| Continuous-wave (CW) laser action is analyzed in detail in random rare-earth-doped nano-powders. Strong multiple scattering of light and interference effects in disordered gain media are shown to be capable of confining light to nano-scale volumes and providing feedback for oscillation. In this research, conditions were close to those needed for strong localization in passive media, but gain is thought to play a key role in the CW laser action from electrically-pumped Nd3+-doped delta-Al2O3 and optically-pumped Nd3+:Y2O3 nano-powder reported here for the first time.; High resolution cathodo- and photo-luminescence studies revealed conventional signs of laser action, such as threshold behavior and linear output above threshold. Other characteristics, such as lack of directionality and the absence of mode selectivity were unexpected. When the penetration depth of incident electrons was increased by one optical wavelength, the threshold of random lasers action dropped significantly, suggesting that the laser cavities were of approximately this size. When fitted with an analytic 1-D model, spectral quenching observations provided a more powerful method of inferring the average cavity size and energy storage time, yielding values of lav = (1.6 +/- 1.0)lambda and tauc = (30 +/- 10) ps respectively. Finite Difference Time Domain (FDTD) simulations confirmed the results from the quenching analysis. All experimental aspects of CW laser action in oxide nano-powders, including their lack of coherence, were shown to be consistent with strong localization dependent on global structure of the random medium, rather than any particular local structure.; Color center emissions were also studied spectroscopically because point defects tended to mask rare earth dynamics of importance in this work. Nonlinear growth of infrared emission near 820 nm was attributed to the production of color centers during pumping and a Cr3+-related Fano resonance feature at 675 nm was identified in Al2O3 for the first time. |
