Fig. 2: Reconstructed exit field of the laser-plasma amplifier.

The complex-valued retrieved exit field of the laser-plasma amplifier is pictured in the inset. Here, the hue and brightness represent the phase and intensity, respectively. The radial profile of the intensity shows a Gaussian-like profile with a dip in the centre. Simulations indicate that an overionized zone in the laser-plasma amplifier leads to decreased amplification in the centre of the channel. The phase profile shows a parabolic shape caused by the radially decreasing refraction index. Note: the high standard deviation of the phase above a 40 µm radius arises from the low intensity and the corresponding random phases during the reconstruction. The diameter of the exit field of 52 ± 5 µm (FWHM) is marked with black dashed vertical lines, where the grey bar represents the error