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Published online: 7 January 2009 | doi:10.1038/nchina.2008.303

Laser physics: The long and the short of it

Tim Reid

Introduction

© (2008) APS

Strong, pulsed, femtosecond lasers can produce periodic rods of light known as laser filaments, which could have applications in remote sensing and lightning control. Now, Jie Zhang and co-workers at the Chinese Academy of Sciences in Beijing and Shanghai Jiao Tong University¹ have explained for the first time how long and short laser filaments might result from very different causes.

Previous studies have suggested that filaments are created when the laser ionizes air, creating plasma with a lower refractive index than the air. This is believed to cause alternate focusing and defocusing of the beam. However, recent calculations have shown that ionization may not be strong enough to generate the longest filaments, which can be 100 metres or more.

Zhang and co-workers ran a simulation of the nonlinear Schrödinger equation to calculate the refractive index field around a femtosecond laser beam, and predicted the ray trajectories (pictured). They found that filament formation is actually dominated by a self-focusing mechanism known as the Kerr effect.

The rays that start in the centre of the beam self-focus earlier than rays starting on the fringes, and the moving foci produce the laser filament. However, ionization still plays a role: certain parts of the beam might collapse without it.

The authors of this work are from:
Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China; Department of Physics, Graduate University of Chinese Academy of Sciences, Beijing, China; Department of Physics, Shanghai Jiao Tong University, Shanghai, China.

Reference

1. Xi, T. T., Lu, X. & Zhang, J. Spatiotemporal moving focus of long femtosecond-laser filaments in air. Phys. Rev. E 78, 055401 (2008).  | Article | ChemPort |