Optically driven materials – very little makes sense

Examples of mechanical action over nanoscale dimensions may be found in a wide variety of modern material and sensors. In the realm of synthetic materials, specific mechanical effects can be produced by the action of light in chemical structures such as flexible polymer isomers that will fold on optical activation. However, the best such systems can be synthetically highly challenging, and they are often slow in response and recovery time. Much wider applications should be possible through a reduction of scale into a regime where intermolecular interactions, and sub-wavelength, near-field electrodynamics can all operate together. Although the predominantly attractive Casimir forces operating at this level commonly produce an effect known as ‘stiction’, a potential obstacle to achieving the optimum performance of nanoelectromechanical devices, it has recently emerged that it is possible to reverse such effects, and take positive advantage of their action.

Proof-of-principle work in the quantum electrodynamics group at UEA has demonstrated the basis for a strategy based on programmed electronic excitation. Laser radiation selectively excites individual components, and the resulting modification to a local energy landscape induces mechanical motion. Achieving finely controlled mechanical motion, through optically induced nanoscale forces, offers the prospect of significant, rapidly responsive effects in relatively simple systems. With the use of suitable laser pulse sequences, the possibilities of such an approach could extend to active, exquisitely detailed modification of surfaces, the programmed generation of localised nanoscale locomotion, or externally controllable rotation within solids. Accordingly, there is a pressing need for a complete and comprehensive theory of nanoscale mechanical action, achieved by all-optical engagement and modification of intermolecular and inter-particle forces.

 

RPG - Andrews

Two nanoparticles tethered to a surface, separated by their equilibrium distance, experience a changed potential energy field when one absorbs light. The changed position of equilibrium produces a motion that distorts local structure.

 

The primary objective is to forge a better understanding of how interactions with light fundamentally affect the various forces that operate between molecules, and between nanoparticles, including the possibility of distinctively quantum mechanical effects. It is known that light will not only modify optical properties, but also the forces that exist between such particles, and securing the principles to best exploit these effects should make it possible to devise more specifically purpose-made systems, designed to exhibit a variety of novel effects. The ultimate aim is to identify general principles that can give a clear direction to future experimental work, to unlock such possibilities as ultrafast responsive surfaces with optical properties actively modified by light itself.

Fundamental progress in this area will further advance an already rapidly developing sub-field of nanotechnology, an area of nanophotonics that represents a vibrant meeting point of mechanics with photonics research. With a variety of opportunities for technical implementation, success in this research programme will provide a general framework offering rich scope for identifying all-optical, ultrafast-response material applications.

Professor David L Andrews
University of East Anglia

David was awarded a Research Project Grant in March 2012; providing £140,424.