Time-resolved photoluminescence spectroscopy of Nb Nb 4 + and O − polarons in LiNbO 3 single crystals

We probe here the optical relaxation properties of Mg-doped wide-band-gap ${\mathrm{LiNbO}}_{3}$ single crystals with both a high spectral and temporal resolution at cryogenic temperatures. Surprisingly, we observe the photoluminescence to decay in a two-step process: a fast relaxation and a slower one centered around an energy ${E}_{max}$ = 2.62 $\ifmmode\pm\else\textpm\fi{}0.05$ eV. Both decays fit well to the stretched-exponential behavior. Moreover, we are able to associate these energies to the recombination of light-induced ${\mathrm{Nb}}_{\text{Nb}}^{4+}$ and ${\mathrm{O}}^{\ensuremath{-}}$ small polarons. Also, we checked the stability of our findings by using ${\mathrm{LiNbO}}_{3}$ single crystals that show on-purpose modified radiative recombination processes, i.e., with a Mg doping both above and below the optical damage resistance threshold, as well as with different poling histories of inverted domains.