It is known that a high-power laser propagating through an underdense plasma can acquire a minimum spot size due to relativistic self-focusing. Beyond the focus, the nonlinear refraction starts weakening, and the spot size of the laser increases, showing periodic self-focusing/ defocusing behavior with the distance of propagation. To overcome the defocusing, we propose the introduction of a localized upward plasma density ramp. In the presence of an upward ramp of plasma density, the laser beam obtains a minimum spot size and maintains it with only a mild ripple. For suitable parameters of the laser and the plasma, we have deduced conditions for the self-focusing. This kind of plasma density ramp may be observed in a gas-jet plasma experiment and resembles a plasma lens.
© 2007 Optical Society of America
Original Manuscript: August 28, 2006
Revised Manuscript: December 22, 2006
Manuscript Accepted: December 26, 2006
Published: April 17, 2007
Devki N. Gupta, Min S. Hur, Ilmoon Hwang, Hyyong Suk, and Ashok K. Sharma, "Plasma density ramp for relativistic self-focusing of an intense laser," J. Opt. Soc. Am. B 24, 1155-1159 (2007)