TOPEX has explored the role of and the crossroads between two recently emerged notions in photonics, topological transport and sub-diffraction spatial confinement of light, for photonic integrated waveguides. In both such fields, the outcomes of the project have pushed the boundaries of the state of the art. First, recent developments in topological photonics had fostered the vision of backscattering-protected waveguides made from topological interface modes, but, surprisingly, measurements of their propagation losses were so far missing. Therefore, the measurement of propagation losses concomitant with those found in conventional waveguide modes, combined with the observation of strong backscattering, are a landmark for topological photonics and, more generally, for integrated photonics. Second, recent developments in nanophotonics had experimentally confirmed that light confinement in dielectric structures is not bound to the diffraction limit but are only limited by the smallest feature that can be made. However, state-of-the-art nanofabrication sets such length-scale to few tenths of nanometers. The development of a simple self-assembly procedure for creating features close to the atomic scale while still preserving the scalability of top-down nanofabrication is therefore a crutial step towards very strong light-matter interaction in bowtie photonic structures. In addition, the method may find applications far beyond nanophotonics, such as in solid-state nanopore sequencing, ultra-high-quality shadow masks for superconducting quantum electronic devices, to name a few.
