Structured light has enabled access to new physics and applications, and has particular potential in nanoscience. Here, we present a concept for creating light fields with constant intensity along all three spatial directions—a light strand—within externally accessible liquid-filled nanochannels integrated into optical fiber cores. We demonstrate all relevant properties of this mode, as well as a universally valid condition for obtaining flat fields and experimental evidence from nanoparticle tracking experiments. This concept can be adapted to other waveguides and could be valuable for characterizing rapidly diffusing nanoparticles in life science, analyzing nanoscale chemical reactions, investigating nanorheological phenomena, and understanding fundamental light–matter interactions.