Funnelling super-resolution STED microscopy through multimode fibres

Holographic multimode fibre endoscopes have recently shown their ability to unveil and monitor deep brain structures with sub-micrometre resolution, establishing themselves as a minimally-invasive technology with promising applications in neurobiology. In this approach, holographic control of the input light field entering the multimode fibres is achieved by means of wavefront shaping, usually treating the fibre as a complex medium. In contrast to other unpredictable and highly scattering complex media, multimode fibres feature symmetries and strong correlations between their input and output fields. Both step-index and graded-index multimode fibres offer a specific set of such correlations which, when appropriately leveraged, enable generating high-quality focused pulses with minimal intermodal dispersion. With this, we funnelled pulsed super-resolution STED microscopy with time-gated detection through a custom multimode fibre probe, combining the correlations of both multimode fibre types. We demonstrate resolution improvements over 3-times beyond the diffraction limit and showcase its applicability in bioimaging. This work provides not only a solution for delivering short pulses through step-index multimode fibre segments but also marks a step towards bringing advanced super-resolution imaging techniques with virtually no depth limitations.