In our last paper on 3D nanolithography in optical crystals we show how to beat the diffraction limit of light, by performing empty nanopores (air, n=1) inside solid-state laser crystals (YAG), with indefinite lengths but width of only hundreds of nanometers and cricular cross-section, something which was completelly impossible until now.
See this and all details in our publication at: https://doi.org/10.1515/nanoph-2022-0665
Light-matter interactions and Extreme Application Photonics Laboratory 