Terahertz wavefront control for extremely bright THz bullet

The brightness of a light source defines its applicability to nonlinear phenomena in science. The SwissFEL laser group has now overcome one of the two principal technological hurdles to produce bright pulses in the Terahertz range (0.1-5 THz). Using a present-technology THz generation scheme based on optical rectification in organic crystal we were able to optimize the THz wavefront such that a spatio-temporal confinement of THz energy at its physical limits has become feasible – the least possible three-dimensional light volume of wavelength cubic. The gain of control on the THz wavefront is a technological breakthrough as it allows to reach the highest ever produced single-cycle Terahertz transients (up to 80 MV/cm, 25 Tesla) in a diffraction-limited spot size. While such field strength is of interest for many applications, the THz spot size might be too small for realistic pump-probe investigations. Therefore further developments are required to overcome the second hurdle being the increase of THz pulse energy. The SwissFEL laser team has proposed a novel approach which shall be explored in near future. The combination of both wavefront control and high THz pulse energy will open a new avenue in nonlinear THz optics and will be a unique tool for controlling properties in condensed matter by impulsive excitation.