Metasurfaces Form Vortex Beams

A metasurface uses circularly polarized light to generate and control new and complex states of light, such swirling vortices of light. The new tool can be used to not only explore new states of light but also new applications for structured light. (Source: Second Bay Studio / Harvard SEAS)

Over the last decade, applied physicists have developed nano­structured materials that can produce com­pletely new states of light exhibiting strange behavior, such as bending in a spiral, cork­screwing and dividing like a fork. These structured beams not only can tell scientists a lot about the physics of light, they have a wide range of appli­cations from super-reso­lution imaging to molecular mani­pulation and communi­cations. Now, researchers at the Harvard John A. Paulson School of Engi­neering and Applied Sciences have developed a tool to generate new, more complex states of light in a completely different way.

“We have developed a meta­surface which is a new tool to study novel aspects of light,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engi­neering at SEAS. “This optical component makes possible much more complex opera­tions and allows researchers to not only explore new states of light but also new appli­cations for structured light.” The new meta­surface connects two aspects of light, known as orbital angular momentum and circular polari­zation. In circularly polarized light, the vibra­tion of light traces a circle.

The fact that light can even carry orbital momentum is a rela­tively recent discovery but it’s this property of light which produces strange new states, such as beams in the shape of cork­screws. Previous research has used the polari­zation of light to control the size and shape of these exotic beams but the connec­tion was limited because only certain polari­zations could convert to certain orbital momentums. This research, however, significantly expands that connec­tion.

“This meta­surface gives the most general connection, through a single device, between the orbital momentum and polari­zation of light that’s been achieved so far,” said Robert Devlin, former graduate student in the Capasso Lab. The device can be designed so that any input polari­zation of light can result in any orbital angular momentum output – meaning any polari­zation can yield any kind of structured light, from spirals and cork­screws to vortices of any size. And, the multi­functional device can be programmed so that one polari­zation results in one vortex and a different polari­zation results in a completely different vortex.

“This is a completely new optical component,” said Antonio Ambrosio, Principal Scientist at Harvard Center for Nano­scale Systems (CNS). “Some meta­surfaces are itera­tions or more efficient, more compact versions of existing optical devices but, this arbitrary spin-to-orbital con­version cannot be done with any other optical device. There is nothing in nature as well that can do this and produce these states of light.”

One potential appli­cation is in the realm of molecular mani­pulation and optical tweezers, which use light to move molecules. The orbital momentum of light is strong enough to make micro­scopic particles rotate and move. “You can imagine, if we illu­minate the device with one polari­zation of light, it will create a force of a parti­cular kind,” said Ambrosio. “Then, if you want to change the force, all you need to do is change the polarization of the incoming light. The force is directly related to the design of the device.” Another appli­cation is high-powered imaging. The black hole in the center of the vortex, known as the zero-light inten­sity region, can image features smaller than the dif­fraction limit, which is usually half of the wave­length of light. By changing the polari­zation of light, the size of this center region can be changed to focus different-sized features.

But these beams can also shed light on funda­mental questions of physics. “These particular beams are first and foremost of funda­mental scientific interest,” said Noah Rubin, graduate student in the Capasso Lab. “There is interest in these beams in quantum optics and quantum infor­mation. On the more applied side, these beams could find appli­cation in free-space optical communi­cation, especially in scattering environ­ments where this is usually difficult. Moreover, it has been recently shown that similar elements can be incor­porated into lasers, directly producing these novel states of light. This may lead to unfore­seen appli­cations.” (Source: SEAS)

Reference: R. C. Devlin et al.: Arbitrary spin-to-orbital angular momentum conversion of light, Science, eaao5392 (2017); DOI: 10.1126/science.aao5392

Link: Group F. Capasso, Harvard School of Engineering and Applied Sciences SEAS, Harvard University, Cambridge, USA

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