Pulse-induced transient blue absorption related with long-lived excitonic states in iron-doped lithium niobate

Transient absorption is studied in Fe-doped lithium niobate single crystals with the goal to control and probe a blue absorption feature related with excitonic states bound to Fe$_\textrm {Li}$ defect centers. The exciton absorption is deduced from the comparison of ns-pump, supercontinuum-probe spectra obtained in crystals with different Fe-concentration and Fe$^{2+/3+}_\textrm {Li}$-ratio, at different pulse peak and photon energies as well as by signal separation taking well-known small polaron absorption bands into account. As a result, a broad-band absorption feature is deduced being characterized by an absorption cross-section of up to $\sigma ^\textrm {max}(2.85$ eV$) = (4\pm 2)\cdot 10^{-22}$ m$^{2}$. The band peaks at about 2.85 eV and can be reconstructed by the sum of two Gaussians centered at 2.2 eV (width $\approx 0.5$ eV) and 2.9 eV (width $\approx 0.4$ eV), respectively. The appropriate build-up and decay properties strongly depend on the crystals’ composition as well as the incident pulse parameters. All findings are comprehensively analyzed and discussed within the model of $\textrm {Fe}^{2+}_{\textrm {Li}}-\textrm {O}^{-}-\textrm {V}_{\textrm {Li}}$ excitonic states.