Light Pulse at 30 Times the Speed of Light

<a href=””>University of Central Florida researchers Abouraddy and study co-author Esat Kondakci demonstrated</a> they could speed a pulse of light up to 30 times the speed of light, slow it down to half the speed of light, and also make the pulse travel backward.

They used a special device as a spatial light modulator to mix the space and time properties of light, thereby allowing them to control the velocity of the pulse of light. The mixing of the two properties was key to the technique’s success.

“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,” Abouraddy said.

Nature Communications – Optical space-time wave packets having arbitrary group velocities in free space

Abstract-Controlling Light Pulses

Controlling the group velocity of an optical pulse typically requires traversing a material or structure whose dispersion is judiciously crafted. Alternatively, the group velocity can be modified in free space by spatially structuring the beam profile, but the realizable deviation from the speed of light in is small. Here we demonstrate precise and versatile control over the group velocity of a propagation-invariant optical wave packet in free space through sculpting its spectrum. By jointly modulating the spatial and temporal degrees of freedom, arbitrary group velocities are unambiguously observed 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 towards it.

They synthesize space-time (ST) wave packets using a phase-only spatial light modulator (SLM) that efficiently sculpts the field spectrum and modifies the group velocity. 

The ST wave packets are synthesized for simplicity in the form of a light sheet that extends uniformly in one transverse dimension over ~25 mm, such that control over  is exercised in a macroscopic volume of space. They measure  in an interferometric arrangement utilizing a reference pulsed plane wave and confirm precise control over  from 30c in the forward direction to −4c in the backward direction. They observe group delays of ~±30 picoseconds (three orders-of-magnitude larger than those in references.), which is an order-of-magnitude longer than the pulse and is observed over a distance of only ~10 mm. Adding to the uniqueness of their approach, the achievable group velocity is independent of the beam size and of the pulse width. All that is needed to change the group velocity is a reorganization of the spectral correlations underlying the wave packet structure. The novelty of our approach is its reliance on a linear system that utilizes a phase-only Fourier synthesis strategy, which is energy efficient and precisely controllable. Their approach allows for endowing the field with arbitrary, programmable spectral correlations that can be tuned to produce—smoothly and continuously—any desired wave packet group velocity. The versatility and precision of this technique with respect to previous approaches is attested by the unprecedented range of control over the measured group velocity values over the , superluminal, and negative regimes in a single optical configuration. Crucially, while distinct theoretical proposals have been made previously for each range of the group velocity (e.g., subluminal, superluminal, and negative spans), our strategy is—to the best of their knowledge—the only experimental arrangement capable of controlling the group velocity continuously across all these regimes (with no moving parts) simply through the electronic implementation of a phase pattern imparted to a spectrally spread impinging on a SLM.

SOURCES- University of Central Florida, Nature Communications

Written By Brian Wang,