While femtosecond-laser-beam has long been pursued as a way to guide lightning (or simply trigger lightning strikes), researchers at the University of Arizona (UA; Tucson, AZ) and the University of Central Florida (UCF; Orlando, FL) have developed a way to send high-energy femtosecond-laser beams through the atmosphere much farther than was previously possible, increasing the potential range for guiding lightning.1
Filaments produced previously by femtosecond-laser setups would essentially disappear over distances greater than a few meters at best when focused tightly, due to diffraction. In the new approach, the primary, high-intensity laser beam is embedded within a second beam of lower intensity. The spatially chirped low-intensity annular "dress beam" is coherent with respect to the high-intensity inner beam; as the primary beam travels through the air, the dress beam acts as a distributed energy reservoir, refueling and sustaining the inner beam over much greater distances than were previously achievable.
Dress beam produced by axicon
In the experiment, both the inner and dress beam are produced by the same Ti:sapphire laser, supplying 40 fs pulses at an 800 nm wavelength and 10 Hz repetition rate. To achieve this, the output beam from the laser is split into two parts. One portion, which becomes the inner beam, has a Gaussian profile and a 4 mm diameter and is weakly focused by a lens with a 2 m focal length; the other part begins with a 12 mm diameter and is made annular by passing it through an axicon having an apex angle of 179.8°.
A motorized delay stage temporally synchronizes the beams, which are spatially recombined. Because the two beams have the same polarization, they can interact; the refueling of the center beam by the dress beam takes place over a distance of approximately 2 m. The peak power of the inner beam is about twice that needed to achieve self-focusing in air.
By itself, the inner Gaussian beam creates a plasma filament about 20 cm long; the annular dress beam by itself is too weak to create any filament at all. When combined, though, the beams create a single plasma channel up to 220 cm long, even if the beams have imperfections. To confirm that this result was not merely due to the increase in total energy, the researchers blocked the annular dress beam and boosted the energy of the inner beam to the previous total energy of the two beams; the result was a short filament on the order of 20 cm long.
Could reach 50 m or more
In a future longer-range setting, the central and dress beams would be closer to being collimated, producing a much longer interaction region. Simulations performed by Matthew Mills at UCF have shown that by scaling up the new laser technology using an axicon with an apex angle of about 179.9° and a clear-aperture radius of about 1 cm, the range of the laser filaments could reach 45 m for a total dressed-beam energy of 28 mJ. Higher energies could create even longer filaments.
In addition to guiding lightning, the technique could be used in remote sensing, attosecond physics and spectroscopy, and channeling microwaves. The development of the new technology was supported by a five-year, $7.5 million U.S. Department of Defense grant.
REFERENCE
1. Maik Scheller et al., Nat. Photon., 8 (2014); doi: 10.1038/nphoton.2014.47.
