Demonstration of a low-frequency three-dimensional terahertz bullet with extreme brightness

The brightness of a light source defines its applicability to nonlinear phenomena in science. Bright low-frequency terahertz (<5 THz) radiation confined to a diffraction-limited spot size is a present hurdle because of the broad bandwidth and long wavelengths associated with terahertz (THz) pulses and because of the lack of THz wavefront correctors. Here using a present-technology system, we employ a wavefront manipulation concept with focusing optimization leading to spatio-temporal confinement of THz energy at its physical limits to the least possible three-dimensional light bullet volume of wavelength-cubic. Our scheme relies on finding the optimum settings of pump wavefront curvature and post generation beam divergence. This leads to a regime of extremely bright PW m−2 level THz radiation with peak fields up to 8.3 GV m−1 and 27.7 T surpassing by far any other system. The presented results are foreseen to have a great impact on nonlinear THz applications in different science disciplines. Bright terahertz sources confined to diffraction-limited spot sizes are needed for nonlinear applications but focusing in this regime is challenging. Here, Shalaby and Hauri use terahertz wavefront manipulation combined with focusing optimization to achieve three-dimensional terahertz bullets.