Possible role of charge transfer vibronic excitons in light induced changeof polarizability in ferroelectric solid solutions KTa1-xNbxO3

The charge transfer vibronic excitons (CTVE) induced by two-photon absorpti on within the pulsed degenerate four-wave mixing (DFWM) study of ferroelect ric solid solution KTa1-xNbxO3 is considered as an origin of the transient optical response. The CTVE are created by charge transfer between oxygen an d tantalum, or oxygen and niobium ions with appearance of O- - Ta4+ or O- - Nb4+ pairs accompanied by self-consistent lattice distortion. This lattice distortion is caused mainly by strengthening of the vibronic interaction i n final recharged ion states of CTVE. Dipole - soft TO-phonon interaction a s well as Jahn-Teller interaction are effective here. The electron-hole "ne gative-U" effect plays important role in CTVE formation in addition. As a r esult, the changes in dielectric polarizability induced by optical pumping and correspondent CTVE creation increase the initial polarizability due to the interaction of CTVE reoriented dipole moments with soft TO-mode. The la tter leads to critical increase of light induced change in polarizability ( LICP) and thus it is connected with critical increase of DFWM absolute diff raction efficiency (ADE) in accord with experimental concentration dependen ce of DFWM for KTa1-xNbxO3.