Fig. 1. (a) Schematic of the generation of long-wavelength IR pulses from a laser-driven nonlinear plasma wake in a density-tailored plasma. (b)–(d) Distributions of the transverse electric field E_y of the modulated laser pulse and the plasma wake density n_e at different positions corresponding respectively to the black dot, blue dot, and red dot in (a). Here, E_y and n_e are normalized to E₀ ≈ 3.2 × 10¹² V/m and n_c ≈ 1.1 × 10²¹ cm⁻³, respectively.

Fig. 2. (a) Density distribution of the designed plasma structure. (b) and (c) Evolutions of the transverse electric field and the radiation wavelength, respectively, of the modulated pulse in a density-tailored plasma as functions of the propagation distance x. (d) Spectral distributions of the initial laser pulse (black dotted line) and the produced IR pulse (blue solid line). The inset is a plot of the temporal waveform of the electric field of the output IR pulse in the long-wavelength part with a central wavelength of λ_c ≈ 20 μm. Here, the electric field E_y of the modulated pulses is normalized to E₀ ≈ 3.2 × 10¹² V/m.

Fig. 3. (a) and (b) Energy conversion efficiency ρ, optical cycle number N, normalized amplitude a, and central wavelength λ of an IR pulse over 10 µm as functions of the length L of the uniform-density part of the stretcher and its density n_e. (c) CEP φ_ir of the long-wavelength IR pulse as a function of the CEP φ₀ of the drive laser pulse. The inset shows the electric field waveform of a λ_c ≈ 20 μm IR pulse for three different CEPs of the initial laser pulse: φ₀ = 0 (blue solid line), π/2 (red dashed line), and π (black dashed line).