Marie Freebody, email@example.com
TEL AVIV, Israel – 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
“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