Origin of picosecond-pulse-induced, degenerate four-wave-mixing signals in KTa1−xNbxO3 crystals

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Abstract
Transient gratings have been produced in KTa1−xNbxO3 by picosecond‐pulse, two‐photon excitation using degenerate four‐wave‐mixing techniques. The excitation process has been characterized through fluorescence studies, and the fluorescence is attributed to the transition between an excited state and the ground state of B4+ ions that are produced in ABO3 perovskite crystals. Strong electron‐phonon coupling gives rise to the luminescence quenching. The observed degenerate four‐wave‐mixing signal is shown to be due predominantly to a phase grating caused by a change in the susceptibility associated with the formation of Nb4+ or Ta4+ ions in the peak region of the grating. The grating signal intensity was found to be dependent on the concentration of niobium ions and the crossing angle of the two laser write beams.