Fig. 5: Experimental time delay of group-velocity-tailored STWPs measured at different distances.

a Frequency-β relationship of an STWP with ΔMG = –1 corresponding to a subluminal Vg. The correlation result shows a 1-ps time delay in 250 μm. b Frequency-β relationship of an STWP with ΔMG = +1 corresponding to a superluminal value Vg. The correlation result shows a 0.1-ps time delay in 250 μm. c Frequency-β relationship of an STWP with ΔMG = +2 corresponding to a negative value Vg. The correlation result shows a –0.7-ps time delay in 250 μm.