Fig. 4: Comparison of measured and simulated Wigner spectrograms of generated IR pulses.

a, b Wigner spectrograms of measured IR pulses when the blade is at 400 and 525 μm, respectively. The dashed lines represent the upper limit of the measurable IR frequency range due to the optics used to transport the LWIR radiation to the XFROG device. The IR components above 200 THz (λ < 1.5 μm) are filtered out to remove strong residual drive pulse. c, d Simulated Wigner spectrograms of one snapshot of the evolved on-axis laser field and the negative refractive index gradient −∂η/∂ζ (orange line) in the plasma downramp (the OC section) for the cases in (a) and (b), respectively. The local frequency of the photons is downshifted (upshifted) when this negative gradient is negative (positive). e, f The corresponding snapshot of plasma density (wake) and the laser electric field for the cases in (c) and (d), respectively. Strong photon frequency down-shifting occurs at the front of the self-compressed pulse generating a LWIR pulse that recedes by GVD into the first wake cavity (c) and partially leaks into the second wake cavity in the case of (d).