Mechanism of the femtosecond domain switching in lithium niobate

Domain switching by femtosecond (fs) light pulses was detected recently in several ferroelectric materials, including lithium niobate (LN), and attracted a strong research interest. Qualitative explanations of this phenomenon by the generation of pyro- and thermoelectric fields, given in the experimental papers, are misleading. We propose a model of fs switching as applied to LN crystals. It includes new ingredients and provides a clear physical picture of the detected nontrivial physical phenomenon. In particular, it dismisses the pyroelectric and thermoelectric fields from the primary reasons of the reversal. The multiphoton band-to-band absorption within the transparency window causes not only the local heating of the crystal but also the generation of free charge carriers. The resulting photo-conductivity quickly compensates the pyroelectric fields and provides, after trapping of free carriers, a negative replica of the compensated electric fields. This replica possesses the sign necessary for the polarization reversal and, under proper parameters of the pulses, exceeds the coercive fields not far from the sample interface.