Ultrafast switching of photoinduced phonon chirality in the antiferrochiral <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>BPO</mml:mi> <mml:mn>4</mml:mn> </mml:msub> </mml:math> crystal

In crystalline systems, chiral crystals cannot interconvert to their enantiomorph postsynthesis without undergoing melting-recrystallization processes. However, recent work indicates that ultrafast terahertz-polarized light has been shown to enable dynamic control of structural chirality in the antiferrochiral boron phosphate $({\mathrm{BPO}}_{4})$ crystal. Here, using first-principles calculations combined with nonlinear phonon dynamics simulations, we investigate the underlying physics of phonon chirality in this system. The results demonstrate that polarized optical pumping not only induces chiral phonons but also establishes a chirality-selective filtering mechanism, both of which can be reversibly switched by tuning the polarization of the excitation pulse. Furthermore, under a temperature gradient, the pump-induced chiral phonons give rise to ultrafast phonon magnetization, with its direction also controllable via light polarization. Our findings establish a new paradigm for ultrafast optical control of phonon chirality via dynamic chirality switching, offering promising opportunities for chiral information transfer and the design of chiral phononic devices.