| Rydberg excitons are highly excited electron–hole pairs in semiconductors that exhibit hydrogen-like energy level structures and pronounced macroscopic quantum characteristics. Owing to their power-law-enhanced interactions and strong nonlinear optical responses, they have attracted increasing attention in recent years. Spectroscopic techniques such as photoluminescence excitation (PLE) and second-harmonic generation (SHG), combined with external field modulation—including electric, magnetic, and microwave fields—have revealed key properties of Rydberg excitons: micrometer-scale spatial extent, large polarizability, long lifetime, and giant dipole moments. Wide-bandgap semiconductors such as cuprous oxide (Cu2O), with low defect density and dipole-forbidden transitions, provide ideal platforms for accessing high principal quantum number excitonic states. In the high-n regime, long-range van der Waals and dipole–dipole interactions between Rydberg excitons become significantly enhanced, giving rise to phenomena such as exciton blockade and nonlinear refraction. External fields can induce sizable energy level splitting, which scale strongly with n, and modify selection rules, enabling selective excitation of specific exciton states. In addition, environmental perturbations can strongly influence the spectral features of Rydberg excitons. Benefiting from their exceptional sensitivity to external fields and local environments, Rydberg excitons show great promise for applications in weak-field sensing, on-chip single-photon devices, quantum simulation, and microwave–optical signal transduction. |
