Valdeci Mestre

TiO2@Silica nanoparticles in a random laser: Strong relationship of silica shell thickness on scattering medium properties and random laser performance

A TiO2@Silica nanoparticle has been introduced in a random laser. TiO2 particles with an average diameter of 0.41 μm were coated with silica shells of different thicknesses. Strong dependency of silica shell thickness on the medium scattering strength was found. A mathematical relationship between the scattering mean free path, random laser threshold, and random laser efficiency was developed. Higher efficiency, lower laser threshold, narrower bandwidth, and longest photobleaching lifetime were obtained in the random laser that had increased silica shell thickness. Optical colloidal stability and light coupling enhancement with scattering particles, provided by silica shell, should lead to improved laser performance.

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

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 (TiO2@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.

Laser stabilization to an atomic transition using an optically generated dispersive line shape

We report a simple and robust technique to generate a dispersive signal which serves as an error signal to electronically stabilize a monomode continuous-wave laser emitting around an atomic resonance. We explore nonlinear effects in the laser beam propagation through a resonant vapor by way of spatial filtering. The performance of this technique is validated by locking semiconductor lasers to the cesium and rubidium D2 lines and observing long-term reduction of the emission frequency drifts, making the lasers well adapted for many atomic physics applications.

Core-shell (TiO2@Silica) nanoparticles for random lasers

Scattering media are of great current interest, due to their potential applications in photovoltaic cells, efficient photocatalyzers, random lasers and novel optical functional devices. Here, we have introduced a core–shell scattering medium for random lasing composed by core-shell nanoparticles (TiO2@Silica) suspended in an ethanol solution of Rhodamine 6G. Higher efficiency, lower laser threshold and long photobleaching lifetime were demonstrated in random laser. A promising method called fraction of absorbed pumping (FAP) has been introduced, which opens a new avenue to characterize and study scattering media. In this article, we also investigate the random laser action at the critical regime of localization by increasing considerably the concentration of TiO2@Silica nanoparticles. Narrow peaks arising in the random laser emission spectrum are observed. 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 the saturation of emission and absorption. The emission spectrum of the peak mode (localized modes) shows approximately equal intensity, 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, which was attributed to an anomalous nonlinear increase of the refractive index by localization.

Localization of light: beginning of a new optics

In recent years, there has been a dramatic progress in the photonics field of disordered media, ranging from applications in solar collectors, photocatalyzers, random lasing, and other novel photonic devices, to investigations into fundamental topics, such as localization of light and other phenomena involving photon interactions. Anderson localization of light is an open researcher frontier, which has greatly attracted the attention of researchers in the past few decades. In this work, we study the transport of light in a strongly disordered optical medium composed by core-shell nanoparticles (TiO2@Silica) suspended in ethanol solution. We demonstrate the crossover from a diffusive transport to a localization transition regime as TiO2@Silica nanoparticle concentration is increased. A striking phenomenon of enhanced absorption, mainly near the input border, arises at the localization transition, from which an increase of refractive index was inferred. An increase of the density of localized states and absorption near the input border is reported when the incidence angle is increased. The specular reflection, measured for the photons that enter the sample, is considerably lower than the effective internal reflection undergone by the coherently backscattered photons in the exact opposite direction, indicating a nonreciprocal propagation of light (parity-symmetry breaking). A theoretical simulation, performed through random-matrix theory, agrees satisfactorily with the experimental results, showing the generality of this approach to address transport phenomena.

High Performance in Random Laser Using a Colloidal Suspension of TiO2@Silica Nanoparticles

A new scattering medium for random laser has been introduced. This random laser is composed of TiO2@Silica nanoparticles suspended in an ethanol solution of rhodamine 6G. TiO2 nanoparticles with average diameter of 0.41 μm were coated with a silica shell of ~40 nm thickness. Random laser study comparing TiO2 and TiO2@Silica suspensions was performed. The study showed a higher performance for TiO2@Silica system. This fact was attributed to an increase of the scattering strength (TiO2@Silica) due to a better colloidal stability and light-coupling enhancement with TiO2 scatter cores. Optical and chemical stability has been combined by coating TiO2 nanoparticles with a silica shell of ~40 nm thickness.

Random Lasing of Rhodamine 6G Solution Containing TiO2:Silica Core:Shell Nanoparticles

High efficiency and low rate of photodegradation was obtained in a random laser suspending TiO2@Silica nanoparticles in ethanol solution of Rhodamine 6G. The TiO2 nanoparticles were coated with a silica shell prepared via Stober method.