LAUSR

DOI: 10.1002/adom.201900681 reason for this interest relies on the fact that THz radiation can couple resonantly to numerous fundamental motions of ions, electrons, and electron spins in all phases of matter. For example, in solids, the THz range overlaps with the frequency of lattice vibrations (phonons), the collision rates of conduction electrons, the binding energy of bound electron–hole pairs (excitons), and the precession frequency of spin waves (magnons). Consequently, THz radiation, both continuous-wave and pulsed, has been used for characterization of and gaining insight into elementary processes in complex materials. The majority of these studies used relatively weak THz fields and, thus, probed the linear response of the material, without inducing notable material modifications. Only recently, however, completely new avenues in THz science were opened up by triggering nonlinear THz responses of materials.[3–13] Instead of using weak fields to primarily observe selected THz modes such as phonons or magnons, strong fields allow one to actively drive them to unprecedentedly large amplitudes, potentially thereby resulting in novel states of matter.[6,8,11] For example, simulations suggest that exciting matter with intense THz transients may lead to massive modifications of electrically[14] or magnetically[15] ordered domains and enable the acceleration of free ions to ≈1 MeV,[16] and postacceleration to 50–100 MeV energies.[17] Remarkable experimental results such as switching of magnetic order,[18,19] parametric amplification of optical phonons,[20] novel insights into spin-lattice coupling,[21,22] and acceleration of free electrons in a THz linear accelerator[23] were achieved only recently. This progress has been made possible by the development of laser-driven table-top THz sources routinely providing pulses with unprecedented energies and peak electric and magnetic field strengths throughout the entire THz spectral range. Different laser-based THz pulse generation techniques can be used to access different parts of the spectral range extending from 0.1 to 10 THz. Some of the recently developed technologies enable the generation of radiation with even larger bandwidth or tuning range up to 100 THz and beyond, which lead to an extension of what is called the THz spectral range. An overview of the approximate spectral coverage and the achieved highest pulse energies and peak electric-field strengths of various laserdriven technologies is given in Figure 1.