A major frontier in strong field laser physics and nonlinear optics is the interaction of powerful terahertz (THz) pulses with matter. A plethora of scientific challenges and applications are presently under study, like table-top electron acceleration, THz-enhanced attosecond pulse generation and strong electric and magnetic THz field interactions with matter. Nevertheless, despite the rapid development of THz science during the last two decades, the majority of available table-top THz sources remain rather weak limiting the interactions of THz radiation with matter mostly in the realm of linear optics.

In a significant breakthrough, an international team of physicists from the Ultrashort Nonlinear laser Interactions and Sources (UNIS) laboratory at the Institute of Electronic Structure and Laser (IESL) of the Foundation for Research and Technology-Hellas (FORTH) in Crete, led by Prof. Stelios Tzortzakis, and from the Ultrafast Laser Group at TU Wien in Vienna, led by Prof. Andrius Baltuška, have developed a novel THz source that exceeds in power performance and conversion efficiency any other THz source known to date.

Currently, the most powerful table-top THz sources are based on the interaction of ultrashort laser pulses with electro-optic crystals through optical rectification and with gases and liquids through two-color filamentation. The first one presents relatively high laser to THz conversion efficiencies reaching values of the order of the percent, but the damage threshold of the material limits the maximum laser energy that one can deposit on the crystal preventing a significant increase of the output characteristics. In turn, two-color filamentation of near-infrared laser pulses offers ultrashort THz pulses (tens of femtoseconds) with a corresponding spectral bandwidth exceeding 50 THz while it is not restricted by any damage threshold since the gas or liquid media recover in-between laser shots.

The breakthrough of the scientific team at FORTH-IESL and TU Wien was the use of intense ultrashort mid-infrared laser pulses to drive laser beam filamentation in ambient air. The researchers report, in their Nature Communications paper, generation of sub-millijoule single-cycle THz pulses with unprecedented THz conversion efficiency of a few percent, exceeding by far any previously reported experimental values for plasma-based THz sources. Moreover, due to the large bandwidth of the generated THz radiation (~ 20 THz) the peak THz electric and magnetic fields exceed the 100 MV/cm and 33 Tesla, respectively.

Based on the reported experimental findings and theoretical estimates, it is projected that soon multi-millijoule THz pulses with peak electric and magnetic fields in the gigavolt per centimeter and kilotesla level, respectively, will become available. Quasi-static ultrashort electric and magnetic bursts at these intensities will enable extreme nonlinear and relativistic science.

The paper is published in Nature Communications: 10.1038/s41467-019-14206-x