Fig. 1. Diagrams of (a) FRS and (b) FWM processes.

Fig. 2. (a) Tempo-spectral diagram of a short (T = 2π/ω_p) laser pulse after propagation in plasma for 250 plasma wavelengths. The initial pulse includes components a49=a50=0.4e−(ζ/T)2. (b) Tempo-spectral diagram of the output of a long (T = 40π/ω_p) laser pulse after propagation in plasma for 800 plasma wavelengths. The initial pulse includes components a49=a50=0.15e−(ζ/T)2. Note the difference in the vertical axis scales. The top panels show the spectral histograms for the whole pulse. The side panels show the temporal envelopes of the initial pulse (red curves) and the final pulse (gray shades). Their full-size envelope plots are shown in Figs. 5 and 6, respectively.

Fig. 3. Plasma wave amplitude at various locations ζ and pulse propagation distances τ. The parameters are the same as in Fig. 2(b).

Fig. 4. Amplitudes of virtual phonons |χ_k| at the pulse peak ζ/(cω_p) = 1. The parameters are the same as in Fig. 2(a).

Fig. 5. (a) Input (red curve) and output (blue shade) pulse envelopes of a short laser pulse T = 2π/ω_p. (b) Contour plot of the time–frequency synchrosqueezed transform. The parameters are the same as in Fig. 2(a).

Fig. 6. (a) Input (red curve) and output (blue shade) pulse envelopes of a short laser pulse T = 40π/ω_p. (b) Contour plot of the time–frequency synchrosqueezed transform. The parameters are the same as in Fig. 2(b).

Fig. 7. PIC simulation results for a supercontinuum [(a) and (c)] generated from a short laser pulse in the FWM-dominated regime, and a frequency comb [(b) and (d)] generated from a long laser pulse in the FRS-dominated regime. In (a) and (b), the red curves show the initial pulse envelope, and the blue shade shows the output signal. (c) and (d) show the power spectral density (PSD) of the corresponding output signals.