Optics & Photonics News - December 2009

PROPAGATION’09
Curved Plasma Channel Generation in Air
Using Ultra-Intense Self-Bending Airy Beams

Pavel Polynkin, Miroslav Kolesik, Jerome Moloney, Georgios Siviloglou and Demetrios Christodoulides

Ultrafast laser filamentation is a rich, interdisciplinary branch of physics that addresses propagation of intense laser pulses in transparent media. 1, 2 Applications range from terahertz generation to lightning control.

When an ultra-intense, ultra-short laser pulse propagates in air, the defocusing e;ect of the plasma generated via multi-photon ionization dynamically balances the self-focusing of the beam and prevents it from collapsing into a singularity. ;e hot core of the beam, composed of the high-intensity laser field and generated plasma, is referred to as the filament. Filaments are typically about 100 µm in diameter and exhibit self-guided, sub-di;ractive propagation over long distances.

High optical intensities inside filaments facilitate e;cient nonlinear wavelength conversion, leading to forward emission of broadband radiation. Analysis of the angularly resolved spectra of this radiation yields insights into the pulse propagation dynamics. 3

In early studies of femtosecond laser filamentation, axially symmetric beam profiles were used, such as Gaussian, flat-top and Bessel beams. Accordingly, filaments were generated along straight lines. ;e broadband forward emissions generated at di;erent points along a straight filament tend to overlap in the far-field, leading to spectra that are difficult to interpret.

We recently reported generating optical Airy beams. 4 ;ese non-axially symmetric beams are approximately di;raction free, and their main intensity features freely self-bend (or accelerate) on propagation in the absence of any refractive-index gradients.

We also conducted experiments on the filamentation of ultra-intense Airy beams in air. 5 We observed an unusual filamentation regime in which the linear

(a)

Beam displacement [mm]

3

2

1

0

(b) 3 2 1 0

Beam displacement [mm]

Distance from Fourier plane [cm] – 40 – 20 0 20 40

(c)

(d)

(Top row) Numerical simulations for plasma density generated along the beam path at low pulse energy ( 5 mJ, panel a) and high pulse energy ( 10 mJ, panel b). (Bottom row) Burn pattern produced by the beam on aluminum foil (c); numerical simulation for the transverse intensity pro;le of intense Airy beam (d).

self-bending property of the beam pattern competed against nonlinear self-channeling e;ects. ;e plasma channel generated by the dominant intensity feature of the beam followed the curved beam trajectory. In this regime, broad-band forward emission by the curved filament is angularly resolved in the far-field, thus enabling detailed study of this emission along the optical path. Extended curved filaments generated by self-bending Airy beams may find applications in remote sensing.

We started with 35-fs pulses with a Gaussian beam profile and transformed the beam into a 2-D Airy beam using a combination of a cubic phase mask and a focusing lens. ;e pulse energy was varied between 5 and 15 mJ. Analysis of forward emission by the curved filament suggests that the plasma channel was continuous at low pulse-energy levels but developed split-o; channels at several locations along the propagation direction when pulse energy was increased.

At high values of pulse energy, the transverse beam profile exhibited a nonlinear reshaping and developed a whisker-like ghost beam and lagging satellite lobe. ;ese features, clearly visible in the burn patterns produced on aluminum foil, were reproduced in numerical simulations. 

;is work was supported by AFOSR under contracts FA9550-07-1-0010 and FA9550-07-1- 0256. ;e contribution of G.S. and D.C. was partially supported by Lockheed Martin Corp.

Pavel Polynkin ( ppolynkin@optics.arizona.edu), Miroslav Kolesik and Jerome Moloney are with the College of Optical Sciences, University of Arizona, Tucson, Ariz., U.S.A. Georgios Siviloglou and Demetrios Christodoulides are with the College of Optics and Photonics, University of Central Florida in Orlando, Fla., U.S.A.