Mechanism of stimulated optical emission from MgO microcrystals with color centers

Recently, we reported that the MgO microcrystals that are prepared via the reaction between magnesium and silicon monoxide under Ar atmosphere exhibit broadband laser emission at room temperature without using cavity mirrors under pulsed laser excitation [T. Uchino and D. Okutsu, Phys. Rev. Lett. 101, 117401 (2008)]. Here we demonstrate that optical gain is obtained through continuous and pulsed pump and probe measurements. It has also been shown that the observed laser emission results from multiple scattering with gain in the MgO microcrystals with color centers such as F and F(+) centers. The observed stimulated optical emission has been interpreted in terms of the photoionization and the subsequent recombination processes of the color centers, which can also induce highly excited neutral states of Mg and the related atomic emissions. We suggest that the emissions from both the color centers (principally the F(+) center) and the excited neutral Mg are optically amplified by the process of multiple scattering, yielding a broad laser emission in the near-ultraviolet to blue-green spectral range.