It is well known that the forces which light imparts on micro- and nanoparticles arise due to intensity gradients and
dielectric mismatch. For laser-irradiated atoms and molecules, optical forces primarily result from close resonance
between the optical frequency and an electronic transition. Recently it has emerged that optically induced pair forces
also arise, through a modification of Casimir-Polder interactions; preliminary assessments of the mechanism have
largely centered on nanoparticle systems. In this paper, we show that a potentially very significant effect can be
anticipated in the condensed phase, an optically induced modification of interatomic forces that is capable of generating
anisotropic patterns of laser-induced compression and expansion. This phenomenon, termed optical electrostriction,
should be measurable and significant when high intensity laser light is transmitted through even an essentially nonabsorptive
material. However, the full conditions for observation of the effect are such that some competing interactions might also arise. Key parameters that determine the size and character of optical electrostriction are delineated and possible applications are considered, including optical actuators for nanoscale electromechanical systems.
KEYWORDS: Ultrafast phenomena, Polarization, Energy transfer, Chromophores, Near field, Resonance energy transfer, Electromagnetism, Radio propagation, Resolution enhancement technologies, Near field optics
In suitably designed nanoscale systems the ultrafast migration of uv/visible electromagnetic energy, despite its near-field
rather than propagating character, can be made highly directional. At the photon level such energy migration
generally takes a multi-step form, with each step signifying the transfer of an electromagnetic quantum between
chromophores playing the transient roles of source/donor and detector/acceptor. There is much interest in nanophotonic
devices based on such mechanisms, although the excitation transfer is usually subject to losses such as radiative decay,
and possible device applications are compromised by a lack of suitable control mechanisms. Until recently it appeared
that only by inefficient and kinetically frustrated means, such as chromophore reorientation or movement, could
significant control be effected. However in a system constructed to inhibit near-field propagation by geometric
configuration, the throughput of laser pulses can facilitate energy transfer through a process of laser-assisted resonant
energy transfer. Suitably configuring an arrangement of dipoles, it proves possible to design parallel arrays of optical
donors and acceptors such that the transfer of energy from any single donor, to its counterpart in the opposing plane, is
switched by throughput laser radiation of an appropriate intensity, frequency and polarization. A detailed appraisal of
some possible realizations of this system reveals an intricate interplay of electronic structure, optical frequency and
geometric factors. In the drive to miniaturize ultrafast optical switching and interconnect devices, the results suggest a
new basis for optically activated transistor action in nanoscale components, with significant parallel processing
With optical tweezer methods now firmly established and the nature of optical forces on individual particles well understood, one of the separate but related issues that has only recently come to the fore concerns the effects of intense optical radiation on inter-particle forces. It has already been established that such forces, which are not dependent on optical field gradients, can effect a weak binding between particles leading in some cases to optical clustering and in others to pattern formation. In this presentation it is shown by quantum electrodynamical analysis that a variety of other optomechanical effects can be produced in materials or systems subjected to the throughput of intense, non-resonant laser radiation. In particular, an optical electrostriction phenomenon is identified and shown to be widely operative in laser optical materials. Although a classical electrodynamical interpretation (in terms of interactions between induced dipoles) comfortably predicts the sign of the resulting mechanical force, it is shown that such a picture has significant limitations in addressing this fundamentally photonic phenomenon. The key parameters that determine the size and character of optical electrostriction are delineated and its significance is quantitatively assessed. The experimental challenges involved in characterizing such phenomena are also given a detailed appraisal.
Resonance energy transfer (RET) is a near-field mechanism for propagating optical energy between particles with suitably matching frequency response. The process communicates electronic excitation between suitably disposed (donor and acceptor) dipoles in close proximity, activated on excitation of the donor. In a multi-component system the transfer of excitation between any given donor and acceptor is usually passive, and it competes with loss mechanisms such as radiative decay and the possibility of transfer to one or more other acceptors. It thus appears that any potential exploitation of RET for optical switching is compromised by the innate passivity of the process. Now it emerges that there is a direct, all-optical route to introduce the necessary control. In a system constructed to satisfy frequency-matching conditions, but designedly to inhibit RET by geometric configuration, the throughput of laser pulses can facilitate energy transfer processes that would otherwise be forbidden, by laser-assisted resonant energy transfer. Suitably configuring an arrangement of transition dipoles, it proves possible to design parallel planar arrays of optical donor and acceptor particles such that the transfer of energy from any single donor, to its counterpart in the opposing plane, can be switched by appropriate laser radiation. As the energy transfer is itself mediated electromagnetically, the device operates as an optical transistor. For simplicity, a pair of two-dimensional arrays is envisaged, each consisting of equally spaced, identical particles arranged on a square lattice. A detailed appraisal of the system, including a consideration of competing processes, suggests that this configuration offers a new basis for the design of optically activated nanoscale transistor arrays.
In the field of optical energy harvesting it has long been known that the efficient capture of radiation by suitably designed absorbers is by no means the sole criterion for an effective collection system. The optical energy acquired by an absorbing medium is of little value at its absorption site; useful devices require that the energy rapidly and non-diffusively relocates to traps or reaction centers. Storage is then achieved by driving charge separation or another more complex reaction. The principles that operate over the crucial mechanisms for inter-site energy transport are now well understood, and materials can be engineered to expedite and control an optimally directed, multi-step flow of energy. In this paper the salient principles drawn from nanophotonics, fluorescence spectroscopy, molecular electronic structure and nonlinear optics are exhibited with reference to a number of recently devised energy harvesting materials and systems, prominently featuring dendrimeric organic polymers. It is also shown how the elementary transfer mechanism can be tailored to more efficiently direct the flow of excitation energy.