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Advanced Optical Materials

Competition and Coexistence of Raman and Random Lasing in Silica‐/Titania‐Based Solid Foams

Preeti Gaikwad, Nicolas Bachelard, Patrick Sebbah, Rénal V. Backov, Renaud A. L. Vallée

Published Sep 2, 2015• 24 citations
Publisher

Abstract

Recently, competing random lasing and stimulated Raman scattering mechanisms have been observed for the same gain medium infiltrated in diffusive silica‐based solid foams. Specifically, the light transport mean free path in such disordered media has been identified to be a parameter of choice to tune the observation of either Raman or random lasing. In this contribution, novel silica‐/titania‐based solid foams are designed that allow tuning this parameter significantly, by one order of magnitude. This being achieved, the transition from “pure” random to “pure” Raman lasing is shown, while going through a state of concomitant existence of both mechanisms. The experimental results are well understood, owing to simulations of a simple 1D diffusive model, which suggests that both gain mechanisms may present a complex interplay, leading to random Raman lasing.

DOI
10.1002/adom.201500247

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Recently, competing random lasing and stimulated Raman scattering mechanisms have been observed for the same gain medium infiltrated in diffusive silica‐based solid foams. Specifically, the light transport mean free path in such disordered media has been identified to be a parameter of choice to tune the observation of either Raman or random lasing. In this contribution, novel silica‐/titania‐based solid foams are designed that allow tuning this parameter significantly, by one order of magnitude. This being achieved, the transition from “pure” random to “pure” Raman lasing is shown, while going through a state of concomitant existence of both mechanisms. The experimental results are well understood, owing to simulations of a simple 1D diffusive model, which suggests that both gain mechanisms may present a complex interplay, leading to random Raman lasing.
We predict the coexistence and competition of random lasing and stimulated Raman scattering (SRS) in active disordered random media. We develop a simple model that includes both mechanisms coupled to diffusion equation. We find that the prevalence of a nonlinear mechanism over the other is determined by the degree of scattering. This is confirmed experimentally in dye-infiltrated three-dimensional porous silica-based disordered samples, with controllable pore size and transport properties. Experimentally, random lasing dominates in a sample with a large transport mean free path, l t ≈ 100 μm, while it gives way to SRS for l t ≈ 25 μm. The competition is explained in terms of disorder-dependent pump depletion and fluorescence saturation.
Random lasing (RL) and stimulated Raman scattering (SRS) of dye in multiple scattering media (MSM) appears simultaneously and each couple with other. This coupling has considerable influence on the SRS regularities of dye in MSM. The main feature of this impact is that RL radiation promotes the Raman lines revealing in the RL spectrum range as part of total radiation. SRS initiation occurs owing to the CARS-like mechanism provided by the two component pump: incident monochromatic radiation (laser pump) and RL radiation arising inside the MSM. It leads to important consequences: the RL spectrum must overlap with the spectral region of the possible Stokes lines of the dye; only those Stokes lines appear which are in a range of the RL spectrum; all conditions which promote RL assist SRS also. It is shown MSM promotes the best conditions for SRS and RL coupling due to optimal matching of RL localization regions and pump radiation.
We investigated several types of random lasing modes against frequency and the excitation of active medium. Random lasing occurring from multiple light scatterings and the interference effect of the scattered light has various complex lasing states in disordered structures, in which light waves are localized in various forms. Modes of random lasing are generally classified into localized, extended and transition modes based on their localization types. Numerical methods are used for analysing such complex phenomena. In this paper, a finite-element method is used to investigate the relationship among lasing modes, frequency and population inversion density of active medium for random lasing. Light amplification defined as the ratio of radiative powers between excited and non-excited states of the active medium is computed by changing both the frequency and the population inversion density of the active medium, which is modelled by the negative imaginary part of relative permittivity. The distributions of random lasing modes is strongly influenced by the lasing frequency. We also found that the localized modes that are strongly confined lasing modes in disordered structures do not appear in the frequency ranges in which mean free paths are approximately equal to the half-wavelength.
We report enhanced emission and gain narrowing in Rhodamine 590 perchlorate dye in an aqueous suspension of polystyrene microspheres. A systematic experimental study of the threshold condition for and the gain narrowing of the stimulated emission over a wide range of dye concentrations and scatterer number densities showed several interesting features, even though the transport mean free path far exceeded the system size. The conventional diffusive-reactive approximation to radiative transfer in an inhomogeneously illuminated random amplifying medium, which is valid for a transport mean-free path much smaller than the system size, is clearly inapplicable here. We propose a new probabilistic approach for the present case of dense, random, weak scatterers involving the otherwise rare and ignorable sub-mean-free-path scatterings, now made effective by the high gain in the medium, which is consistent with experimentally observed features.
The spatial formation of coherent random laser modes in strongly scattering disordered random media is a central feature in the understanding of the physics of random lasers. We derive a quantum field theoretical method for random lasing in disordered samples of complex amplifying Mie resonators which is able to provide self-consistently and free of any fit parameter the full set of transport characteristics at and above the laser phase transition. The coherence length and the correlation volume respectively is derived as an experimentally measurable scale of the phase transition at the laser threshold. We find that the process of stimulated emission in extended disordered arrangements of active Mie resonators is ultimately connected to time-reversal symmetric multiple scattering in the sense of photonic transport while the diffusion coefficient is finite. A power law is found for the random laser mode diameters in stationary state with increasing pump intensity.