Influence of Cr3+ doping on the enhanced dielectric and nonlinear optical features of pyroelectric Pb5Ge3O11 single crystals

It was shown a possibility to enhance the output laser stimulated second harmonic susceptibility for Pb5Ge3O11 single crystals in their ferroelectric phase using an external laser light and appropriate doping by Cr3+ ions. The improvement of the Nonlinear Optical effect is achieved due to local enhancement of the local charge density non-centrosymmetry (acentricity), changes of the effective excitation of the occupied states and additional photo-polarization of the 3d-localized Cr3+ states. The addition of Cr3+ ions leads to additional enhancement of hyperpolarizability. To clarify the origin of the observed effect, and to understand the origin of its interesting properties, the calculations of the band structure of Pb5Ge3O11 were performed using, for the first time, the first-principles DFT calculations. These reveal that Pb5Ge3O11 has an indirect energy band gap Eg equal to about 2.6 eV and that the highest occupied valence band is very flat and is separated from the remaining valence bands by a small energy gap equal to about 0.3-0.4 eV. This discovery could explain many unexplained puzzling experimental results and especially the different photo-activated effects found previously in Pb5Ge3O11. The possible origin of the DFT calculated band structure is discussed. The enhancement of the second order optical susceptibility was explained within a framework of additional charges of the particular dopants during the coherent laser illumination.