Multi‐Pathway Upconversion Emission in Symmetry‐Broken Nanocavities: Broadband Multiresonant Enhancement and Anti‐Correlated Interfacial Sensitivity

Abstract The development of efficient, biocompatible, and broadband solid‐state nanocavity platforms with enhanced upconversion emission remains a critical challenge in nanophotonics, particularly for biosensing and bioimaging applications. Here, it is introduced symmetry‐broken nanolaminated plasmonic nanoantenna arrays (NLPNAs) that overcome the limitations of current approaches by achieving the desired balance of multiresonant enhancement, interfacial sensitivity, and scalable fabrication. The strategic disruption of axial symmetry within metal‐insulator‐metal nanocavities enables broadband multiresonant enhancement across multiple upconversion emission pathways, including second harmonic generation, third harmonic generation, and upconversion photoluminescence, under femtosecond laser excitation across a broad near‐infrared (NIR) wavelength range (1000–1600 nm). The fabricated asymmetric NLPNAs demonstrate substantial enhancements compared to their symmetric counterparts, displaying up to 30‐fold increase in upconversion emission. The unique, anti‐correlated interfacial sensitivities exhibited by these distinct emission pathways offer a new avenue for improving the reliability and specificity of multiphoton nanoplasmonic biosensing and bioimaging techniques.