Physicists have taken advantage of the properties of periodic systems to efficiently time-reverse ultrashort electromagnetic pulses for applications in medical ultrasound, optical communications, superlensing, ultrafast plasmonics and biological imaging. Because a time-reversed pulse evolves as if time were running backward, time reversal eliminates any distortions or scattering that has occurred at earlier times, regardless of the medium the pulse has propagated through. Research of this time reversal was conducted at Imperial College London and appeared in the May 10, 2011, issue of Physical Review Letters (doi: 10.1103/PhysRevLett.106. 193902). So far, time reversal has been successfully demonstrated for pulses of a relatively narrow spectrum. However, schemes that enable time reversal of broadband pulses have required complicated techniques, making them difficult to implement and yielding low efficiencies. Paving the way for efficient reversal of truly few-cycle pulses with devices that are easy to fabricate and implement, the new scheme is based on dynamically tuning the wave speed in photonic crystals that contain “zero gaps” – bandgaps with a zero width. When a bandgap has zero width, instead of being reflected, an incident pulse can propagate through almost perfectly. This zero-gap structure has two features, the researchers said. First, it serves as an effective homogeneous medium, admitting all incident light. Second, the existence of two bands in proximity allows for transfer of energy between them using relatively slow modulations. A prime example of a perfectly symmetric zero-gap system is a 1-D photonic crystal with two layers, each exactly λ/4 wide. These structures are frequently used for antireflection coatings. The researchers tuned the wave velocity in the medium in real time and determined that the short pulses propagating inside the structure could be reversed with up to 100 percent efficiency. With possible applications extending beyond optical implementations, including quantum systems, the researchers are hopeful that they can extend and improve this method of time-reversed electromagnetic pulses.