SPICE Workshop on Chiral Phonons, July 29th - 31st 2025
Michael Först
Functionally relevant order in solids can be manipulated by coherently driving their crystal lattices using mid-infrared and terahertz (THz) pulses [1–3]. In this tutorial, I will provide an introduction to this emerging field and focus on two recent examples that highlight its potential.
First, I will show how chirality of either handedness can be transiently induced in the achiral material boron phosphate (BPO₄) [4]. This is a nontrivial task, as it requires breaking all mirror and roto-inversion symmetries simultaneously. However, this process is enabled by the resonant excitation of one of two orthogonal, degenerate phonon modes and their nonlinear coupling to a set of symmetry-lowering vibrational modes. Remarkably, the sign of the induced chiral order parameter—that is, the handedness—is controlled by the polarization of the THz field. The resulting structure exhibits a strength of optical activity comparable to the static value observed in prototypical chiral α-quartz.
In the second example, I will demonstrate ultrafast switching of ferroaxial order by engineering an effective axial field composed of circularly driven phonon modes [5]. Unlike ferroelectric or ferromagnetic systems, ferroaxial materials host bistable states that preserve both spatial-inversion and time-reversal symmetries, making them resistant to manipulation by conventional means. In the prototype compound rubidium iron dimolybdate (RbFe(MoO₄)₂), a single circularly polarized THz pulse can switch a ferroaxial domain to its opposite state. This new state remains stable for many hours and can be reversed by applying a second THz pulse with opposite helicity.
References
[1] M. Först et al., Nature Physics 7, 854 (2011).
[2] P. G. Radaelli, Physical Review B 97, 085145 (2018).
[3] A.S. Disa et al., Nature Physics 17, 1087 (2021).
[4] Z. Zeng et al., Science 387, 431 (2025).
[5] Z. Zeng et al., arXiv 2506.10682 (2025).