Generation of Electromagnetic Pulses from Plasma Channels Induced by Femtosecond Light Strings

Abstract

We present a model that elucidates the physics underlying the generation of an electromagnetic pulse from a femtosecond laser induced plasma channel. The radiation pressure force from the laser pulse spatially separates the ionized electrons from the heavier ions and the induced dipole moment subsequently oscillates at the plasma frequency and radiates an electromagnetic pulse. 33.80.Wz,42.65.Re,52.40.Db Recent investigations of the propagation of intense femtosecond infrared (IR) laser pulses in air show that the dynamical interaction between nonlinear self-focusing, plasma defocusing, and group-velocity dispersion can cause an initial beam to break up spatially into several filaments, or light strings, with diameters around a hundred microns that can maintain themselves over long distances [1][2][3][4][5]. It has been observed experimentally that femtosecond light strings in turn produce plasma channels by multi-photon ionization (MPI) along their direction of propagation with lengths on the order of centimeters [6][7][8][9][10]. Observations of the electromagnetic pulses (EMPs) from light string induced plasmas suggest that these channels attain dipole moments during the laser pulse which subsequently oscillate at the plasma frequency and radiate [10].In this letter we present a simple model for EMP generation from femtosecond light string induced plasmas to elucidate the underlying physics, in particular how the initial dipole moment is produced in the plasma channel, and the characteristic field strengths and spectrum of the EMP. In this model electrons and ions are initially created by MPI due to the light string, and a dipole moment is subsequently induced in the plasma via the radiation pressure force resulting from the velocity-dependent Lorentz force. For a laser field of frequency ω this force contains a second-harmonic component at 2ω in addition to a low frequency component, and it is this low frequency component of the force that creates the initial dipole moment in the plasma channel by longitudinally separating the light electrons from the heavy ions on the short time scale of the light string. Following the laser pulse the electron-ion system then starts to oscillate longitudinally at the plasma frequency [11] and generates an EMP propagating radially away from the plasma channel [10]. We stress that this mechanism for creating a dipole moment in a plasma is new and distinct from that due to the ponderomotive force from a tightly focused Gaussian beam [12,13]: In particular, the usual ponderomotive force acting on electrons is proportional to the spatial gradient of the applied light intensity [14]. For light strings of diameter 100 µm and a few centimeters length the ponderomotive force should be negligible, and this was our motivation for looking for a new physical process that could create a dipole moment in the plasma channel.In our model we consider a femtosecond laser pulse linearly polarized along the x-axis, and propagating in air along the positive z-axis, with a central frequency ω ...

Femtosecond filamentation in transparent media

“…In this model, the ionization front has frequency components propagating at superluminal velocity, giving rise to a Cerenkov-like emission. Other authors have proposed another model, in which the ponderomotive force is not the main mechanism for THz emission (Cheng et al, 2001;Keskinen et al, 2004). In this model, radiation pressure separate axially electrons from ions after multiphoton ionization, inducing an oriented dipole moment aligned in the direction of the filaments (Proulx et al, 2000).…”

Physics and applications of atmospheric nonlinear optics and filamentation

“…The THz emission was first interpreted as originating from the oscillation of the electrons in the plasma channel, due to a longitudinal charge separation induced by the radiation pressure of the laser pulse [217]. However, the polarization of the THz signal is independent of the laser polarization, instead of being parallel to the driving electric field of the incident laser.…”

Femtosecond Filamentation in Air

“…According to Cheng et al [6], a longitudinal charge separation is initially induced in the plasma channel by the radiation pressure due to the pulse. Electrons then oscillate at the plasma frequency ω 2 p = ρ e e 2 / 0 m, and generate the plasma current which constitute the sources for the EMP.…”

“…This EMP contains frequency components up to the GHz and beyond [6,34,68,77]. e) Filamentation also occurs in other media such as transparents liquids or solids [19,23,78,84].…”