Energy-harvesting materials

It is shown how key features of natural photosynthesis can be emulated in novel materials based on photoactive multichromophore arrays and crystals. A major consideration in the design of such systems is the means of channeling electronic excitation from sites of light absorption to centers where it is stored or released. Storage is often achieved by driving charge separation or, for the longer term, a more complex chemical reaction whilst rapid release is commonly associated with frequency up-converted emission. In each case channeling to the conversion site generally entails a multi-step energy transfer mechanism whose efficiency is determined by the arrangement and electronic properties of the array chromophores or ions, guided in the more complex systems by a spectroscopic gradient that promotes overall directionality. The functional cascade molecules known as photoactive dendrimers are exemplars of this approach. The latest developments involve new mechanisms for concerted excitation transfer in multichromophore systems, leading towards the tailoring and exploitation of optical nonlinearities for high intensity energy pooling applications

Electromagnetic Radiation

Electromagnetic radiation, commonly referred to as light, underpins all spectroscopic techniques, ranging from the highly energetic gamma rays, through x-rays, ultraviolet, visible, infrared, microwaves to the low-energy radio waves. The principles of wave- and particle-like behaviour determine the nature of the radiation and its interaction with matter, whether in the form of subatomic, atomic, molecular or macromolecular structures

The analysis of fluorophore orientation by multiphoton fluorescence microscopy

The accessibility of tunable, ultrafast laser sources has spurred the development and wide application of specialized microscopy techniques based on chromophore fluorescence following two- and three-photon absorption. The attendant advantages of such methods, which have led to a host of important applications including three-dimensional biological imaging, include some features that have as yet received relatively little attention. In the investigation of cellular or subcellular processes, it is possible to discern not only on the location, concentration, and lifetime of molecular species, but also the orientations of key fluorophores. Detailed information can be secured on the degree of orientational order in specific cellular domains, or the lifetimes associated with the rotational motions of individual fluorophores; both are accessible from polarization-resolved measurements. This paper reports the equations that are required for any such investigation, determined by robust quantum electrodynamical derivation. The general analysis, addressing a system of chromophores oriented in three dimensions, determines the fluorescence signal produced by the nonlinear polarizations that are induced by multiphoton absorption, allowing for any rotational relaxation. The results indicate that multiphoton imaging can be further developed as a diagnostic tool, either to selectively discriminate micro-domains in vivo, or to monitor dynamical changes in intracellular fluorophore orientation

Controlling nanoscale optical emission with off-resonant laser light

In the optical excitation of many nanoscale systems, the primary result of photon absorption is an electronic excitation that is typically followed by ultrafast relaxation processes. The losses associated with such relaxation generally produce a partial degradation of the optical energy acquired, before any ensuing photon emission occurs. Recent work has shown that the intensity and directional character of such emission may be significantly influenced through engagement with a completely off-resonant probe laser beam of sufficient intensity: the mechanism for this optical coupling is a secondorder nonlinearity. It is anticipated that the facility to actively control fluorescent emission in this way may lead to new opportunities in a variety of applications where molecular chromophores or quantum dots are used. In the latter connection it should prove possible to exploit the particle size dependence of the nonlinear optical dispersion, as well as that of the emission wavelength. Specific characteristics of the effect are calculated, and suitable experimental implementations of the mechanism are proposed. We anticipate that this all-optical control device may introduce significant new perspectives to fluorescence imaging techniques and other analytical applications

Resonance energy transfer and interface forces:Quantum electrodynamical analysis

On the propagation of radiation with a suitably resonant optical frequency through a dense chromophoric system - a doped solid for example - photon capture is commonly followed by one or more near-field transfers of the resulting optical excitation, usually to closely neighboring chromophores. Since the process results in a change to the local electronic environment, it can be expected to also shift the electromagnetic interactions between the participant optical units, producing modified inter-particle forces. Significantly, it emerges that energy transfer, when it occurs between chromophores or particles with electronically dissimilar properties (such as differing polarizabilities), engenders hitherto unreported changes in the local potential energy landscape. This paper reports the results of quantum electrodynamical calculations which cast a new light on the physical link between these features. The theory also elucidates a significant relationship with Casimir-Polder forces; it transpires that there are clear and fundamental links between dispersion forces and resonance energy transfer. Based on the results, we highlight specific effects that can be anticipated when laser light propagates through an interface between two absorbing media. Both steady-state and pulsed excitation conditions are modeled and the consequences for interface forces are subjected to detailed analysis

Optomechanical control of molecular motors

The majority of mechanisms that can be deployed for optical micromanipulation are not especially amenable for extension into the nanoscale. At the molecular level, the rich variety of schemes that have been proposed to achieve mechanical effect using light commonly exploit specific chemical structures; familiar examples are compounds that can fold by cis-trans isomerization, or the mechanically interlocked architectures of rotaxanes. However, such systems are synthetically highly challenging, and few of them can realistically form the basis for a true molecular motor. Developing the basis for a very different strategy based on programmed electronic excitation, this paper explores the possibility of producing controlled mechanical motion through optically induced modifications of intermolecular force fields, not involving the limitations associated with using photochemical change, nor the high intensities required to produce and manipulate optical binding forces between molecules. Calculations reveal that significant, rapidly responsive effects can be achieved in relatively simple systems. By the use of suitable laser pulse sequences, the possibilities include the generation of continuous rotary motion, the ultimate aim of molecular motor design