Fast electrons can make materials glow. This process, known as cathodoluminescence, is widely used in electron microscopy because electron beams can be focused with nanometer precision, far below the wavelength of visible or infrared light, to study nanostructured materials. Yet controlling where the emitted light goes remains a central challenge. One route is the Smith-Purcell effect, in which an electron traveling near a periodic structure produces directional radiation. As the electron moves over the array, it excites the elements sequentially, but with some time delay between consecutive elements. If all elements respond equally, the emitted waves add up only along a specific angle. In conventional Smith-Purcell emission, the emission angle is mainly fixed by the electron speed, the light wavelength, and the spacing of the structure. Once the device is made, its response is largely predetermined.
In a new paper published in Light: Science & Applications, a team of scientists from the University of Southern Denmark, with collaborators from ICFO (Spain) and AMOLF (The Netherlands), propose a generalized Smith-Purcell effect based on programmable metasurfaces. Instead of using an array of identical elements, they consider finite periodic arrays in which each nanoscale element can have its own optical response. By controlling how strongly each element is excited, and with what phase, the collective light emission can be redirected into chosen angles.
The idea is similar to the operating principle of a phased-array antenna, but miniaturized to nanometer length scales and driven by a passing electron beam. “As the electron flies over the array, it excites the elements one after another. If all array elements are equal and respond uniformly, the emitted waves add up only along the usual Smith-Purcell directions. However, if their responses are deliberately varied, new interference channels appear, allowing the cathodoluminescence to be steered into additional directions that are set by the non-uniform array structure”, the authors explain.
The researchers developed a rigorous theory for this generalized emission and used it to design steering patterns. Their calculations show that arrays of nanoscale scatterers can redirect electron-induced light over a broad angular range at a chosen wavelength. Because Smith-Purcell radiation is naturally broadband, the same approach also shapes how emission direction changes with color.
A key feature of the proposal is active tunability. “The important step is to replace a fixed grating by a structure with external knobs,” the authors explain. “Electrical gating in graphene or thermal switching in vanadium dioxide gives access to different optical responses within the same device. This means that electron-induced light could be redirected dynamically, rather than being locked to a single design chosen at fabrication.”
Although the work is theoretical, the predicted effects should be testable with angle-resolved cathodoluminescence measurements. The external control does not create the light directly; rather, it programs the nanoscale array so that light generated by the electron is emitted in selected directions.
The results point toward programmable electron-driven light sources, nanoscale spectroscopy tools, and electron-based optical technologies spanning frequencies from the terahertz and infrared to the visible. More broadly, the work provides a design strategy for shaping free-electron radiation using active metasurfaces.
“Our goal is to give researchers a new way of designing how fast electrons emit light,” the authors conclude. “By combining fast electrons with programmable metasurfaces, we open a route toward compact light sources and spectroscopy tools whose emission can be shaped on demand.”
Light: Science & Applications
Active steering of cathodoluminescence through a generalized Smith-Purcell effect