Observation of orbital angular momentum from an ultrathin topological insulator metasurface

Orbital angular momentum (OAM) existing in the vortex light beam with isolated singularities and spiral phase distribution presents significant applications in optical communications and light-field manipulation. The generation of OAM based on plasmonic metasurfaces is generally limited by the large optical loss and weak tunability of metal materials. Three-dimensional (3D) topological insulators (TIs) with insulating bulk states and topologically protected surface states allow the excitation of surface plasmons with low loss in the high-frequency region. Herein, we designed and fabricated an ultrathin Sb₂Te₃ TI plasmonic metasurface using the magnetron sputtering deposition and focused ion beam lithography. The results show that the 18 nm thick TI metasurface can efficiently generate surface plasmon resonances (SPRs) in the visible spectrum, which can effectively modulate the spatial phase of incident light for the generation of OAM. We find that the OAM conversion efficiency of the TI-based metasurface is remarkable compared with that of the gold-based metasurface. The experimental results obtained by a self-built OAM testing system demonstrate that the ultrathin TI metasurface can generate a distinct vortex beam with a first-order topological charge. This work will provide a new approach for generating OAM in ultrathin structures and exploring the applications of TIs in light-field manipulation.