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

Abstract

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...

Random Laser Action in Dye-Doped Polymer Media with Inhomogeneously Distributed Particles and Gain

“…A defect introduced in such a monodisperse medium concentrates and enhances light in the region around it [12]. The size and shape of the scattering particles affect the coherent and incoherent random lasing [13][14][15]. The experimental results demonstrate that the lasing threshold [13] and spectral linewidth [14] depend on the particle size, whereas the shape of the scatterers affects the peak intensity of the laser emission [14].…”

“…The experimental results demonstrate that the lasing threshold [13] and spectral linewidth [14] depend on the particle size, whereas the shape of the scatterers affects the peak intensity of the laser emission [14]. Considering the particle size distribution, polydispersed neodymium-doped powders exhibit a high lasing efficiency when compared with monodispersed powders [15]. The inhomogeneous distribution of scatterers also changes the efficiency of random lasing [16]; the lasing threshold of an RL with clustered particles decreases as the degree of particle aggregation is increased [17].…”

Dynamic random lasing in silica aerogel doped with rhodamine 6G

“…For example, a 3D RL with 68 mrad beam divergence and another RL with 50% optical efficiency were demonstrated. 30,31 Some applications such as wavelength-division multiplexing, optical sensing of single or multiple species and terahertz generation would benet from stable, tunable, dual or even triple wavelength laser emission. 32,33 As pointed out above, the appearance of equidistant thin peaks has been observed in weakly scattering media.…”

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

“…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.…”

“…leO is defined as the average photon path length inside the scattering medium before being backscattered. leO can be expressed as leOla(NIB)ln(FAP) [27,43,44]. For this calculus, we did not take into account the enhancement absorption factor (0) by localization [6].…”