikes in the spectrum at random positions [8]. On the other hand, the diffusion approximation does not allow to predict the existence of RL peaks that have been more recently measured in numerous experiments [9]. These peaks, which appear at a fixed wavelength and whose light is emitted from localized portions of the disordered structure, may be associated with the presence of Anderson localized states [10] or with highly scattering resonances dwelling in the disordered structures [11], that are commonly referred to as Resonant Feedback Random Lasers (RFRL). This kind of lasing emission has been observed in very strong scattering systems (Zinc Oxide or Gallium Phosphide) with very small spatial extension, very small pump spot, or also with stripe shaped spot [12]. In particular it has been shown [13] that by increasing the pump spot of a RFRL a smooth spectrum is recovered.Up to now different theoretical approaches have been suggested to account for the different aspects of RLs. The RL spectrum has been investigated through the strong nonlinear interaction of RL individual lossy modes [14], while an analogy with condensed matter physics a spinglass-like model [15] predicts the existence of various thermodynamic phases including a mode locked condition, in which lasing resonances are synchronized. A different approach valid in the condition in which the This review follows a recent letter [1] in which it has been demonstrated that the particular regime (so far called either resonant or diffusive) in...

Non-locality and collective emission in disordered lasing resonators

“…Appropriate spatial filtering makes it possible to collect only light emitted from the random lasing cluster, eliminating unwanted contributions from amplified spontaneous emission. 24 Here, in fact, we are interested in the non-locality generated by modes sharing an individual cluster. Thus, we compared spectra collected at different locations in the same disordered structure because distinct regimes are expected when varying H: the interaction is low for small H and grows when H is increased.…”

“…We performed the isolation procedure described in Ref. 24, selecting only 'lonely' clusters (clusters for which the nearest neighbor is more than 800 mm away). In this condition, the spectra measured on neighboring clusters do not show any type of correlation or coupling.…”

“…By pumping the area surrounding the cluster, one can generate a flux of directional stimulated emission capable of pumping only modes that are well coupled with the pump shape, and the lasing threshold may be surpassed ( Figure 2). The number of activated modes is controlled by selecting, through a spatial light modulator, the angular aperture H of the wedge-shaped pumped area (see Figure 1b and studies [23][24][25] for further details).…”

Dynamics of phase-locking random lasers

“…When the number of modes is relatively low, it is possible to follow experimentally the transition to a collective state, which has been associated with phase locking [20,21]. When the number of modes is huge, mean-field approximations are possible [22] and allow us to predict the existence of various phases including a mode-locked one [23].…”

“…1). We use a micron-sized assembly and a digital pump shaping (see [21]) to actively control the intermode interaction.…”