Photoexcitation, trapping, and recombination processes of theF-type centers in lasing MgO microcrystals

Abstract

We report results of a detailed temperature dependence of photoluminescence (PL) decay time and continuous emission properties of the $F$ and ${F}^{+}$ centers in recently reported lasing MgO microcrystals [T. Uchino and D. Okutsu, Phys. Rev. Lett. 101, 117401 (2008)]. Thermally induced ionization and carrier trapping play a vital role in the entire emission processes of the $F$-type centers, especially at time scales longer than microseconds. In these time scales the PL decay profiles tend to exhibit a power-law behavior over more than 3\ensuremath{-}4 decades of time, implying thermally activated hopping and tunneling of the trapped electrons. Such delayed PL signals show two maxima in intensity at temperatures of \ensuremath{\sim}130 and \ensuremath{\sim}300 K. This apparently anomalous temperature dependence is also indicative of the thermally stimulated emission processes of the originally photoexcited $F$ and ${F}^{+}$ centers. The PL spectra above lasing threshold exhibit temperature-dependent broadening, demonstrating that the thermal vibrations of the crystal lattice affect the stimulated emission scheme as well. On the basis of these experimental results, a model of the photoexcitation, trapping, and recombination processes of the $F$-type centers is presented.