Imaging the temporal profile of structured optical modes

Spatially structured optical modes exhibit a group velocity lower than c, resulting in a measurable temporal delay with respect to plane waves. Here, we develop a technique to image this temporal delay and measure it across a set of optical modes. An inevitable consequence of spatially varying delay is temporal broadening of the mode. As such, for a focused Gaussian, we observe an ≈ 1 % increase in the temporal profile, corresponding to a narrowing of the optical spectrum by ≈ 0.03 nm. This work shows that imaging is essential to fully understanding the changes to the group velocity for structured modes.