We present a combined theoretical and experimental analysis to describe the interplay between polarization field, charge screening, and radiative and nonradiative recombinations in AlGaN/GaN-based nanostructures. We perform the study of photoluminescence (PL) in both stationary and time-resolved PL (TR-PL) conditions, considering also the effect of temperature on the different recombination mechanisms, and especially on nonradiative recombinations. The theoretical analysis is obtained by coupling a complete self-consistent solution of Schrödinger and Poisson equations to determine the electronic states in the nanostructure with a rate equation model to account for time-dependent effects of charge rearrangement. We review the topic of polarization field screening taking into account the most recent results on the evaluation of the very strong coefficients for spontaneous and piezoelectric polarizations that have recently been predicted for III–V nitride semiconductors with natural wurtzite symmetry, and we show how the screening influences both static and dynamic recombination mechanisms. Experimental results are obtained for high-quality GaN/AlGaN multiple quantum wells by means of both continuous-wave (cw) and TR-PL techniques. The case of single quantum well is also considered. The PL measured decays show a time dependence that is not only controlled by radiative lifetimes, which depend on the fields inside the GaN wells, but also on the carrier recombinations through nonradiative channels. We demonstrate that PL emission is influenced by charge accumulation in the well, and a loss of carriers from the ground level induced by an interplay between radiative and nonradiative recombination processes. Moreover, from the analysis of the temperature dependence of the TR-PL decays, we deduce important confirmations of a thermally activated detrapping mechanism that strongly affects the nonradiative recombination processes.
We present a high-power, high-efficiency and low threshold laser prototype based on doped ceramic Yb(3+):YAG. We achieved an output power of 9 W with a slope efficiency of 73% and a threshold of 1 W at 1030 nm in quasi-Continuous Wave (QCW). Moreover, we obtained an output power 7.7 W with a slope efficiency of 60% in Continuous Wave (CW). Finally, a characterization of a low losses tunable cavity for several laser wavelengths with an output power exceeding 5 W is reported.
We report the tunable, CW and quasi CW laser operation at room temperature of an highly doped (30% at.) Yb:YLF crystal longitudinally pumped by a fiber coupled laser diode array. The CW output power is 1.15 W vs. an absorbed pump power of 6 W, with a slope efficiency of 31%. In quasi-CW operation (20% duty factor @10 Hz) an output power of 4 W with an absorbed power of 9.5 W, and a slope efficiency of 62.8% were obtained. The tuning range spans from 1022 to 1075 nm. To our knowledge, these are among the best experimental results obtained at room temperature with Yb doped YLF.
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