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Curved light bends the rules

Photonics Spectra
Sep 2010
Marie Freebody,

Conventional thinking leads us to believe that light only follows a straight path and that it diffracts as it travels. But some scientists are breaking all the rules by creating a new class of nondiffracting optical beam that can bend around corners.

These rays are called “Airy beams,” a name that honors the English astronomer Sir George Biddell Airy for his studies of the parabolic trajectories of light in rainbows. The leap from mathematical predictions to experimental observation was made in 2007 by a group of researchers at CREOL – the College of Optics & Photonics at the University of Central Florida in Orlando.

In this setup for nonlinear generation of Airy beams, the nonlinear photonic crystal is placed on a temperature-controlled stage tuned to the phase-matching conditions. Images courtesy of professor Ady Arie.

Now, researchers at Tel Aviv University have demonstrated new ways to generate and control Airy beams, using new algorithms and nonlinear optical crystals. Their research is reported in the July 2009 (online June 21, 2009) issue of Nature Photonics and in two subsequent publications, Applied Physics Letters in November 2009 and Optics Letters in May 2010.

To date, Airy beams have been generated only with linear diffractive elements that project a single color of light. But, the aptly named professor Ady Arie and his graduate students Tal Ellenbogen, Noa Voloch-Bloch, Ayelet Ganany-Padowicz and Ido Dolev of Tel Aviv University’s faculty of engineering have discovered that a brand-new approach yields greater control of these unique beams.

This photographed profile of the green Airy beam was projected onto the laboratory wall.

By using three-wave mixing processes, which occur in asymmetric nonlinear photonic crystals, Arie’s team generated beams at new wavelengths, opening up new possibilities for all-optical switching and manipulation of Airy beams.

“The most interesting application in my opinion is optical manipulation of small particles,” Arie said. “The Airy beam can trap microparticles and drag them along the parabolic trajectory of the central lobe. This may enable [sorting of] particles into different locations in space by varying the acceleration slope and direction of the beam.”

The team’s approach involves sending light through a special kind of crystal, whose nonlinear coefficient is spatially modulated. The light waves bounce inside the crystal, changing their wavelength and color and providing the researchers with greater control of the trajectories as well as the ability to create light beams at new wavelengths.

Shown is an artist’s concept of simultaneous multiple nonlinear generations of Airy beams.

“We have designed and fabricated a special nonlinear photonic crystal that converts a near-infrared Gaussian beam into a visible accelerating Airy beam,” Arie said. “We have shown how the nonlinear process can be used to control the parameters of the generated beam.”

In particular, the shape and peak intensity location of the beam can be varied by either changing the operating temperature of the nonlinear crystal or by changing the wavelength of the pump laser. What’s more, the team also has demonstrated how to change the acceleration direction of the beam: If the nonlinear process is an up-conversion one, the beam is accelerated to one direction; if it is a down-conversion process, it is accelerated to the opposite direction.

In this illustration of the experiment, ω and 2ω represent the angular frequencies of the pump and second-harmonic laser beams, respectively. The black-and-white regions of the nonlinear photonic crystal represent the sign modulation of the nonlinear coefficient of the stoichiometric lithium tantalate crystal used in the experiment. The lens is used for optical Fourier transformation of the beam, which is generated in the crystal into an Airy beam.

Their next step is to mix two – or even three – Airy beams in a nonlinear crystal. An interesting possibility would be to mix beams with different acceleration coefficients because the acceleration of the generated beam depends upon the accelerations of the input beams.

“In addition, we plan to expand to study nonlinear conversion of other types of accelerating beams,” Arie said. “The beam we studied so far is only the simplest member of a family of nondiffracting and freely accelerating beams.”

accelerating beamsAdy Arieairy beamsAll-optical switchingArieAyelet Ganany-PadowiczBiophotonicsCollege of Optics & PhotonicsCREOLGeorge Biddell AiryIdo DolevMarie FreebodynanoNature PhotonicsNoa Voloch-BlochOptical trappingResearch & TechnologyTal EllenbogenTech PulseTel Aviv Universitythree-wave mixingUniversity of Central Florida

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