Coupling of optical resonances in a compositionally asymmetric plasmonic nanoparticle dimer.

Electromagnetic coupling between plasmon resonant nanoparticles follows principles of molecular hybridization, that is, particle plasmons hybridize to form a lower energy bonding plasmon mode and a higher energy antibonding plasmon mode. For coupling between equivalent particles (homodimer), the in-phase mode is optically allowed, whereas the out-of-phase mode is dark due to the cancellation of the equivalent dipole moments. We probe, using polarized scattering spectroscopy, the coupling in a pair of nonequivalent particles (silver/gold nanoparticle heterodimer) that allows us to observe both in-phase and out-of-phase plasmon modes. The hybridization model postulates that the bonding modes should be red shifted with respect to the gold particle plasmon resonance and the antibonding modes blue shifted with respect to the silver particle plasmon resonance. In practice, the antibonding modes are red shifted with respect to the silver plasmon resonance. This anomalous shift is due to the coupling of the silver particle plasmon resonance to the quasi-continuum of interband transitions in gold, which dominate in the spectral region of the silver particle plasmon resonance. The hybridization model, which considers only free-electron behavior of the metals, fails to account for this coupling.