Researchers in Singapore and China have uncovered a hidden property in the well-studied Smith-Purcell effect: chirality, or handedness. In a paper titled Observation of inherent chiral Smith-Purcell effect via symmetry breaking, Associate Professors Wu Lin and Dong Zhaogang from the Singapore University of Technology and Design (SUTD) demonstrated that radiation emitted by moving electrons is inherently chiral and can be extracted using a plain silicon grating.
Symmetry Hides Chirality in Electromagnetic Fields
A moving electron carries a twisting electromagnetic field around it, and a grating can convert this field into circularly polarised light. Based on three-dimensional simulations, the central region yields linearly polarised emissions, while one side of the beam shows left-handedness and the other side displays right-handedness. Earlier two-dimensional models had only looked at the middle.
“The main reason it was missed is symmetry,” said Associate Prof Wu. “The chirality was always there, but the left- and right-handed components are distributed symmetrically in space. In conventional measurements, we usually look only along the symmetry plane, so the two opposite contributions cancel each other.”
Breaking Symmetry Through Rotation and Shifting
To tip the scales, the researchers broke the symmetry in two distinct ways: by rotating the grating relative to the electron beam, or by keeping the grating fixed and moving the beam from the centre towards one edge. “Twisting lets us tune the chirality, while shifting lets us select a stable handedness over a broad wavelength range,” added Associate Prof Dong.
Both approaches were tested on silicon gratings inside a scanning electron microscope, where the emitted light was sorted into left- and right-handed parts. Using the standard scale—where 0 indicates an absence of net handedness and 1 denotes exclusively one-handed light—the recorded figure increased alongside the twist angle, reaching 0.726 at 30 degrees. Reversing the twist flipped the handedness.
Scaling Up Performance With Silicon Gratings
Shrinking the grating pitch from 120 to 100 nanometres pushed the measured value to 0.87, which is the highest yet reported for Smith-Purcell radiation and more than double the previous best. Shifting the electron beam produced one handedness along one edge and the opposite along the other, with simulations predicting values of up to 0.98.
“The key insight was that we did not need to make the silicon grating itself chiral,” noted Associate Prof Dong. “The chirality was already hidden in the electromagnetic field carried by the moving electrons.” The shifting experiments also provided a surprise for the team, as emission was strongest when the electron passed beside the grating rather than directly above it, due to the sharp corners of the grating scattering the electron’s field into light remarkably well.
Frequently Asked Questions About Chiral Smith-Purcell Radiation
What is the Smith-Purcell effect?
The Smith-Purcell effect occurs when a charged particle, such as an electron, passes close to a periodic grating and emits light. Recent research shows this radiation also carries inherent chirality.
How was the hidden chirality revealed?
Researchers broke spatial symmetry by either rotating the silicon grating relative to the electron beam or shifting the beam from the centre toward the edge. This stopped the opposite handedness components from cancelling each other out.
What performance levels did the SUTD team reach?
By shrinking the grating pitch from 120 to 100 nanometres, the team pushed the measured handedness value to 0.87, more than double the previous best for Smith-Purcell radiation.