Using a technique that someday could help alleviate data congestion and prevent information loss, University of Central Florida (UCF) researchers are able to speed up and slow down a light pulse in free space. The new technique allows the speed of the pulse to be adjusted without the use of any pass-through material to speed it up or slow it down. The researchers used a phase-only spatial light modulator (SLM) to synthesize space-time (ST) wave packets and control the velocity of the pulse. The SLM was used to efficiently sculpt the field spatio-temporal spectrum and modify the group velocity. In experiments, when the researchers modulated the spatial and temporal degrees of freedom jointly, they observed arbitrary group velocities in free space above or below the speed of light in vacuum, whether in the forward direction propagating away from the source or even traveling backward toward it. “We’re able to control the speed of the pulse by going into the pulse itself and reorganizing its energy such that its space and time degrees of freedom are mixed in with each other,” professor Ayman Abouraddy said. The results could help point the way to schemes for phase matching in nonlinear optical processes and new types of laser-plasma interactions. “This is the first clear demonstration of controlling the speed of a pulse light in free space,” Abouraddy said. “And it opens up doors for many applications, an optical buffer being just one of them, but most importantly it’s done in a simple way that’s repeatable and reliable.” The research was published in Nature Communications (https://doi.org/10.1038/s41467-019-08735-8).