Scientists from the Max Born Institute, in collaboration with foreign cooperation partners, have theoretically elucidated the basic mechanism in the generation of terahertz radiation in gases, provided experimental proof of this, and demonstrated possibilities for controlling the THz spectral parameters. As terahertz radiation (1Terahertz = 1THz = 1012Hz = 1012 oscillations per second) refers to light with an extremely large wavelength of about 0.3 mm. A frequency of 1 THz is about 50 times greater than the frequency at which cell phones work. Terahertz radiation is now widely used in technology, such as wireless data transmission or the analysis of materials. Even the so-called 'nude scanner' at airports uses THz radiation to illuminate objects. In research extremely short duration THz pulses are used to study basic properties of solids and liquids, e.g. the charge transport and the electrical resistance.
Generation of terahertz radiation by ionizing two-color femtosecond pulses in gases
Fig. 1 Mechanism of the THz emission. The two-color laser field E (t) (red) generates free electrons by tunnel ionization with a stepwise density modulation (green) near the maxima of the field amplitude. This leads to the creation of a current (blue dotted), which acts as a source of THz emission. The insertion shows the experimental arrangement.
These studies require the generation of short THz flashes, for which one can exploit the ionization of gases by ultrashort laser pulses. Among the various sources, THz generation using two-color femtosecond pulses in a gas is characterized by high field strengths (down to the MV / cm range) and large spectral widths (above 100 THz). Although discovered in 2000 and now studied and applied in many works, the basic mechanism of their emergence in the literature is still controversial. Initially interpreted by rectification due to non-linearity of third order in gases, later the THz generation was associated with the generation of a plasma current in the two-color laser field. Our theoretical investigation  and the associated experiments at MBI  showed that the THz emission is inseparable from the stepwise increase in plasma density in tunnel ionization near the maxima of the field strength amplitude of the pump pulses, resulting in ionized electrons in one temporal sequence of discrete ionization events in the attosecond region, which leads to the emission of THz pulses. Their spectrum is therefore determined by the interference of the contributions of different ionization events, yielding a notable analogy with the diffraction theory of light on a lattice.
Comprehensive (3+1) -dimensional numerical computer simulations confirmed this model, which is not only an explanation of a number of experimental observations, but also new ways of controlling THz parameters and shaping desired THz spectra by timing ionization events (e.g. B. by a frequency detuning of the two two-color pump pulses) allows . The realization of experiments at the MBI  made it possible to test this new understanding of THz radiation by experimental observations. The measurements of the THz spectra showed a sensitive dependence on the gas pressure in accordance with (3+1) -dimensional simulations, which provided important insights into the basic mechanism of the THz emission and the influence of propagation effects of the pump waves. In this case, a plasma-induced blue shift of the pump pulses plays a key role in the broadening of the THz spectra with increasing gas pressure. On the other hand, this provides experimental confirmation for the mechanism described above, where the THz emission is directly related to the stepwise modulation of the ionization current.