Page 213 - Principles and Applications of NanoMEMS Physics

202 Chapter 5

time of 7fs, and a double-peak resonance structure. The field enhancement
was explained as caused by the excitation of the SP resonance by the
evanescent field of the fiber aperture and subsequent radiation, by the
particle, of propagating modes into the far field, much like an antenna. The

Laser
Laser
Fiber er
Fib
Au- -
Tip
Tip Au
A l-C oatin N anoparticle
A l-C oating g
N anoparticle
TiO Film
TiO Film
2 2
G lass Substrate te
G lass Substra
Photodiode e
Photodiod
Figure 5-7. Sketch of setup for measuring surface plasmon resonances in single metallic
nanoparticles. The fiber tip has an aperture diameter of about 80 nm and positioned 7 nm
away from the 200 nm thickTiO 2 film, which is supported by a 1 mm-thick glass substrate.
(After [217].)
double-peak feature was explained as denoting the electromagnetic coupling
of two close-lying particles.

5.3.2.4 SP Coupling of Metallic Nanoparticles

The properties of SP coupling between close-lying metallic nanoparticles
were studied by Krenn et al. [219] and Kottmann and Martin [220]. Krenn et
al. [219] utilized PSTM to elucidate the evolution of the optical near-field
pattern when a large number of identical particles are arranged in a linear
chain. Comparison with theoretical calculations lead them to confirm the
unexpected squeezing of the optical near field due to SP coupling above a
chain of half oblate Au spheroids nanoparticles with sizes averaging 100 x
2
100 nm in section, by 40 nm height.
Kottmann and Martin [220] conducted a theoretical investigation of the
plasmon resonances of interacting silver cylindrical nanoparticles with 50
nm diameter at various separations, e.g., see Fig. 5-8. This figure shows that
at a separation of 5 nm and incidence along the major axis (i.e., along the
horizontal arrow) a single cylinder exhibits a resonance (dotted line) at
λ = 344 nm . This resonance has the same magnitude, although shifted down
to λ = 340 nm , for two cylinders (dashed line). In addition, an extra
resonance at about 372 nm is observed (dashed line) for this latter case,
showing the coupling of the two cylinders. In this case, an enhancement in
gap field amplitude, with respect to the incident field amplitude, by a factor