Fig. 4: The spatio-temporal spectral structure of 3D ST wave packets.

Measurements for a superluminal wave packet (\(\widetilde{v}\,\approx \,1.37c\)) are plotted in the left column, and those for its subluminal counterpart (\(\widetilde{v}\,\approx \,0.83c\)) are plotted in the column on the right. a Measured spatial spectrum \(|\widetilde{\psi }({k}_{x},{k}_{y},\lambda ){|}^{2}\) by a wavelength-insensitive camera showing an annular structure. b Measured temporal spectra at selected radial positions revealing the radial chirp and the spectral uncertainty. c Measured radial chirp by plotting the central wavelength λ_c of the spectrum with radial spatial frequency k_r. Error bars in c represent the spectral resolution of the optical spectrum analyzer (OSA; Advantest AQ6317B) we made use to perform spectral measurements.