This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In 1953, two physicists showed that an electron beam skimming past a grating emits light whose color depends only on the electron's speed and the grating's spacing. Known as Smith-Purcell radiation, this phenomenon has become a useful tool for building compact, tunable light sources and tracking particle beams without touching them.
Now, researchers from Singapore and China have discovered that this well-studied effect has been hiding a property nobody thought to look for: chirality, or "handedness." In their paper "Observation of inherent chiral Smith-Purcell effect via symmetry breaking," published in Nature Communications, associate professors Wu Lin and Dong Zhaogang from the Singapore University of Technology and Design (SUTD) showed that the radiation is inherently chiral and demonstrated two simple ways to bring that chirality out with a plain silicon grating. "The main reason it was missed is symmetry," said 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." A moving electron drags a twisting electromagnetic field around it, and a grating can convert this into circularly polarized light. In three-dimensional simulations, the emission is left-handed on one side of the beam, right-handed on the other, and linearly polarized in the middle, the only place earlier two-dimensional models had looked. To tip the scales, the researchers broke the symmetry in two ways: by rotating the grating relative to the electron beam, or by keeping the grating fixed and moving the beam from the center toward one edge.
"Twisting lets us tune the chirality, while shifting lets us select a stable handedness over a broad wavelength range," added Dong. Both approaches were tested on silicon gratings inside a scanning electron microscope, with the emitted light sorted into left- and right-handed parts. On the standard scale, where 1 means purely one-handed light and 0 means no net handedness, the measured value climbed with the twist angle to 0.726 at 30 degrees.
Reversing the twist flipped the handedness. Shrinking the grating pitch from 120 to 100 nanometers pushed the value to 0.87, the highest yet reported for Smith-Purcell radiation and more than double the previous best. Shifting the beam gave one handedness along one edge and the opposite along the other, and simulations predict values of up to 0.98.
"The key insight was that we did not need to make the silicon grating itself chiral," noted Dong. "The chirality was already hidden in the electromagnetic field carried by the moving electrons." The shifting experiments gave the researchers a surprise. The emission was strongest with the electron beside the grating rather than above it, because the sharp corners of the grating scatter the electron's field into light remarkably well.
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