Three-phonon mixing as a source of light-induced chirality

Mid- and far-infrared optical pulses have found wide applicability in experiments in which collective modes of solids are driven nonlinearly, as a means to manipulate functional properties of materials on ultrafast time scales. In the illustrative case of boron phosphate (BPO$_4$), mid-infrared excitation has been used to induce chirality in an otherwise achiral crystal. The pump frequency dependence of photo-induced chirality reveals that, while on-resonance excitation is understood in terms of the well-documented phonon rectification from two-phonon mixing, additional pathways become relevant when the pump pulse is detuned from resonance. We find that a three-mode mixing mechanism becomes important in this case, whereas the contributions from other impulsive Raman terms are negligible. The results reported here provide a quantitative foundation for nonlinear phononic lineshapes in transparent materials, and open up new opportunities for lattice control.