Robust Hybrid Plasmon–Photon Modes in Colloidal Metasurfaces Probed by Angle-Resolved SERS

Plasmonic-photonic metasurfaces enable large-area, spectrally selective field enhancement for surface-enhanced Raman scattering (SERS). However, identifying low-loss hybrid plasmon-photon modes under confocal, near-normal excitation remains elusive. Angle-resolved SERS provides direct access to their dispersion and degeneracies under stationary, ultrahigh numerical aperture excitation (NA ≈ 1.0 in air) by independently scanning polar and azimuthal incidence angles. SERS additionally demands modes that tolerate defects and periodicity fluctuations. We therefore fabricate colloidal metasurfaces across particle concentrations and lattice periods to quantify robustness. Using template-assisted self-assembly, we reveal a direct correlation between nanoparticle concentration, structural order, and the emergence of hybrid modes. At the optimized periodicity, SERS is maximized (up to 2.32× higher SERS intensity), which we can modulate through resonance detuning. Angle-resolved SERS at 633 and 785 nm under oblique incidence shows maximal enhancement when the hybrid guided-mode resonance is tuned to the laser line, yielding up to 36× higher intensity than off-resonant conditions for the same sample. An analytical model attributes these modes to phase-coupled waveguide coupling and collective plasmonic chain resonances. Thus, we yield practical design rules for robust, tunable colloidal plasmonic-photonic SERS metasurfaces via hybrid-mode dispersion engineering.