Analysis of ZnO random laser spectra under nanosecond pumping

Structural instability and dynamic emission fluctuations in zinc oxide random lasers

“…As others before us, 4,5,7,[11][12][13] we find strong spectral fluctuations in the lasing output when nanosecond pulses are used for pumping, while picosecond pumping generates very reproducible lasing spectra. From the results presented here, it appears unlikely that the shot-to-shot spectral fluctuations in nanosecond pumping are caused by structural damage.…”

“…For the case of long excitation pulses, i.e., pulse durations of several nanoseconds, we find a very low degree of reproducibility in the emission spectra, similar to what has been observed by others. 4,5,7,[11][12][13] This stochastic behavior contrasts to the high reproducibility 4-7 observed with picosecond excitation pulses. While one might suspect the stochastic behavior also to be caused by material damage, our results indicate that this is not the case.…”

“…Also, the observation of much higher reproducibility in picosecond pumping at pulse fluences well above the damage threshold indicates that damage is not the cause for stochastic lasing behavior. Alternative causes have been discussed in several publications [4][5][6][7][11][12][13] which point to the shorter recombination lifetimes in the ZnO nanoparticles as compared to bulk ZnO: Essentially, when the recombination lifetimes are substantially shorter than pump pulse times, temporal sequences of un-coupled lasing events can occur within a single pump pulse, and the dynamic evolution of these sequential lasing events is suggested to produce the large shot-to-shot variations in nanosecond pumping.…”

“…[1][2][3] Efficient excitonic emission from ZnO nanoparticles and comparably long exciton lifetimes make the material an interesting candidate for ultraviolet optically pumped random lasing. 1,[4][5][6][7][8][9][10][11][12][13] Ease of fabrication, small mode volumes, and a variety of spectral characteristics lend themselves to three basic applications for ZnO random lasers: (A) The possibility of speckle-free lasing originating from a non-resonant amplification or the superposition of a large number of optically independent lasing events, (B) the possibility of generating unique spectral distributions from the combinations of just a few random lasing spots, and (C) the possibility of preparing coherent nanoscale lasing sources in simple thin film arrangements. These applications utilize coherent and incoherent lasing regimes, and both of these regimes are accessible in random lasers.…”

“…Instead, we agree with work that has attributed the low reproducibility to temporal fluctuations that occur when pump pulse times are greater than the exciton lifetime. 4,5,13 Our main emphasis, however, is on describing the extent and cause of a lightinduced instability in the operation of ZnO random lasers. We show that lasing thresholds and intensities depend sensitively on total accumulated light exposure.…”