Threshold of random lasers in the incoherent transport regime

Diffusive to quasi-ballistic random laser: incoherent and coherent models

“…The vastly different length scales on which random lasing may occur, and the many different physical systems in which they have been realized, have triggered the development of different theoretical approaches to describe this phenomenon [19][20][21][22][23][24][25][26]. Whereas it might appear reasonable that a radiative transfer approach, which does not incorporate interference effects, may be appropriate for astronomical length scales with long amplifying paths and few scattering events, and a diffusive model may be suitable to describe strongly scattering media in the diffusive limit [27], it has so far remained unexplored how to describe the crossover between such different regimes.…”

Tuning random lasing in photonic glasses

“…For uncorrelated random systems in which interference between the scattered waves can be neglected, diffusive models are very accurate even in presence of optical gain [12,13] and they provide the time evolution of the lasing process and a smooth lasing spectrum with no spiking lasing behaviour [14]. The radiative transport model with gain can also be solved for * Corresponding author: michele.gaio@kcl.ac.uk instance with Monte Carlo simulations which consider a random walk of photons [15,16] and in which amplification of single paths can be important in defining the spectral properties [17], and by solving the complete radiative transfer equations [18]. These approaches allow the study of large systems (> 100s mean free paths) and geometries similar to real experiments.…”

Controlling directionality and the statistical regime of the random laser emission

“…Moreover, an optimization procedure of the pump profile to select extended modes with different orientations has been proposed to obtain a directionality tuning in weak scattering systems [9]. A systematic experimental study on magentamode selection and tunability of a random laser by a proper pump spatial shape was reported in small clusters of T i0 2 nanoparticles embedded in dye solution [10], Another relevant feature in which random laser emission differs from a conventional laser resides in the possible presence of large fluctuations [11], Random spikes in the spectrum have been observed in weakly scattering media and several theoretical and experimental works have evidenced that such behavior has statistical origin and does not require localization or interference [12,13], In such regime theoretical results obtained from a time-dependent model based on radiative transfer equations and disordered-averaged modes has been also reported [14]. Extended modes, identified as rare long paths that are uncoupled from gain competition (lucky photons), can experience a large gain, causing random narrow 'Corresponding author: federico.tommasi@unifi.it spikes in the output spectrum [15], Crossovers among differ ent statistical regimes of random laser emission have been identified and studied by applying a theoretical model based on phase-insensitive intensity feedback and random walks [16,17], A Levy statistics in the emission spectrum emerges in the presence of large fluctuations, while a Gaussian statistics is typical of smooth and regular spectra.…”