Terahertz light has the ability to control solids at the atomic level, forming chiral structures of left- and right-handedness. [Image: Zhiyang Zeng (MPSD)]
Chirality is a property of an object whose mirror image cannot be superimposed onto itself through any combination of rotations or translations. Typically, crystalline materials exist in either left- or right-handed structures and must be melted down to form the other chiral state.
Now, researchers in Germany and the United Kingdom say they have discovered that a finite chiral state on an ultrafast timescale can be induced in a nonchiral crystal by irradiation with terahertz pulses (Science, doi: 10.1126/science.adr4713). In addition, they report that they could selectively induce either a left- or right-handed chiral structure by simply rotating the polarization of the terahertz light by 90 degrees.
“Many aspects of symmetry breaking in chiral and other systems can be used to store information, and the dream would be to have a nonvolatile memory operating at ultrahigh speeds based on these principles,” said study author Andrea Cavalleri, director of the Max Planck Institute for the Structure and Dynamics of Matter, Germany. “The fact that the chiral state does not switch back when it is not driven can make memories of this kind be very stable.”
Emergence of chirality
Cavalleri and his colleagues demonstrated this feat in boron phosphate, an antiferro-chiral material in which the unit cell comprises equivalent amounts of left- and right-handed fragments. A type of nonlinear photonic interaction occurs by exciting a specific terahertz frequency vibrational mode, which displaces the crystal lattice along the coordinates of other modes in the material.
“One can use selective excitation of certain phonons with midinfrared light to make one chiral fragment bigger than the other, inducing chirality at ultrahigh speeds in either direction, despite the fact that this change is not possible using conventional means such as strain or electric fields,” said Cavalleri.
If the findings extend to ferri-chiral systems as well, the approach could be leveraged for ultrafast switching of chirality.
The transient crystal structure survives for several picoseconds. When the polarization of the terahertz excitation pulse is oriented to excite the orthogonal vibrational mode, the induced displacement changes direction, and the system is driven into the opposite chiral state.
Potential applications
If the findings extend to ferri-chiral systems as well, the approach could be leveraged for ultrafast switching of chirality, with applications for ultrafast memory devices and sophisticated optoelectronic platforms.
“This is early days, but it is possible to envisage situations in which the electronic and magnetic properties, influenced by the chirality of the lattice, can also be switched with similar methods, developing optoelectronic platforms based on chiral elements,” Cavalleri said.