Negative refraction lenses trap particles

29 Mar 2006

Perfect lenses with a negative index of refraction at microwave frequencies find a practical application - trapping and manipulating particles.

Three dimensional negative refraction flat lenses (3DNRFL) are ideal for electromagnetic trapping and manipulating of neutral particles, according to Dennis Prather and his research group from the University of Delaware, US. Carried out at microwave frequencies, the team trapped a cluster of particles and translated it 9.2 mm without moving the lens. (Optics Express 14 2228)

"Using a full 3DNRFL, we have been able to demonstrate, for the first time, electromagnetic gradient force trapping, similar to that of optical tweezers, but at microwave frequencies," report the authors. "The 3DNRFL provides a unique mechanism to create a highly focused, strongly convergent beam independent of a single optical axis."

Materials with a negative refractive index essentially bend light in the opposite direction to their conventional counterparts. Over the past 12 months, several research groups have succeed in fabricating artificially engineered materials, also known as metamaterials, which have a negative refractive index at optical frequencies.

Although this experiment was carried out at microwave frequencies, the team believes the technique can be adopted at optical wavelengths. "In the optical regime, the challenge lies in the fabrication of such a 3DNRFL," says Prather's team. "For optical wavelengths, the flat lens would become a thin patterned film, which would provide a focused beam with size limited only by the size of the source."

The flat lens used in this current work is a body-centred cubic photonic crystal made from a low-loss microwave material. Negative refraction is obtained by carefully engineering the dispersion properties of the photonic crystal, which has 20 layers in total each with a thickness of 6.35 mm.

Prather's team place the photonic crystal lens 1 mm above a microwave monopole source emitting between 16 and 17 GHz. On the other side of the lens, a first petri dish is used to create a 10 mm air gap while a second contains the sample. A stereomicroscope equipped with a digital camera records the results.

The sample consists of polystyrene particles dispersed in a liquid called dioxane. Mounted on a motorised stage, the monopole is translated and the team found that the cluster of trapped particles follows the source and reforms at a new position. Using this approach, the researchers dragged the particles over 9.2 mm.

"The cluster size changes little as it move over long distances," say the authors. "This is expected - because the flat lens has translation symmetry for imaging and is devoid of a unique optical axis. As a result, there is no field curvature and the quality of the monopole image is independent of its lateral position. This is a very advantageous property for trapping as it also allows for effective manipulation without lens movement."