Fig. 2: Phonon-driven enhancement of second-harmonic generation.
From: Ultrafast simultaneous manipulation of multiple ferroic orders through nonlinear phonon excitation
a We used MIR pulses with a center frequency of 16.9 THz (69 meV) to drive the high-frequency A1 phonon in BiFeO3 at normal incidence. A probe beam at 800 nm was used for SHG with a 45° incidence in reflection geometry. b (Top panel) Electric field component of the MIR pump and Fourier spectrum (inset). (Middle panel) The MIR pump induces a 1.5% transient enhancement of the SHG intensity when polarized along PFE. (Bottom panel) Rotating the MIR pump polarization by 90° results in no pump-probe signal. c Dependence of the SHG signal on the center frequency of the MIR pump pulse. The signal becomes nonzero as the frequency of the pump pulse approaches that of the high-frequency A1 mode, indicating a phonon-driven mechanism. Vertical error bars represent the standard deviation. Horizontal bars are the bandwidths of the mid-infrared pump pulses. d Linear dependence of the light-induced change in second-harmonic polarization on the pump fluence, consistent with nonlinear phononic rectification and ionic Raman scattering34. Vertical error bars represent the standard deviation.
