Abstract
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$_\textrm {Li}$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$^{2+/3+}_\textrm {Li}$Li2+/3+-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 $\sigma ^\textrm {max}(2.85$σmax(2.85 eV$) = (4\pm 2)\cdot 10^{-22}$)=(4±2)⋅10−22 m$^{2}$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 $\approx 0.5$≈0.5 eV) and 2.9 eV (width $\approx 0.4$≈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 $\textrm {Fe}^{2+}_{\textrm {Li}}-\textrm {O}^{-}-\textrm {V}_{\textrm {Li}}$FeLi2+−O−−VLi excitonic states.
Highlights
The microscopic understanding of pulse-induced transient absorption phenomena, such as greeninduced infrared absorption (GRIIRA) [1] or blue-induced infrared absorption (BLIIRA) [2], is mandatory for the control of laser-induced damage mechanisms in lithium niobate (LiNbO3, LN) and, for applications of LN in nonlinear photonics [3]
Transient absorption is studied in Fe-doped LN (Fe)-doped lithium niobate single crystals with the goal to control and probe a blue absorption feature related with excitonic states bound to FeLi defect centers
This study reveals the striking impact of ns-pump, supercontinuum-probe spectroscopy for the microscopic understanding of strongly localized electronic states in oxide crystals with lithium niobate as an example
Summary
The microscopic understanding of pulse-induced transient absorption phenomena, such as greeninduced infrared absorption (GRIIRA) [1] or blue-induced infrared absorption (BLIIRA) [2], is mandatory for the control of laser-induced damage mechanisms in lithium niobate (LiNbO3, LN) and, for applications of LN in nonlinear photonics [3]. The appearance of transient infrared absorption was successfully explained by the coupling of optically generated electrons with phonons, i.e., by the formation of Nb4N+b/Li small, strong-coupling electron polarons [4]. Important knowledge to control the appearance of blue absorption by adjustment of, e.g., Fe-concentration, of the Fe2L+i /3+-ratio, and/or by means of pulse peak energy and/or photon energies is missing in literature completely, so far. The presence of transient absorption in the blue-green spectral range in LN was discovered in 2005 by Herth et al in Fe-doped LN using single probe-laser lines [5] and – as a first attempt – has been attributed to the coupling of optically generated holes with the lattice in direct vicinity of VLi lithium vacancies. Messerschmidt et al discussed excitonic states at the origin of long-lived transient blue absorption [9]
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