One of the earliest lessons in science that students learn is that a ray or beam of light travels in a straight line. Students also learn that light rays fan out or diffract as they travel. Recently it was discovered that light rays can travel without diffraction in a curved arc in free space. These rays of light were dubbed "Airy beams," after the English astronomer Sir George Biddell Airy, who studied what appears to be the parabolic trajectory of light in a rainbow.
Now, scientists with the Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated the first technique that provides dynamic control in real-time of the curved trajectories of Airy beams over metallic surfaces. This development paves the way for fast-as-light, ultra-compact communication systems and optoelectronic devices,and could also stimulate revolutions in chemistry, biology and medicine.
The key to the success of this work was their ability to directly couple free-space Airy beams – using a standard tool of optics called a "grating coupler" - to quasi-particles called surface plasmon polaritons (SPPs). Directing a laser beam of light across the surface of a metal nanostructure generates electronic surface waves – called plasmons – that roll through the metal's conduction electrons (those loosely attached to molecules and atoms). The resulting interaction between plasmons and photons creates SPPs. By directly coupling Airy beams to SPPs, the researchers are able to manipulate light at an extremely small scale beyond the diffraction limit.
In addition, the unique properties of the plasmonic Airy beams open new opportunities for on-chip energy routing along arbitrary trajectories in plasmonic circuitry, and allows for dynamic manipulations of nano-particles on metal surfaces and in magneto-electronic devices."
Dynamic control of the plasmonic Airy beams is provided by a computer-controlled spatial light modulator, a device similar to a liquid crystal display that can be used to offset the incoming light waves from a laser beam with respect to a cubic phase system mask and a Fourier lens. This generates a plasmonic Airy beam on the surface of a metal whose ballistic motion can be modified.
IMAGE: The GIF animation shows the computer-based dynamical control of the trajectory and peak intensity position of plasmonic Airy beams achieved by Berkeley Lab’s Xiang Zhang.
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"The direction and speed of the displacement between the incoming light and the cubic phase mask can be controlled with ease simply by displaying an animation of the shifting mask pattern as well as a shifting slit aperture in the spatial light modulator," Peng Zhang says. "Depending on the refresh rate of the spatial light modulator this can be done in real time. Furthermore, our spatial light modulator not only sets the plasmonic Airy beam into a general ballistic motion, it also enables us to control the Airy beam's peak intensity at different positions along its curved path."
The ability of the spatial light modulator to dynamically control the ballistic motions of plasmonic Airy beams without the need of any permanent guiding structures should open doors to a number of new technologies, according to Xiang and Peng Zhang and their collaborators. For example, in nano-photonics, it enables researchers to design practical reconfigurable plasmonic sensors or perform nano-particle tweezing on microchips. In biology and chemistry, it allows researchers to dynamically manipulate molecules.
Says Sheng Wang, second lead author of the Optics Letters paper, "The ultrafine nature of SPPs is extremely promising for applications of nanolithography or nanoimaging. Having dynamic tunable plasmonic Airy beams should also be useful for ultrahigh resolution bioimaging. For example, we can directly illuminate a target, for example a protein, bypassing any obstacles or reducing the background."
Adds co-author Yongmin Liu, "Our findings may inspire researchers to explore other types of non-diffracting surface waves, such as electron spin waves, in other two-dimensional systems, including graphene and topological insulators."
This work was supported by the U.S. Army Research Office, the U.S. Air Force Office of Scientific Research, and the National Science Foundation Nanoscale Science and Engineering Center.
Lawrence Berkeley National Laboratory addresses the world's most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab's scientific expertise has been recognized with 12 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit
Contact: Lynn Yarris firstname.lastname@example.org 510-486-5375 DOE/Lawrence Berkeley National Laboratory