Figure 17 Control of light fields using metallic structures. Recent advances have permitted
the concentration of light on a very fine length scale by using nanoscale structures. The figure
shows an experimental image of white light scattering from silver nanoparticles. The color
differences arise from the limits imposed by particle shape on the interactions of light and
electrons within a nanoparticle. Enhancements in the light intensity exceeding 1,000 times
can be achieved by using appropriately shaped nanostructures, as has been demonstrated by
single-molecule Raman scattering and other enhanced optical effects. (Source: Michaels,
Nirmal, and Brus 1999)Photonic crystals are periodic structures — analogous to the usual crystals formed of atoms —
that can be tailored to modify the propagation of light (Johnson and Joannopoulos 2002).
Researchers have recently identified and demonstrated many fascinating properties of these
systems, including new schemes for light guidance and localization. Photonic crystals may be
applied to solar energy conversion in a variety of ways, starting with the production of highly
effective optical filters, anti-reflecting coatings, and mirrors that exhibit engineered responses as
a function of the angle of incidence. Photonic crystals can also channel the light to areas where
the absorbing molecules are located. The interaction of the photonic cavity with the absorption
material may also be a mechanism to control the absorption properties of the absorbing material.
A promising approach to tailoring the absorption properties of materials is through control of
dimensions on the nanoscale. The well-known quantum size effects in nanoparticles are only the
first example of nanoscale control of the optical properties of materials (Alivisatos 1996;
Empedocles and Bawendi 1999). Multi-component systems could enable another level of
sophistication in the design of optical properties (Wu et al. 2002; Redl et al. 2003).