Picoscale Light-Matter Interactions in Plasmonic, Polaritonic, and van der Waals Nanostructures.

Picophotonics describes light-matter interactions in which atomic-scale structure, sub-nanometer confinement, or picometer-scale structural variations significantly influence optical response. This review establishes a unified framework for understanding these interactions across plasmonic picocavities, molecular systems, and van der Waals (vdW) materials and heterostructures. We examine key mechanisms governing extreme optical confinement, including nonlocality, electron tunneling, charge-transfer effects, excitonic interactions, and phonon/plasmon polaritons. Experimental approaches, including scanning tunneling microscopy-based optical measurements, tip-enhanced Raman spectroscopy, near-field optical microscopy, and ultrafast spectroscopy, are discussed together with electromagnetic and quantum-mechanical modeling methods. Particular emphasis is placed on how atomic-scale structural variations, interfacial geometry, and material composition regulate optical response and enable applications in single-molecule spectroscopy, chemical sensing, cavity-modified chemistry, and quantum photonics. We further compare metallic picocavities with vdW polaritonic and hybrid plasmonic-vdW platforms in terms of confinement, losses, tunability, reproducibility, and scalability. Finally, key challenges involving experimental artifacts, heating, fabrication, and quantitative metrology are critically assessed, providing perspectives toward reproducible and programmable picoscale light-matter interactions.