Possible role of charge transfer vibronic excitons in light induced change of polarizability in ferroelectric solid solutions KTa_1-xNb_xO₃

Abstract

The charge transfer vibronic excitons (CTVE) induced by two-photon absorption within the pulsed degenerate four-wave mixing (DFWM) study of ferroelectric solid solution KTa_1-xNb_xO₃ is considered as an origin of the transient optical response. The CTVE are created by charge transfer between oxygen and 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 in final recharged ion states of CTVE. Dipole—soft TO-phonon interaction as well as Jahn-Teller interaction are effective here. The electron-hole “negative-” effect plays important role in CTVE formation in addition. As a result, 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 latter leads to critical increase of light induced change in polarizability (LICP) and thus it is connected with critical increase of DFWM absolute diffraction efficiency (ADE) in accord with experimental concentration dependence of DFWM for KTa_1-xNb_xO₃.

Key Words:

• charge transfer vibronic exciton
• ferroelectric phase transition
• ferroelectric solid solutions
• laser-induced polarizability