Crystalline Hollow Microrods for Site‐Selective Enhancement of Nonlinear Photoluminescence

We describe a new class of one-dimensional hollow microstructure that is formed through a kinetically controlled crystal growth process. We show that a hexagonal-phase NaYbF4 microrod comprising isolated holes along the longitudinal axis can be synthesized using a one-pot hydrothermal method with the assistance of citrate ligands. The unique structural feature is found to create modulation in light intensity across the microrods as a result of interference arising from light scattering and reflection by the inner walls. By doping upconverting lanthanide ions, we demonstrate that the structural feature can be harnessed to produce spatially resolvable optical codes in a single crystal upon near-infrared excitation. Our study thus highlights a new paradigm to control light emission and opens a fresh frontier for hollow-structural materials.