Čerenkov-type second-harmonic microscopy has recently attracted much attention because of its ability to resolute the ferroelectric domain walls even in disordered crystals. As reported in , ferroelectric domain structures are from a few hundred of nanometers to several microns thick and they can be resolved and visualized in both two and three dimensions. In general, Čerenkov-type second-harmonic emission is one of the most interesting non-collinear parametric patterns in far-field because of its spatial properties and spectral dependencies . Much efforts have been devoted to investigate the physical origin of this phenomena and its relation to the Fourier spectrum of the domain structures . However, it has been proved that the sharp modulation of the χ2 nonlinearity constitutes a sufficient condition for the emission of Čerenkov second harmonic in bulk materials . So its high sensitivity to the ferroelectric domain walls offers an excellent tool to monitor the ferroelectric domain growth and switching, even in bulk and in three dimensions. Such a dynamic was until now a challenge to be visualized in real time.
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