Random Lasing at Localization Transition in a Colloidal Suspension (TiO₂@Silica)

Abstract

Anderson localization of light and random lasing in this critical regime is an open research frontier, which besides being a basic research topic could also lead to important applications. This article investigates the random laser action at the localization transition in a strongly disordered scattering medium composed of a colloidal suspension of core–shell nanoparticles (TiO 2 @Silica) in ethanol solution of Rhodamine 6G. The classical superfluorescence band of the random laser was measured separately by collecting the emission at the back of the samples, showing a linear dependence with pumping fluence without gain depletion. However, frontal collection showed saturation of the absorption and emission. Narrow peaks of approximately equal intensity are observed on top of the classical superfluorescence band, indicating suppression of the interaction between the peaks modes. The linewidth of these peaks is lower than that of the passive modes of the scattering medium. A method called fraction of absorbed pumping allowed us to infer that this peak’s mode (localized modes) is confined to a shallow region near the input-pumping border.

Polydispersed Powders (Nd3+:YVO4) for Ultra Efficient Random Lasers

“…On the contrary, a random laser composed by a homogeneous scattering medium with short transport mean free path would be pumped only from the front side of the sample, reducing considerably the volume and intensity of pumping and, consequently, its efficiency. A promising method called fraction of absorbed pumping (FAP) was used to study the absorption and diffusion processes in the samples, which allowed us to determine that the absorbance increases appreciably with the introduction of smaller grains between the big particles.…”

Influence of silicon nanocrystals on the performance of Yb3+/Er3+: Bi2O3-GeO2 pedestal waveguides for amplification at 1542 nm

“…This enhancement of the refractive index must decrease the intrinsic losses of BGO waveguides associated to border defects and irregular morphology and enhance the guiding effect within the waveguides. In addition, Si nanostructures behave as a source of scattering, which could be linked to an increase of pump photon path length inside the waveguide and, therefore, to an increase of absorption and gain [24][25][26][27][28] . Figure 7.…”

Anomalous transport of light at the phase transition to localization: strong dependence with incident angle

“…Thereby, owing to the inhomogeneity at microscopic scale, localized and extended modes, coming from different regions with klT values lower and higher than unity, respectively, can coexist in a same sample. This picture is what we have called in our previous works as the localization transition regime [6,7,24,43,54]. In this way, the average klT value, extracted from the coherent backscattering experiment, would not provide a definitive criterion for the critical phase of localization transition.…”

“…Thereby, the absorption of reflected light should come from a layer (near the input surface) with thickness shallower than 10 m (supplementary material). From the FAP values, we can estimate the average photon path length (leO) inside the scattering medium before being backscattered leOla(NIB)ln(FAP) [27,43,44], which would yield us an estimative of the increase of light confinement near the input border (≤10 m depth). An increase of the FAP value is observed as the incidence angle is increased (figure S3g supplementary material), reveling an increase of leO and absorption near the input border as the incidence angle is increased.…”

“…A detailed polarization study of the backscattering cone is called for, in order to determine the polarization of the coherently backscattered photons and its relationship with the incidence angle. The increase of localization with incidence angle near the input border could be also interpreted as that photons from superficial localized states that would be emitted by nonlinear effects (non-equilibrium) [43,55] can be again trapped in another superficial localized state, due to the increase of internal reflection. This latter implies in that the density and residence time (Q factor) of superficial localized states would increase as internal reflection (incidence angle) increases, which can be inferred from the theoretical predictions of Mirlin [39,40] and Ramos and co-workers [42] in disordered electronic media.…”