Friction-induced polarization dynamics in Bi$_4$Ti$_3$O$_{12}$ thin films

The results of an experimental study by the method of terahertz pumping – nonlinear optical sensing of the picosecond dynamics of ferroelectric polarization induced by the action of a terahertz pulse in a ferroelectric film Bi$_4$Ti$_3$O$_{12}$ on a silicon substrate in a wide range of the electric field strength of a THz pulse are presented. Numerical modeling based on the Landau-Khalatnikov equation of the dependences of the intensity of the second optical harmonic on the electric field shows the presence of a threshold THz field with an amplitude of $E_{\mathrm{th}}$ = 4.5 MV/cm. It is shown that for small fields $E<E_{\mathrm{th}}$, the polarization fluctuates within a certain minimum of the two-minimum potential, although in time the second harmonic signal follows the terahertz pulse. At $E>E_{\mathrm{th}}$, polarization is switched and the time dependence of the intensity of the second harmonic becomes more complicated. Based on a comparison of theoretical and experimental data, the conditions for switching polarization by a strong electric field of a picosecond HZ pulse are analyzed.