  Hopfions, higher-dimensional topological quasiparticles with sophisticated 3D
knotted spin textures discovered in condensed matter and photonic systems, show
promise in high-density data storage and transfer. Here we present crystalline
structures of hopfions lying in space-time constructed by spatiotemporally
structured light. A practical methodology using bichromatic structured light
beams or dipole arrays to assemble 1D and higher dimensional hopfion lattices
is proposed and a technique for tailoring topological orders is elucidated. The
birth of photonic hopfion crystals heralds a new era in high-dimensional,
condensed, and robust topological information processing.
