Amanda D. Francoeur, email@example.com
TUCSON, Ariz. – Researchers have devised a way to curve laser beams in the air by converting a standard directional beam into a self-bending Airy beam. The development may help with atmospheric applications, particularly with the ability to redirect lightning to keep it from striking sensitive areas such as industrial plants or airports.
This complex Airy beam profile is arranged with a series of spikes, or lobes, propagating along a curved trajectory. The main peak of the profile, or dominant lobe, becomes a light bullet when it reaches sufficient intensity.
Scientists led by Dr. Pavel Polynkin, associate research professor of optical sciences at the University of Arizona, and by professor Demetrios Christodoulides of the Center for Research and Education in Optics and Lasers–College of Optics and Photonics at the University of Central Florida in Orlando have created complex laser beam profiles to produce ultrafast self-bending laser beams.
“Self-bending laser beams have been demonstrated before, but only with low-intensity, continuous-wave laser beams,” Polynkin said.
Dr. Jérôme Kasparian and professor Jean-Pierre Wolf, researchers at the University of Geneva in Switzerland, reviewed a paper on the group’s study that provided a perspective on its work and prospects for the discovery. Both groups’ articles were published in April in the journal Science.
To create the bent laser beam, Polynkin’s group used a Ti:sapphire oscillating laser system that emits short pulses at a duration of approximately 35 fs at 800 nm. The beam generates a visible white light continuum and, as it propagates, a ripple effect, or Airy profile, occurs. Interferences between the ripples influence the main peak of the profile to move away from a straight line by 2 mm.
“Airy beam is a name of the transverse intensity pattern which is described in terms of an Airy function,” Polynkin said. “What we call ‘light bullet’ is that dominant lobe of the Airy pattern, in the corner of the pattern.”
Kasparian explained, “An Airy laser beam has a profile with a peak on one side, some ripples, and a broad trail on the other side.” As the beam propagates, interferences occur at various locations of its profile, and it takes on a curved trajectory.
The advantages of an Airy beam are that it can resist diffraction throughout long distances and can self-bend, or accelerate, during propagation. It also is self-healing in that, if a portion of the beam hits a solid object, energy is transferred from the intact portion of the beam to reform the asymmetric wave pattern. This could benefit remote sensing because it would allow the laser beam to propagate through clouds.
When the beam’s powerful pulses reach a sufficient energy level, they ionize the air locally, forming a light bullet – plasma channel – the main peak of an Airy profile. In so doing, when propagated into a thundercloud, the beam could trigger lightning strikes and act as a rod to control the electrical discharges. The lightning then would be contained to be guided safely to the ground.
Interferences between ripples in the Airy beam pattern force the light bullet away from a straight line. The interferences occur at various locations in the profile, pushing the light bullet sideways.
In laboratory experiments, the laser filaments were several meters long, but they can lengthen to hundreds of meters in atmospheric experiments, and their diameter is about 100 μm. The plasma channels left in the wake of the laser pulses last only a few nanoseconds once a pulse has propagated, fading away before the next pulse is emitted.
The light bullets are self-guiding. The self-focusing of the beam counteracts the diffractive spreading, enabling diffraction-free propagation. With ultrahigh-intensity beams, self-focusing potentially could collapse a beam, but the plasma produced has a negative contribution to the refractive index and defocuses the beam to compensate for self-focusing. Thus, “three effects – diffraction, self-focusing and plasma defocusing – act together and result in this self-guided propagation of filaments,” Polynkin said.
An infrared laser beam, invisible to the eye, is propagated from the upper left corner to the bottom right. A plasma channel, shown in the center of the image, is left in its wake. The channel is curved only slightly by a few millimeters per meter of propagation within the lab and, therefore, is not evident in the photo.
By controlling beam curvature, the technique could accommodate other applications, including particle acceleration, terahertz generation, remote spectroscopy, lidar, and the observance of global warming, ozone loss and tropospheric pollution.
Researchers also could develop techniques for laser machining of various guiding structures to include wavelength division multiplexers, and interferometer beamsplitting and beam-coupling.