Cover story: Boneberg J, Leiderer P. Optical near-field imaging and nanostructuring by means of laser ablation. Opto-Electron Sci 1, 210003 (2022).

Since the days of Ernst Abbe, the diffraction limit which bears Abbe´s name has been a famous principle in classical optics. It states that it is not possible to focus a light beam to a diameter smaller than about half of the beam´s wavelength. While this is still true for far-field optics, where focusing is achieved by a lens at a macroscopic distance from the focus, the optical near field of dielectric micro- or nanospheres allows one to reach a focus distinctly tighter than given by the Abbe limit. This substantial near-field enhancement, nowadays called photonic nanojet, has been used to structure surfaces with short laser pulses by nano-ablation. Since millions of micro- or nanolenses can easily be deposited from a colloidal suspension as monolayers on a surface by means of self-organization, it is possible to pattern surfaces with a vast amount of regularly arranged holes with diameters in the hundred-nanometer range with just one laser pulse. Near-field hotspots that are even one order of magnitude more tightly confined can be obtained by metallic nanoparticles utilizing plasmonic resonances. An example is metallic triangles, as they can be produced by “nanosphere lithography” - employing colloid monolayers - on arbitrary substrates in a wide range of sizes, thickness, and materials. This nano-ablation in the plasmonic near field can not only be used for structuring purposes; since the ablation patterns represent a (nonlinear) image of the near field distribution of the plasmonic particles, they also provide a valuable testbed for numerical simulations of these near fields.