Laser filaments — thin 'strands' of laser-induced plasma that form in the atmosphere — can induce air vortices in their wake, report researchers in the USA (Opt. Express 22, 26098–26102; 2014). Such filaments form when the peak power of femtosecond laser pulses is sufficient to ionize the air and create a plasma. When the plasma's defocusing effects are compensated by self-focusing due to the nonlinear optical Kerr effect, the result is a natural 'self-channelling' of the laser light and the generation of spatially confined laser filaments.

Anton Ryabtsev and co-workers from Michigan State University have now observed that these filaments can create vortices in dry air. The team created filaments by focusing laser pulses with a duration of 40 femtoseconds (with an energy of 0.7 mJ and a repetition rate of 1 kHz) from an 800 nm regenerative amplifier into a hollow glass tube (4 or 14 mm in diameter). A green light sheet formed by a 532 nm laser diode was then used to image a cross section of the tube. To visualize the air flow in the tube, a small amount of butanol was sprayed nearby to act as a distribution of small scattering particles. The scatter of the green laser light was then imaged by a digital camera.

Credit: © 2014 OSA

The results show that, following the generation of a laser filament, pairs of vortices form in its wake. The precise number and location of the vortices depends on the position of the filament relative to the central axis of the tube. When the laser is blocked or no filament is formed, no motion of the air is observed.

The researchers believe that the vortices are formed due to filament-induced heating of the air and resulting convection currents. Indeed, experiments with a resistance wire placed at the location of the filament indicate very similar air-flow patterns and rotation speeds when the temperature of the wire reaches 150 °C. It is known that the temperature at the core of laser-induced plasmas can reach 3,000–5,000 K.

The findings suggest that filaments can offer a means for affecting fluid flow dynamics in the air and that such effects need to be taken into account when performing filament-based experiments, especially molecular tagging velocimetry.