Due to a physical constraint called the diffraction limit of light, the smallest space in which optical tweezing can trap a particle is approximately half the wavelength of the light beam. In the visible spectrum this would be about 200 nanometers half the shortest visible wavelength of 400 nanometers.
Thus, if the specimen in question is only 2 nanometers wide the size of a typical protein trapping it in a space of 200 nanometers allows only very loose control at best. Scale-wise, it is akin to guiding a minnow with 20-meter-wide fishing net.
Additionally, the optical force that light can exert on an object diminishes as the objects get smaller. "If you want to trap something very small, you need a tremendous amount of power, which will burn your specimen before you can trap it," Saleh said.
Some researchers get around this problem by attaching the specimen to a much larger object that can be dragged around with light. Dionne noted, however, that important molecules like insulin or glucose might behave quite differently when attached to giant anchors than they would on their own. To isolate and move a tiny object without frying it, the researchers needed a way around the limitations of conventional optical trapping.
The promise of plasmonics
Dionne says that the most promising method of moving tiny particles with light relies on plasmonics, a technology that takes advantage of the optical and electronic properties of metals. A strong conductor like silver or gold holds its electrons weakly, giving them freedom to move around near the metal's surface.
When light waves interact with these mobile electrons, they move in what Dionne describes as "a very well-defined, intricate dance," scattering and sculpting the light into electromagnetic waves called plasmon-polari
|Contact: Andrew Myers|
Stanford School of Engineering