Opt. Express 26, 31706–31716 (2018)

By adding a nanoscale central hole to the core of an optical fibre scientists in Jena, Germany have realized a convenient lens-free scheme for focusing light exiting the fibre. Henrik Schneidewind and co-workers fabricated silica glass fibres with a 4-μm-diameter GeO2-doped core and a 400-nm-diameter hollow bore. Measurements and simulations show that the intensity distribution of visible (650 nm) light leaving the fibre starts with an annular shape close the fibre end (distance <3 μm), but then evolves into a bright spot for distances of 4–15 μm, before diverging. A spatial plot of this intensity profile versus distance resembles the shape of a candle’s flame, thus inspiring the team’s name for the innovation of ‘photonic candle’. The team says that the scheme’s key attraction is its simplicity. No post-processing or nanostructuring is required for the focusing, it is simply a case of a high-quality flat cleave of the fibre end. Potential applications that may benefit from the approach include optical trapping, bioanalytics or experiments in quantum optics.