THz Radiation from Subwavelength Volumes on Water\nSurfaces and in Air: Effects of Mass Density Gradients and External\nMagnetic Fields

High-intensity\n∼10 PW/cm<sup>2</sup> laser pulses with a\n35 fs pulse duration are tightly focused into ∼ 8 μm\nin diameter focal spots to induce ionization and dielectric breakdown\nof 1) water microfilms and 2) air in the focal region and generate\nX-ray and THz waves. At the used focusing, there was no filamentation,\nand energy deposition took place within the geometrical confinement\nof the focal volume in air and on the water surface in the case of\nwater films, and we explored very different mass-density conditions\nof medium for THz generation. The shockwave-induced mass-density gradients\nin air and at the front side of the water surface enhanced the X-ray\nand THz radiation when a prepulse (one or two) was time and space\nsynchronized with the main pulse. At the optimized separation in space\nand the corresponding timing of the main pulse, X-ray and THz emissions\nwere observed to be orders of magnitude stronger. Visualization of\nthe main pulse interaction with the prepulse-triggered shockwave front\nwas optically resolved and its handedness was measured in transmission\nand reflection from water targets. The mechanisms of X-ray and THz\nradiation are explained, and the effect of an externally applied magnetic\nfield in different orientations is tested. It was observed that magnetic\nfields up to 0.6 T had only a minor effect on current transients,\nwhich were responsible for THz emission from the water microfilm target.