"Optical Vortices" Might Extract Abundant Information From Matter

"Optical vortices" might extract abundant information from matter, providing a new and potentially wide-ranging optical tool, a Spain-US team has proposed theoretically. An ordinary light beam, when viewed head-on, looks like a bright circle. But a special light beam called an "optical vortex," when viewed head-on, looks like a bright ring surrounding a dark central core (see www.aip.org/png/2001/133.htm). Optical vortices are the simplest kind of beam carrying a property called "orbital angular momentum" (see Update 639).

Extensively studied since the early 1990s, such light beams, when viewed from the side, trace out a three-dimensional corkscrew pattern (see figure at www.aip.org/png/2005/229.htm); the pattern represents regions of constant phase (for example, regions of maximum electric field). This spiraling of light represents an extra "degree of freedom" that researchers can use as a new handle to optically encode information and subsequently to retrieve information from objects the beam strikes. In conventional laser beams, the energy flows parallel to the beam axis, like water in a jet.

However, for light with orbital angular momentum (OAM), the energy spirals around the beam axis. Ordinary beams carry only "spin angular momentum," encoded in the polarization of light. All possible spin states can be constructed with just two polarization states (vertical and horizontal, or clockwise and counterclockwise). For light with nonzero OAM, however, many states are possible, with higher states denoting tighter corkscrews (and consequently, a faster spiraling of energy; see figure at www.aip.org/png/2005/229.htm). For this reason, one can encode a huge amount of information in an OAM beam by creating light made of a superposition of many OAM states.

The researchers call the different OAM components "spiral spectra." In the "digital spiral imaging" concept now put forward by Lluis Torner at the new Institute for Photonic Sciences (ICFO) in Barcelona and his colleagues, a light beam of a convenient shape illuminates a sample to be probed. The sample scatters the beam and alters its spiral components. Breaking down the altered beam into its individual orbital-angular momentum components (and thereby analyzing the “spiral spectrum” of the scattered beam) can yield a wealth of information from the object.

The spiral spectra would, for example, be sensitive to nonuniformities in geometrical and structural properties of objects, and could be potentially useful for detecting biological and chemical agents, for probing biological specimens sensitive to OAM light, and might even aide recent proposals to increase the amount of data that can be imprinted on a compact disk using OAM. (Torner, Torres, Carrasco, Optics Express, Feb. 7, 2005; contact Lluis Torner, http://www.icfo.es ; for more background on OAM light, see Physics Today, May 2004, and New Scientist, 12 June 2004).