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...

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