DIFFRACTION SOLUTION SUPERLUMINAL FREE
In addition to temporal spectral modulation, it has been recently appreciated that structuring the spatial profile of a pulsed beam can impact its group velocity in free space 14, 15, 16. In general, resonant systems have limited spectral bandwidths that can be exploited before pulse distortion obscures the targeted effect, with the pulse typically undergoing absorption, amplification, or temporal reshaping, but without necessarily affecting the field spatial profile.
Additionally, nanofabrication yields photonic systems that deliver similar control over the group velocity through structural dispersion in photonic crystals 10, metamaterials 11, tunneling junctions 12, and nanophotonic structures 13. Modifying the temporal spectrum in this manner is the basic premise for the development of the so-called slow light and fast light 3 in a variety of material systems including ultracold atoms 4, hot atomic vapors 5, 6, stimulated Brillouin scattering in optical fibers 7, and active gain resonances 8, 9. Indeed, the group velocity v g of an optical pulse in a resonant dispersive medium can deviate significantly from the speed of light in vacuum c, without posing a challenge to relativistic causality when v g > c because the information speed never exceeds c 1, 2. The publication of Einstein’s seminal work on special relativity initiated an investigation of the speed of light in materials featuring strong chromatic dispersion 1.