New spin on the optical vortex
Researchers at San Diego State University (San Diego, CA) have demonstrated how a phase-only liquid-crystal display (LCD) patterned with a helical structure can be used as a so-called "vortex lens."
Researchers at San Diego State University (San Diego, CA) have demonstrated how a phase-only liquid-crystal display (LCD) patterned with a helical structure can be used as a so-called "vortex lens."1 The device can be used to generate donuts of light from points and to cause interesting deformations in input images. Though the current work is exploratory rather than being aimed at a specific application, it highlights the emergence of the optical vortex as an important tool in several fields including optical trapping, astronomical observation, and optical pattern recognition. In addition, using a real-time display to implement these functions may allow engineers an additional means of control and perhaps feedback.
With respect to wavefront phase, the main difference between the effect of an ordinary lens (a) and that of a vortex lens (b through d) lies at the center, where the close intermingling of opposite phases encourages destructive interference at the center of the focused spot. This is true for differing numbers of spiral arms and for clockwise or counterclockwise rotation.
An optical vortex has a spiral or helical phase structure (see figure). The structure interacts with itself such that, when the light beam is focused, optical power can be enhanced at positions away from the center of the focus. Though optical vortices have been understood for 30 years, it is only in the last 10 that they have started to make an impact in practical systems. Among the best known is the so-called "optical spanner" developed in the mid-1990s. This is a variation of the conventional optical trap in which a particle is trapped by the electric field at the focus of a laser beam. In the spanner configuration, the particle is not only trapped, but made to spin because of the rotational momentum of the incoming light field. It also has the advantage of applying less optical power to the center of the captured object, making it a potentially less-destructive method.
More recently, researchers have started to use the vortex for image processing. One particularly interesting application, which has uses throughout optics and especially astronomy, is to prevent dazzle from a bright central object (such as a star) from obscuring weaker light sources (such as planets).1 Because it is now known that this effect could be used at x-ray as well as visible wavelengths, this could eventually become a widely used technique.2 Other image-processing applications range from simple edge enhancement to aiding location of the center of a correlation peak in pattern recognition.
The latest contribution to this area is the demonstration of the vortex on a pixelated liquid-crystal display, in which a vortex pattern is converted into phase modulation. By using the device in different optical setups, the San Diego State team has been able to explore the patterns that emerge. For instance, by putting a vortex lens in the Fourier-transform plane of a 4-f optical system (a type of optical information-processing system), they were able to show how two circular apertures a lateral distance apart were transformed into doughnuts at the output plane of the 4-f system, how their diameters grew with the number of spiral arms in the vortex pattern, and how the edges of the two doughnuts eventually interfered in the plane where they intersected. In imaging experiments, they showed how a single slit input was mapped to two displaced slits with a discontinuity at their center.3 The amount of displacement grew with the number of arms in the spiral.
What makes this work particularly interesting is that it demonstrates how a powerful optical technique can be combined with a mechanism for feedback, and thus show a route to further progress. Though the San Diego State research was concerned with the products of static vortex lenses, their LCD-based approach could eventually be used to provide dynamic images and thus a mechanism for automatic or experimenter control. This could be used, for instance, to electronically change the rotation of a trapped particle while it is being examined, or to adaptively filter out a bright central source that is changing over time.
- G. A. Swartzlander Jr., Optics Lett. 26(8), (April 15, 2001).
- A. G. Peele and K. A. Nugent, Optics Express 11(19) (Sept. 22, 2003).
- K. Crabtree et al., Applied Optics 43(6), (Feb. 20, 2004).
SUNNY BAINS is a scientist and journalist based at Imperial College, London; e-mail:[email protected]