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Plasmonics and optical tweezers - nanotechnology that manipulates with light

(Nanowerk Spotlight) Ever since Roman glass blowers made the Lycurgus cup, some 2,400 years ago, researchers and engineers have figured out to do all kinds of things with light, be it in optical fiber communications; the use of lasers for welding, cutting and surface modification of materials; photonic gyroscopes in aviation; or optical switches in computing. As a general field of science, photonics defines the knowledge devoted to the generation, transmission, detection, control and handling of light. One sub-domain, nanophotonics deals with the manipulation and emission of light using nanoscale material and devices. One of the sizzling hot topics within nanophotonics is plasmonics, which holds the promise of a class of subwavelength-scale optoelectronic components that could form the building blocks of a chip-based optical device technology that is scaleable to molecular dimensions. Here, we report on the latest achievements of a Spanish-French group of researchers that brings the exiting concept of an optically driven lab-on-a-chip closer.
Lycurgus cup green Lycurgus cup red
The Lycurgus cup is on display at the British Museum in London. (Images: British Museum)
The Lycurgus cup, when illuminated from outside, appears green. However, when Illuminated from within, it glows red. The glass contains metal nanoparticles, gold and silver, which give it these unusual optical properties. The underlying physical phenomenon for this is called surface-plasmon excitation.
Dr. Harry Atwater, in an excellent overview article on plasmonics in the April issue of Scientific American explains it this way: "Because of plasmonic excitation of electrons in the metallic particles suspended within the glass matrix, the cup absorbs and scatters blue and green light – the relatively short wavelengths of the visible spectrum. When viewed in reflected light, the plasmonic scattering gives the cup a greenish hue, but if a white light source is placed within the goblet, the glass appears red because it transmits only the longer wavelengths and absorbs the shorter ones."
In physics, a plasmon is the quasiparticle resulting from the quantization of plasma oscillations just as photons and phonons are quantizations of light and sound waves, respectively. Plasmons can also couple with a photon to create a third quasiparticle called a plasma polariton. As the name indicates, 'surface plasmons' are those plasmons that are confined to surfaces. In particular, the control of these surface plasmons has become increasingly attractive for optical signal processing, surface enhanced spectroscopy and sensor nanotechnology.
"Parallel to current research efforts, confinement and intensity of surface plasmon fields suggest new promising breakthroughs in the topical area of optical manipulation and transport of tiny amounts of matter" Dr. Romain Quidant explains to Nanowerk. "In this context, fully integrated optical tweezers able to produce versatile and controllable optical force landscapes are highly desirable for the development of new lab-on-a-chip devices. Such an alternative method replaces single-beam three-dimensional (3D) optical trapping technology, which usually operates with cumbersome bulk optics and relatively high laser intensity, with engineered plasmonic patterns able to work with non-focused illumination and a lower laser intensity threshold."
Quidant, ICREA researcher an assistant professor at ICFO – Institut de Ciències Fotòniques – in Barcelona, Spain, and a colleague from CEMES in Toulouse, France, were able to forge a bridge between the fields of optical manipulation and surface plasmon optics, exploiting the enhancement and confinement of surface plasmon fields bounded at metal surfaces to achieve strong optical forces able to manipulate with light small amounts of matter. They reported their findings in a recent article in Nature Physics ("Parallel and selective trapping in a patterned plasmonic landscape").
In previous work ("Surface Plasmon Radiation Forces"), using a photonic force microscope, the researchers have directly measured the forces induced on single dielectric micro-beads by a surface plasmon field generated at the surface of a thin homogeneous gold layer. When working at the surface plasmon resonance, the total force magnitude on the probe bead was found to be 40 times stronger than the force measured in the absence of surface plasmon excitation.
"A flat gold surface illuminated by an asymmetrical non-focused laser beam leads to a homogeneous in-plane optical potential that does not allow localized trapping of single objects to be carried out" says Maurizio Righini, a PhD student in Quidant's group and first author of the most recent paper. "Local trapping requires an additional confinement in the surface plane which can be introduced by patterning the metal surface to control the distribution of surface plasmon fields and thus the force landscape seen by the objects."
In this way, the researchers have demonstrated that illuminated microscopic metal pads can act as novel 2-D optical tweezers, able to manipulate single dielectric micro-beads.
"The advantages of this approach over conventional optical tweezers are multiple" says Quidant: 1) It does not require laser focusing through a bulk objective; 2) it enables parallel trapping from a single beam; and 3) it benefits from the surface plasmon force enhancement which significantly reduces the requirements on incident power which have up to now strongly limited the applicability of optical manipulation."
Even more astonishing, says Quidant, they observed that, unlike conventional optical tweezers, surface plasmon tweezers can be designed to become selective to specific objects out of a mix acting as an efficient optical sieve.
This study provides a first proof of concept of a novel generation of miniaturized and integratable 2-D optical tweezers based on surface plasmons. These tweezers open new perspectives to transport, trap and sort with light small objects at the surface of a chip. Their simplicity and flexibility could pave the way towards new analytical devices entirely operated with light, where specific nano-analytes can be extracted from a complex sample, manipulated and inspected at the chip surface. Being able to have such nanoscale features on microscale lab-on-chip devices could have materializations in many areas, from low cost and parallel analysis in health care, to drug tracking and food control.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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