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A team of researchers has discovered how a little-known type of symmetry in quantum materials, called nonsymmorphic symmetry, governs the way these materials interact with intense laser light.

The work is in Â鶹ÒùÔºical Review Applied as a Letter. The findings reveal surprising effects—including the suppression of even-order response and striking polarization-dependent responses—that could enable the design of next-generation lightwave-based electronics and quantum devices.

When broken symmetry doesn't behave as expected

In most materials, breaking inversion symmetry (a mirror-like property) allows the generation of even-order responses, such as second-harmonic signals, when exposed to light. But in nodal-line semimetals (NLS), the researchers observed the opposite: All even-order responses vanish, leaving only odd-order optical responses.

This counterintuitive effect arises from nonsymmorphic symmetry, a subtle structural feature where mirror reflections are combined with fractional atomic shifts. Despite the lack of inversion symmetry, this hidden rule enforces the cancellation of even harmonics.

Light as a fingerprint of symmetry

The present study also uncovered striking patterns in how these quantum materials emit light. When driven by intense laser, the emitted light exhibits two-fold anisotropy, i.e., the response of the emitted light changes dramatically with light polarization, forming a "butterfly-like" emission pattern.

Additionally, some of the emitted signals appear along the direction of the incoming laser, while others emerge at right angles, reminiscent of the nonlinear Hall effect. Moreover, the intra-chain and inter-chain electrons in NLS motion leave different imprints on the emitted light, depending on laser orientation.

"These results show how hidden crystal symmetries can control light-matter interactions in unexpected ways," said Prof. Gopal Dixit from IIT Bombay. "By tuning light polarization, we can selectively enhance or suppress optical signals, opening up powerful new possibilities for ultrafast technologies."

Quantum semimetals such as nodal-line, Dirac, and Weyl systems are already seen as candidates for future electronic, optical, and quantum devices due to their unusual electronic properties. By demonstrating how nonsymmorphic symmetry uniquely shapes their nonlinear optical response, the study points to strategies for symmetry-engineered optoelectronic platforms.

"Lightwave-driven devices are the frontier of ultrafast science," added Navdeep Rana, first author of the study. "The present work shows that the key to unlocking their full potential lies in the hidden symmetries of quantum materials."

The research bridges the fields of quantum materials, ultrafast laser science, and nonlinear optics, demonstrating how fundamental discoveries about symmetry can translate into real technological potential.