Calculations of stimulated emission dynamics are shown to be in good qualitative agreement with experimental observations of nonlinear quenching on rare earth transitions competing for gain in continuous-wave random laser media.
Gain competition is analyzed theoretically on coupled stimulated emission transitions in Λ-type configurations of excited atoms and compared with experimental observations in electrically and optically pumped random media. Under steady-state conditions, we predict and observe dramatic spectral quenching among 2F5/2 transitions in electron-beam-irradiated Nd3+:δ-Al2O3 nanopowders, identifying a simple mechanism that promotes laser action at the longest wavelength (405 nm). In optically pumped Nd3+:Y2O3 nanopowder, simultaneous laser action is observed at two previously unreported wavelengths of Nd3+ (763 and 821 nm), but spectral quenching is not observed, owing to the absence of Λ coupling among the inverted transitions. Quantitative analysis of quenching and threshold behavior in the case of Nd3+:δ-Al2O3 indicates that in wavelength-sized volumes of these light-confining media the average storage time of light may be as long as 30 ps.
Nonradiative relaxation and photoluminescence quenching in nanocrystalline powders doped with rare-earth elements are of interest in optical bistability, random laser, and other optoelectronic applications. Here, the luminescence quenching of a one-dimensional random medium made of multilayer nanoparticles (Y2O3) doped with rare-earth elements (Yb3+) is analyzed by considering the transport, transition, and interaction of the fundamental energy carriers. The nonradiative decay and luminescence quenching in random media are enhanced compared to single crystals, due to multiple scattering, enhanced absorption, and low thermal conductivity. The coherent wave treatment is used to calculate the photon absorption, allowing for field enhancement and photon localization. The luminescent and thermal emission is considered as incoherent. The size-dependent absorption coefficient and penetration depth are observed. The nonradiative decay is identified as a multiphonon relaxation process, and is found to be enhanced compared to bulk materials. The luminescence quenching and nonlinear thermal emission, occurring with increasing irradiation intensity, are predicted.
The spatial and spectral evolution of emission from gain media with homogeneously distributed scatterers is modeled using a Monte Carlo simulation of the random walk of pump and emitted photons. We find a sharp transition to ultrafast, narrow-linewidth emission for a 10-ps incident pump pulse and a rapid approach to steady state for longer pulses. The threshold for this transition is raised significantly when the transverse diffusion of photons out of the excited region is substantial. The quantitative agreement of the results with recent observations of emission from random gain media lends further support to the interpretation of ``random lasers'' in terms of amplified spontaneous emission within a multiply-scattering medium.