Nanoscale Al₂O₃ Core with Ag Shell-Based Ultranarrow and Symmetric Cavity Plasmons for a Sub-nm Spectral Shift and Radius Differential Resolution Measurements

We report a plasmon ruler based on an ultranarrow and near-symmetric cavity plasmon mode in dielectric–metal core–shell resonators (DMCSRs), demonstrating the ability to probe a sub-nanometer spectral shift and radius differential resolution. Specifically, the ultranarrow and near-symmetric cavity plasmon mode (TE1) is experimentally achieved via the opening engineering of a Ag shell, which is engineered only by incorporating nanometric uniform aluminum oxide layer (Al2O3, ∼5 to 15 nm) growth using atomic-layer deposition technology into a polystyrene microsphere core of DMCSRs. The TE1 cavity plasmon mode with an ultranarrow linewidth (∼12 nm) and a near-symmetric lineshape demonstrates excellent identification capabilities for probing a 0.23 nm spectral shift and a 0.75 nm radius spatial differential resolution in in situ thermal expansion experiments. Our findings offer an unprecedented opportunity for using the ultranarrow and symmetric cavity plasmons in ultrasensitive sensing applications.