There has been growing interest in scattering media in recent years, due to their potential applications as solar collectors, photocatalyzers, random lasers and other novel optical devices. Here, we have introduced a novel core-shell scattering medium for a random laser composed of TiO2@Silica nanoparticles. Higher efficiency, lower laser threshold and long photobleaching lifetime in random lasers were demonstrated. This has introduced a new method or parameter (fraction of absorbed pumping), which opens a new avenue to characterize and study the scattering media. Optical chemical and colloidal stabilities were combined by coating a suitable silica shell onto TiO2 nanoparticles.

In recent years, there has been dramatic progress in the photonics field in disordered media, ranging from applications in solar collectors, photocatalyzers, random lasing, and other novel photonic functions, to investigations into fundamental topics, such as light confinement and other phenomena involving photon interactions. This paper reports several pieces of experimental evidence of localization transition in a strongly disordered scattering medium composed of a colloidal suspension of core-shell nanoparticles (TiO2@silica) in ethanol solution. We demonstrate the crossover from a diffusive transport to a localization transition regime as the nanoparticle concentration is increased, and that an enhanced absorption effect arises at localization transition.

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.

Aqueous colloidal silver nanoparticles have substantial potential in biological application as markers and antibacterial agents and in surface-enhanced Raman spectroscopy applications. A simple method of fabrication and encapsulation into an inert shell is of great importance today to make their use ubiquitous. Here we show that colloids of silver-core/silica-shell nanoparticles can be easily fabricated by a laser-ablation-assisted chemical reduction method and their sizes can be tuned in the range of 2.5 to 6.3 nm by simply choosing a proper water-ethanol proportion. The produced silver nanoparticles possess a porous amorphous silica shell that increases the inertness and stability of colloids, which decreases their toxicity compared with those without silica. The presence of a thin 2 to 3 nm silica shell was proved by EDX mapping. The small sizes of nanoparticles achieved by this method were analyzed using optical techniques, and they show typical photoluminescence in the UV-vis range that shifts toward higher energies with decreasing size.

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.

This paper reports the synthesis of silver nanoparticles coated with porous silica (Ag@Silica NPs) using an assisted laser ablation method. This method is a chemical synthesis where one of the reagents (the reducer agent) is introduced in nanometer form by laser ablation of a solid target submerged in an aqueous solution. In a first step, a silicon wafer immersed in water solution was laser ablated for several minutes. Subsequently, an AgNO3 aliquot was added to the aqueous solution. The redox reaction between the silver ions and ablation products leads to a colloidal suspension of core-shell Ag@Silica NPs. The influence of the laser pulse energy, laser wavelength, ablation time, and Ag+ concentration on the size and optical properties of the Ag@Silica NPs was investigated. Furthermore, the colloidal suspensions were studied by UV–VIS-NIR spectroscopy, X-Ray diffraction, and high-resolution transmission electron microscopy (HRTEM).

inherent cost advantages become attractive for the large majority of applications.In order to overcome the problems of random lasers associated to nondirectional output and lack of efficiency, the main approach has been to choose lowdimensional random lasers. 1D fiber random lasers are well suited for this purpose and have achieved up to several watts of continuous output. [10] 2D distributed feedback (DFB) lasers have demonstrated highly efficient and directional output in the microjoule range. [11] These low-dimensional random lasers are generally quite large (like in the case of random fiber lasers) and require sophisticated production methods that are in stark contrast to the simplicity and practicality of the 3D random laser production.Noginov and co-workers have studied the dependence of random laser emission in neodymium doped powders (Nd 0.5 La 0.5 Al 3 (BO 3 ) 4 ) on the particle size, the powder volume density, and the pump spot size. [12,13] Best reported efficiency was below half a percent. An impediment for increasing the efficiency is the surface reflectivity of the compacted powders. The bulk reflection coefficient of Nd 0.5 La 0.5 Al 3 (BO 3 ) 4 at λ = 532 nm for medium to high powder density is ≈0.7. [13] Using a fiber-coupled random laser, where the pump fiber terminates deep inside the scattering medium in order to deliver the pump energy directly into the gain volume without reflection loss at the surface, Noginov et al. achieved a higher efficiency of ≈0.7%. [14] Azkargorta et al. achieved 20% and 42% slope efficiencies with respect to pump power using Nd: yttrium aluminium garnet (YAG) and Nd 3 Ga 5 O 12 crystal powders. [15,16] The stimulated random laser (RL) emission of these rare earth doped powder pellets comes in the form of a Lambertian emission with a linewidth that decreases around laser threshold and becomes much smaller than typical amplified spontaneous emission (ASE). [17] Output power also shows a typical laser threshold and slope efficiency. As we have shown for yttrium vanadate doped with neodymium (Nd:YVO 4 ), the emission decay after a pump pulse follows two different exponentials corresponding to a fast laser emission decay of a few microseconds and a slower fluorescence emission decay which is shorter than the intrinsic decay time, which should amount to 73 µs for 1.33 mol% neodymium doping concentration, because of upconversion. [18] We therefore expected some decrease in laser efficiency due to energy transfer upconversion.Random lasers hold the potential for cheap, coherent light sources that can be miniaturized and molded into any shape with several other added benefits such as speckle-free imaging; however, they require improvements specifically in terms of efficiency. This paper details for the first time a strategy for increasing the efficiency of a random laser that consists in using smaller particles, trapped between large particles to serve as absorption and gain centers whereas the large particles control mainly the light diffusion into the sample. Measurements...

Silica aerogel is a lightweight material, well known for its good mechanical and thermal characteristics, but its optical properties have received less attention, because it is weakly scattering. Here we present for the first time the lasing properties and their complex dynamics of silica aerogel doped with R6G. It is shown that the Q factors of the lasing modes determine the operation of the laser, being either resonant or ASE-lasing.For resonant lasing, the number of resonators is easily varied and the number of modes in a single resonator and their emission frequency can be dynamically adjusted, making this a truly versatile photonics material.