Powerful terahertz waves from long-wavelength infrared laser filaments

Strong terahertz (THz) electric and magnetic transients open up new horizons in science and applications. We review the most promising way of achieving sub-cycle THz pulses with extreme field strengths. During the nonlinear propagation of two-color mid-infrared and far-infrared ultrashort laser pulses, long, and thick plasma strings are produced, where strong photocurrents result in intense THz transients. The corresponding THz electric and magnetic field strengths can potentially reach the gigavolt per centimeter and kilotesla levels, respectively. The intensities of these THz fields enable extreme nonlinear optics and relativistic physics. We offer a comprehensive review, starting from the microscopic physical processes of light-matter interactions with mid-infrared and far-infrared ultrashort laser pulses, the theoretical and numerical advances in the nonlinear propagation of these laser fields, and the most important experimental demonstrations to date. Terahertz light: taking plasma filaments on a power trip An experimental technology that can reduce the cost and complexity of generating powerful terahertz waves could benefit fields such as wireless communication. Stelios Tzortzakis from the IESL-FORTH in Heraklion, Greece, and colleagues review efforts to understand and optimize a process that uses ultrafast laser pulses to ionize gases into thin filament plasma antennas. Careful laser frequency mixing then enables the filaments to emit terahertz radiation, which is broadband and powerful enough to pass through buildings and walls without risk to human health. Theoretical and experimental studies have highlighted the importance of using laser sources with specific wavelengths to enhance the power conversion efficiency of terahertz radiation and enable long-range propagation in air. Improved schemes for controlling frequency mixing are projected to offer order-of-magnitude increases in terahertz power output compared to current approaches.