Chapter 4 Properties of nanomaterials

5- Interpar;cle coupling
Systems composed of mul;ple par;cles may exhibit op;cal proper;es different than that of an isolated
par;cle, even if each par;cle in the group is iden;cal. In the near-field, evanescent fields of proximate
nanopar;cles can interact. At larger distances nanopar;cles interact in the far-field through their
scatered fields, which is par;cularly relevant for periodic arrays. Jensen et al. inves;gated near-field
coupling between two iden;cal Ag nanospheres using DDA simula;ons (Fig 2.23). For a par;cle radius of
30 nm, an interpar;cle spacing of 30 nm results in a 10 nm red-shiv of the resonance peak. For much
smaller spacing the effect is much more drama;c, resul;ng in 100 nm and 125 nm red-shivs for a spacing
of 4 nm and 2 nm respec;vely.
Closely-spaced nanopar;cle pairs exhibit strong polariza;on sensi;vity, as coupling effects are much
weaker for incident light polarized perpendicular to the par;cle pair axis. Complete transfer of energy
from one nanopar;cle can occur via dipole-dipole interac;on, leading to the idea of sub-wavelength
waveguides based on linear chains of metal nanopar;cles. The principle interest in coupled metal
nanopar;cle dimers is the intense electric field enhancement observed in the gap between the par;cles,
which is far larger than those obtained around isolated par;cles. These are found to be strongly related to
the interpar;cle separa;on, with smaller distances producing larger field enhancement.
Par;cles arranged in a periodic patern will scater photons coherently, leading to a change in the
observed far-field scatering spectrum due to interac;on between scatered photons. Periodic
arrangements of nanopar;cles form a diffrac;on gra;ng, and coupling of LSPs to evanescent or radia;ng
gra;ng modes alters the ex;nc;on spectrum from that of isolated nanopar;cles. Essen;ally the
diffrac;on gra;ng imposes restric;ons on the angular distribu;on of scatering, and so inhibits or
strengthens scatering at given wavelength. Therefore the op;cal proper;es of a periodic array of
nanopar;cles depend on the gra;ng period in addi;on to the proper;es of the individual nanopar;cles.