A team has experimentally observed the inherent chiral Smith–Purcell effect, a version of Smith–Purcell radiation in which the emission depends on the chirality of the electron beam and of the structure it passes. The effect arises naturally when the symmetry of the system is broken, without the need for external chiral materials or added magnetic fields.

The classical Smith–Purcell effect occurs when an electron beam passes close to a periodic metallic grating: the electrons induce charges in the structure and emit radiation at angles that depend on the wavelength and the grating period. The chiral version, now observed, adds an asymmetry between emission to the left and to the right, so that the polarization and intensity of the radiated light change according to the relative arrangement between the beam and the structure.

The key to the experiment lies in designing a periodic structure whose symmetry is broken in a controlled way, so that the interaction with the electron beam becomes sensitive to chirality. By varying the beam conditions and the grating geometry, the researchers were able to characterize the chiral asymmetry of the emission and compare it with the predictions of the proposed model.

The result paves the way to compact, tunable radiation sources in which the chirality of the emission can be controlled through the structure design and the beam conditions. Potential applications include generating light with defined polarization in the terahertz and microwave ranges, as well as characterizing electron beams and developing photonic devices sensitive to chirality.