Symmetry control by Raman-mode excitations via anharmonic couplings with terahertz field–induced infrared modes in ferroaxial PbWO 4

Ultrafast conversion from a high- to low-symmetry structure is important for optical-switching devices. To realize such a conversion, we used nonlinear phononics. Exciting infrared-active modes with an intense terahertz pulse generates symmetry-reducing Raman modes via anharmonic couplings with the infrared-active modes. A terahertz-pump optical-Kerr-ellipticity-probe method was applied to a ferroaxial material, PbWO 4 , using terahertz pulses with two different bandwidths, revealing that Raman-mode dynamics induced by an anharmonic mode coupling can be distinguished from those due to the stimulated Raman scattering. Our results showed that the anharmonic mode coupling comprises two components. Under terahertz light, forced oscillations of the infrared-active modes drive coherent Raman mode oscillations. After the terahertz light disappears, the square of the infrared-active mode amplitudes induces Raman-mode displacements. By narrowing the bandwidth and broadening the temporal width of the terahertz pulse, the Raman mode displacements are rectified over several picoseconds. This approach is promising for generating a new state with lower symmetry.