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Plasma density oscillations (Langmuir waves) in the wake of an intense (Ipeak-3 x 1017W/cm2) laser pulse (100 fs) are measured with ultrafast time resolution using a longitudinal interferometric technique. Phase shifts consistent with large amplitude (Sne/ne-1) density waves at the electron plasma frequency were observed in a fully tunnel-ionized He plasma, corresponding to longitudinal electric fields of-10 GV/m. Strong radial ponderomotive forces enhance the density oscillations. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, , manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the *

It is found that dissipative types of stable soliton structures can exist in nonlinear optical media with broadband gain and group-velocity dispersion (GVD). These structures resemble ionization or combustion waves and are essentially self-accelerating pulses with a stationary-envelope form and a permanently shifting wave spectrum. Contrary to the conservative solitons, the dissipative ones exist for any sign of GVD. Being an attractor in the development of arbitrary initial distributions, the dissipative structures cause the fundamental Schrodinger solitons to disappear in the course of evolution in weakly nonconservative systems. PACS number(s): 42.50.Rh, 42.81. Dp, 42.65.ReRecent progress in developing ultrashort-laser techniques attracts the interest of studying interaction between powerful optical pulses and broadband active media. This problem is of particular importance for the dynatnics of femtosecond laser generators [1], amplifiers of supershort pulses [2], and nonlinear active optical fibers [3]. A common theoretical aspect in these applications is the long-term evolution of wave packets in dispersive active media that exhibits simultaneously both conservative and dissipative nonlinearities.It is we11 known that the propagation of wave pulses in a lossless and dispersive dielectric medium with its self-action incorporated is described by the nonlinear Schrodinger (NLS) equation, which normally includes the conservative cubic nonlinearity of the refractive index. The interplay between nonlinearity and group-velocity dispersion (GVD) then may lead to soliton generation [4] or quasishockwave formation [5]. On the other hand, in two-level optical media solitons of induced transparency can be formed due to the nonconservative character of the resonant light-matter interaction [6]. Specific nonconservative mechanisms of soliton generation, caused by a delicate balance between saturable absorption and amplification and/or transverse phase modulation and gain inhomogeneity, play an important role in the passively modelocked short-pulse laser operation [1,7,8].The purpose of the present paper is to show that the long-term evolution of a wave pulse propagating in a nonlinear two-level optical medium containing broadband gain elements results in the formation of new types of single soliton structures (named here dissipative opti-

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