Page 208 - Principles and Applications of NanoMEMS Physics

5. NANOMEMS APPLICATIONS: PHOTONICS 197

Raether [215] has pointed out that at visible wavelengths in silver, L may
i
be as high as 22µ m at λ = 5145 Å , and 500µ at λ = 10 , 600 Å . In
m
addition to characterizing the SP decay by a distance, it may also be
characterized by its lifetime. This is related to the SP group velocity by,
T = L v and, is a complex frequency ω = ω ' − iω and real k are
' '
'
i i g x
π
assumed, may be expressed as T = 2 ω , where from (9), one obtains,
' '
i
ε ' ' ε ' ε
ω ' ' = k ' c 1 1 2 . (15)
x () 2 ε ' + ε
2 ε ' 1 2
1
Since SPs are associated with both a field and electron motion, their lifetime
is influenced by mechanisms giving rise to attenuation. These include,
radiation damping (conversion of the SP into light due to scattering),
electron scattering processes giving rise to ohmic losses, and chemical
interface damping due to high interface state densities [217]. Two steps are
essential, therefore, in the miniaturization of optics by exploiting SPs,
namely, the processes of exciting the SPs by light, and of transporting SPs
with minimum loss. These subjects are taken up by nanophotonics.

5.3 Nanophotonics

Nanophotonics deals with the realization of nanometer-scale optical
components and signal processing functions. While the goal is to produce
miniaturized optical components, it is conceivable that components in the SP
domain, while performing equivalent optical functions, might take different
forms not derivable from a direct downscaling of their optical counterparts.
Nevertheless, functions such as light-to-SP conversion, SP wave guiding,
and SP-to-light conversion are expected to be fundamental to these pursuits.

5.3.1 Light-Surface Plasmon Transformation

Schemes for converting light into SPs, and vice versa, derive from
circumventing the incompatibility of their dispersion relations, which do not
intersect, see Fig. 5-2 below, and the necessity to conserve momentum.
Accordingly, there are two fundamental elements to supply the additional
momentum, namely, the grating coupler, and the ATR prism.