|
In the nanoscale structure of a wide variety of material systems, a close juxtaposition of optically responsive components
can lead to the absorption of light by one species producing fluorescence that is clearly attributable to another. The
effect is generally evident in systems comprising two or more light-absorbing components (molecules, chromophores or
quantum dots) with well-characterised fluorescence bands at similar, differentiable wavelengths. This enables the
fluorescence associated with transferred energy to be discriminated against fluorescence from an initially excited
component. The fundamental mechanism at the heart of the phenomenon, molecular (resonance) energy transfer, also
operates in systems where the product of optical absorption is optical frequency up-conversion.
In contrast to random media, structurally organised materials offer the possibility of pre-configured control over the
delocalization of energy, through molecular energy transfer following optical excitation. The Förster mechanism that
conveys energy between molecular-scale components is strongly sensitive to specific forms of correlation between the
involved components, in terms of position, spectroscopic character, and orientation; one key factor is a spectroscopic
gradient. Suitably designed materials offer a broad scope for the widespread exploitation of such features, in
applications ranging from chemical and biological sensing to the detection of nanoscale motion or molecular
conformations.
Recently, attention has turned to the prospect of actively controlling the process of energy migration, for example by
changing the relative efficiencies of fluorescence and molecular energy transfer. On application of static electric fields
or off-resonant laser light – just two of the possibilities – each represents a means for achieving active control with
ultrafast response, in suitably configured systems. As the principles are established and the theory is developed, a range
of new possibilities for technical application is emerging. For example, applications can be envisaged for new forms of
all-optical switching and transistor action. There is also interest in engaging with the interplay of optical excitation and
local nanoscale force, exploiting local responses to changes in dispersion forces, accompanying molecular energy
transfer.
|
