Plasmonics

Physics: Surface plasmon polariton (SPP): coupled EM wave + electron plasma oscillation at a metal/dielectric interface. Dispersion: k_SPP = (ω/c)√(ε_m·ε_d/(ε_m+ε_d)), always greater than k_free photon → field confined to interface (evanescent in both media). Field enhancement |E|²/|E₀|² ~ 100 at Ag/air. Propagation length 10–100 μm at visible. Excitation: prism coupling (Kretschmann geometry) or grating. SPR biosensor: shift in resonance angle ∝ Δn at surface → detects protein binding ~1 pg/mm². SERS (surface-enhanced Raman): hotspots in gap junctions can enhance Raman cross-section by 10¹⁴.