Solitonlike Electromagnetic Waves behind a Superintense Laser Pulse in a Plasma

We present strong evidence, based on 2(1/2)D particle-in-cell simulations of the interaction of ultrashort, high-intensity laser pulses with underdense plasmas, of the formation of long-lived, slowly moving $(0.1c)$, low-frequency solitonlike electromagnetic waves. These nonlinear waves consist of electron-density depressions and intense cylindrical electromagnetic field concentrations with a larger amplitude and a lower frequency than those of the laser pulse.