Plasmon Ruler: Nanoparticle Dimer Coupling

Two gold nanospheres, each ringing with its own localized surface plasmon, start to talk to each other once their gap shrinks below a couple of diameters. Their dipole fields hybridize into a red-shifted "bonding" mode and the gap between them lights up as a near-field hot spot — the same physics behind surface-enhanced Raman spectroscopy. This 2D simulator drives that dimer with the universal plasmon-ruler equation (Jain, Huang & El-Sayed, 2007): drag the spheres together and watch the fractional resonance shift decay exponentially with the gap normalized by particle diameter, with a live extinction spectrum showing the resonance peak split and redshift — precisely why biosensors use a nanoparticle pair's color shift as a nanoscale ruler for measuring molecular distances and conformational changes label-free.