Programming crystal chirality by mechanical strain

The origin of mechanically induced material responses dates back to 1880 when Jacques Curie and Pierre Curie discovered piezoelectricity. This phenomenon, which converts mechanical deformation into electric polarization, underpins technologies such as ultrasound transducers, pressure sensors, and precision actuators. Now our recent discovery expands this concept to a different fundamental property of matter: chirality, also known as geometric handedness.

An object is chiral if its atomic or physical arrangement differs fundamentally from its mirror image—like a pair of human hands. No amount of rotating or translating can align a left hand perfectly over a right one. An object without this property, like a perfect sphere or cube, is achiral. In crystalline solids, this handedness dictates how a material interacts with light and modifies its lattice vibrations, electronic states, and magnetic order.

Control over crystal chirality has long been sought because of its potential as a versatile tool for engineering material functionality, including tunable optical activity, enantioselective chemical interactions, and spin-dependent responses in quantum materials. But the handedness of a chiral crystal is a trait typically determined during its growth and is not easily changed. Attempting to change a crystal’s intrinsic chirality had previously seemed as improbable as turning a right hand into a left hand.

Our study demonstrates that chirality can be induced and controlled in certain already-formed achiral crystals. In these specific crystals, mechanical strain can induce chirality with a selected handedness and reverse it on demand. This phenomenon, which we term the piezochiral effect, offers a new way to program a crystal’s handedness and opens several promising new directions, most immediately in the field of optics.

Breaking symmetry between left and right

The key to controlling chirality lies in the distinction between symmetry of the local structure and that of the crystal as a whole. We start from the observation that some achiral crystals contain locally chiral substructures with oppositely handed motifs. Such systems offer an ideal platform for engineering chirality.

Typically, the equal and opposite contributions of these chiral substructures compensate for each other, leaving the overall symmetry of the crystal intact. But if an external perturbation, like a strain, is applied, it changes the two oppositely handed substructures in different ways. That process removes the symmetry that relates the left- and right-handed parts and turns the entire crystal chiral.

This concept provides the foundation for the piezochiral effect. The figure illustrates the idea through a model composed of two sets of elliptical spirals. The red and blue spirals have opposite handedness, and the long axes of their ellipses point along perpendicular directions. In the unstrained structure, a rotation followed by a reflection maps one set onto the other. By rotating and reflecting these spirals, the complete structure can therefore be superimposed on its mirror image, making it achiral by definition.

When we compress the system along the horizontal axis, the strain distorts one of the spiral sets into a more elongated oval shape while compressing the other set into a more rounded circular shape. After the deformation, a rotation followed by a reflection no longer maps the red spirals onto the blue ones. The symmetry that related left and right is removed, and the structure is rendered chiral.

The same mechanism also allows the handedness of the induced chirality to be controlled. Alternating the strain between compression and tension reverses the relative changes in the two sets of spirals and, consequently, switches the handedness of the overall structure. Applying the strain along the perpendicular axis can produce the same reversal.

Seeing chirality with light

To turn this thought experiment into reality, we identified crystals of silver gallium sulfide (AgGaS₂) that contain chiral substructures. The unit cell of these crystals can be decomposed into oppositely handed, spiral-like atomic substructures with elongated cross sections that point along perpendicular directions. The material, therefore, contains precisely the structural ingredients envisioned in our model.

Optical activity is a sensitive tool that can be used to probe chirality. In our experiment, linearly polarized light was sent through a crystal while a uniaxial strain was applied to compress or stretch the internal structures. In a chiral crystal, a rotation of the direction of polarization is expected as the light traverses the crystal, and a reversal of the direction of rotation is expected when the crystal’s handedness is reversed. A familiar example is a sugar solution, in which chiral sugar molecules rotate the direction of polarization of transmitted light. To differentiate the effect from trivial birefringence induced by the strain field, this rotation must be independent from the polarization of the incoming beam.

In a thin AgGaS₂ crystal, the measured polarization rotation changed linearly when strain was applied along a specific axis and reversed direction when compressive strain was replaced by tensile strain. That behavior revealed an opposite chiral response. Applying the same type of strain along the perpendicular crystal axis also reversed the rotation, in a manner consistent with the spiral model. Together, these observations confirmed our predictions and marked the experimental discovery of the piezochiral effect.

New directions for chiral materials

Our discovery naturally raised the question of how widely the effect can occur. To address this, we also developed a symmetry-based framework for classifying piezochiral responses across crystal systems. Some crystal symmetries allow chirality to vary linearly with strain, whereas others permit only quadratic or higher-order responses. Still, others forbid piezochirality entirely. The classification has been incorporated into a materials database. By identifying the response allowed by each material’s crystal symmetry, the database provides a practical guide for selecting promising materials for further study.

Optics is one area where piezochirality could have immediate applications. In that setting, strain could be used to tune optical activity or control how a material responds to polarized light and changes to its polarization. A chiral structure can also connect an electron’s motion to its spin, which opens a route to mechanical control of spin and charge responses. In chemistry, mechanically controlled handedness suggests possible routes toward enantioselective sensing, separation, and catalysis.

These future directions also raise many open questions. How large can the piezochiral response become, and how rapidly can it be driven? Can programmable chirality be used to control electronic, magnetic, optical, or chemical behavior? The answers will determine the extent to which piezochirality can be developed, but the discovery itself has already established a new principle for materials control. Our study shows that a crystal doesn’t need to be born chiral, but it can become so by simply being squeezed.