In their conventional form, hair-thin glass fibres ensure fast internet speeds – but conducting light with very low loss isn’t the only thing they can do. Philip Russell, Director at the Max Planck Institute for the Science of Light in Erlangen, and his colleagues are using a novel type of optical fibre, known as photonic crystal fibre (PCF), to manipulate the properties of laser light and control its interactions with matter. Their work is leading to many applications, for example in advanced ultraviolet light sources and optical fibre sensors for medicine, industrial manufacturing, structural engineering and environmental monitoring.