Research Issues
Advances in the synthesis of high-quality novel materials ultimately underlie potential progress
in both photon management and the control of optical absorption. Giant field enhancements will
require materials with controlled properties on the nanoscale, as well as new capabilities for
assembling these components into precise nano-optical structures. Potential photon up-
conversion or down-conversion schemes will require new semiconductor materials with carefully
controlled electronic transitions that provide intermediate band states. In addition, advances in
plasmonic waveguide development will require new approaches to the synthesis and fabrication
of high-quality nanoshaped structures. Finally, new hard and soft materials must be developed
with controlled absorption, reflection, and emission properties. For certain applications, like
thermo-photovoltaics, high-temperature stability is also an important issue. Photostability is a
ubiquitous issue for all solar energy conversion schemes.
The science of electric-field concentration in nano-optical structures is not fully developed.
Theoretical and experimental investigations are needed to establish a full understanding of the
field enhancement in nanostructured optical systems and to develop an optical design
methodology for these systems, analogous to what is now available for conventional optical
elements (Figure 18). The use of surface plasmon-polariton waves to capture and deliver energy
to target receptors is a promising approach, but further research is needed to understand and
optimize such (energy capture and delivery) processes. The use of photonic-band-gap structures
requires a deeper understanding of the interaction of the photonic structure with the solar energy
conversion target.
Research opportunities also exist in establishing a more complete understanding of what controls
optical absorption in light-harvesting biological systems composed of complex ensembles of
chromophores, as well as in studying and developing analogous constructs using hard materials,
such as nanoparticles. General principles for self-consistent design of the photon control, optical
absorption, and subsequent solar energy conversion steps must be established.
Impact
The primary processes of photon delivery, in terms of both spatial and spectral distribution, and
optical absorption are common to all approaches to solar energy conversion. Advances in photon
management and the tailoring of optical absorption properties can consequently impact the
efficiency of any solar energy scheme.