Helical light: Interference and tweezers
- Published: Jun 15, 2012
- Author: David Bradley
- Channels: Atomic
Dark laser helices
A scientist in the UK has demonstrated theoretically that "dark" helices formed by laser beams could be useful in lithography, chiral separations of molecules and optical tweezers and other applications.
Researchers have known for some time of monochromatic light beams, such as Laguerre-Gauss beams, which carry orbital angular momentum also have a helical phase profile. According to Ole Steuernagel of the University of Hertfordshire's Science and Technology Research Institute, UK, have helical wave fronts and can be made visible using interferometry. Others have suggested that it might be possible to generate helical beams using various approaches.
Bright and dark interference fringes can extend across a standing wave's cross-section and this phenomenon is ubiquitous and exploited in Lippmann's photography and Gabor's holography. Such fringes form in laser cavities and interferometry and, explains Steuernagel, are used as transporters and imaging elements in atom optics.
Steuernagel points out that laser beams can be made to form dark as well as bright intensity helices of light with the dark helices potentially have many applications beyond their brighter cousins. Applications in lithography and the manipulation of particles through optical forces, such as particle trapping and particle transport are possible. In lithography, for instance, light helices could be used to make meta materials that have helical imprints embedded within them of left- or right-handed optical activity. Conversely, a left or right twist in a material lithographed using helical laser light might be used to create a membrane to filter chiral molecules based on their chirality. There are also potential applications of twisted molecules and waveguides for trapped quantum particles. The dark helices will do less damage to sensitive quantum systems.
Such helices are of the order of half a wavelength of the interfering light and can overlap to form structures resembling, what Steuernagel refers to as "very tall stacks of pancakes". These stacks can be deformed and novel "structures" produced by causing two monochromatic, collinear, counterpropagating partial waves with different orbital angular momenta to interfere.
Of course, a dark helix while having a similar shape to a bright helix counterpart is a curling thread of relative darkness in a background of bright light. This means that such a helix is not limited by the common optical limits on feature resolution and can generate greater intensity contrast than bright helicies. There is also the option of creating them either one-by-one as with bright helices or in a massively parallel fashion on a tight grid.
Special dark helices
Steuernagel explains how dark helices are special because of their sharper intensity contrasts. "Through my research, I have shown that they tend to outperform bright helices," he says. "This is potentially important for lithographic applications to create sharply defined and contoured helical imprints." His work shows that for many applications, dark helices might be able to do what bright helices will not be able to do.
Optics Express, 2012, 20, 14371-14379: "Bright and dark helices of light"
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.