Although science fiction literature and art portray extraordinary stories of people interacting with their images behind a mirror, we know that they are not real and belong to the realm of fantasy. However, it is well known that charges or magnets near a good electrical conductor experience real attractive or repulsive forces, respectively, originating in the interaction with their images. A team of researchers from Spain have now shown that this property can be extended to an optical microcavity under external illumination.
Schematic view of the mirror image method. Image from Optics Express, Vol. 20, Issue 10, pp. 11247-11255 (2012)
Specifically, silicon nanospheres with a high refractive index, when excited with a laser in the vicinity of a metal surface, produce attractive and repulsive forces which have been demonstrated to be significantly larger than competing forces such as Van der Waals and Brownian motion.
Whereas electrostatic and magnetic interactions with a conductive surface are known to be always attractive and repulsive, respectively, the character of the interaction between the surface and an optical microcavity can be either, and depends primarily on the multipolar electromagnetic resonance modes within the cavity. These modes are determined by the wavelength of the incident laser light, and can therefore be modelled and selected to produce attractive or repulsive forces as required.
These intense photonic forces could be used to develop a new kind of optical levitation, allowing accurate manipulation of nanoparticles. Two lasers tuned to repulsive and attractive modes of the cavity should allow full control over the position of a particle. This discovery has important consequences for subwavelength microscopy, optical sensing and numerous other applications.
Additionally, the particles can be decorated with biomarkers capable of attaching to specific biomolecules. This suggests a new way of performing molecule-specific biosensing, with the advantage of a high degree of spatial control over the position of the probe.
In a separate direction, a particle trapped with a laser of a particular colour could interact with a probing laser of a different colour, producing a photonic switch which could achieve speeds in the MHz range and above.