SUTD researchers have discovered that the light electrons emit as they skim past a grating has always carried a hidden handedness, and that twisting the grating or nudging the beam sideways turns a plain silicon structure into a chiral, broadly tunable light source.
Unveiling the inherent chirality of Smith–Purcell effect by breaking the symmetry between an electron beam and a periodic grating.
SUTD
In 1953, two physicists showed that an electron beam skimming past a grating emits light whose colour 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”, 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 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.”
A moving electron drags a twisting electromagnetic field around it, and a grating can convert this into circularly polarised light. In three-dimensional simulations, the emission is left-handed on one side of the beam, right-handed on the other, and linearly polarised 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 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, 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 nanometres 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 Associate Prof 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.
“Twisting lets us tune the chirality, while shifting lets us select a stable handedness over a broad wavelength range,”
Associate Professor Dong Zhaogang, Singapore University of Technology and Design
“Once we understood the role of the grating edge, the result made physical sense,” Associate Prof Dong shared. “It showed us that where the electron travels relative to the structure can be just as important as the structure itself.”
As neither approach relies on a delicate optical resonance, the effect is naturally broadband. The emitted light could be tuned from about 450 to 900 nanometres while the chirality changed only slightly. Avoiding resonance also promises simpler fabrication and greater freedom in choosing materials. Associate Prof Wu sees chiral light sources as the application closest to practical use, with extreme ultraviolet light—prized in lithography—an attractive target.
However, challenges remain. Materials must be chosen carefully, since some glow under electron bombardment and can swamp the signal. Reaching X-ray wavelengths would demand even smaller structures or much higher electron energies, and quantum effects may then complicate the picture.
“The challenge is not as simple as generating X-rays,” Associate Prof Dong said. “It is to generate them while preserving and controlling their chirality, and to understand the new quantum physics that may emerge at these much smaller scales.”
Nonetheless, these findings point to a new way of designing free-electron light sources. Said Associate Prof Wu: “Instead of making the structure increasingly complicated, we can create new functionality by controlling the symmetry of the electron-structure interaction.”
She added that the project benefited greatly from interdisciplinary collaboration. As she puts it: “Domain knowledge helps us discover what is possible, design turns that insight into a solution, and technology makes it real.”
“Instead of making the structure increasingly complicated, we can create new functionality by controlling the symmetry of the electron-structure interaction.”
Associate Professor Wu Lin, Singapore University of Technology and Design
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