Publication date: 15th December 2025
Hunting optical phenomena at the nanometer scale—performing nano-optics—is inherently paradoxical. On one hand, optical processes are governed by length scales comparable to the wavelength of visible light, typically a few hundred nanometers. On the other hand, the optical properties of nanostructures start to deviate from their bulk counterparts precisely when their size becomes smaller than this scale. Metallic nanoparticles, for instance, support plasmon resonances whose spectral features are extremely sensitive to subtle variations in size and shape. In other systems, such as quantum dots or quantum wells, atomic-scale structural details can become decisive.
To probe these effects, a whole class of experimental approaches has emerged over the past two decades to circumvent the optical diffraction limit, enabling access to the optical response of individual nanostructures and to the new physics they reveal.
In this talk, I will introduce a family of such techniques that use focused beams of free electrons—such as those available in a transmission electron microscope—to perform optical spectroscopy at the nanometer scale, sometimes in conjunction with laser beams. I will show how these methods can map plasmons, excitons, photonic modes, and even phonons with unmatched spatial resolution. Beyond the beauty of the resulting images, I will discuss how these experiments can now be interpreted quantitatively in purely optical terms—extinction and scattering cross-sections, or electromagnetic local density of states. Finally, I will illustrate how recent developments in this field open the way to new classes of free-electron experiments, from quantum optical measurements to nanoscale thermometry.
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