This award will support continued work based on the recent discovery that coherent rotational wavepackets in oxygen and nitrogen molecules in air can exert surprisingly strong control over high power femtosecond laser filamentation. The wavepackets manifest themselves in air as refractive index lenslets that propagate as quantum echo wakes following the filamenting laser pulse at its group velocity. A secondary pulse injected into these wakes, depending on the injection delay, can either be trapped and steered or scattered out of the beam axis. These results promise to allow much higher filament laser power, greatly extended filament lengths, increased electron density, and enhanced filament continuity, all essential for applications of femtosecond filaments, including terahertz generation, remote ionization, nonlinear light generation and atmospheric monitoring, and channels for electrical discharges. The proposed experiments represent a novel direction in plasma physics of some intellectual merit: the interaction of quantum coherent ensembles with plasmas. The award will also support another set of experiments in laser-nanoparticle interactions. The goal is to control the nanoplasma dynamics with the strong laser ponderomotive force, the high field analogue of inertial confinement. This highly interdisciplinary experimental and theoretical work provides an excellent training experience for graduate and undergraduate students. The physical ideas underlying the experiments, and the diagnostic techniques developed to measure ultrafast processes under extreme conditions, have a broad science and technology impact, from national defense to industry.
In this project, we measured literally all of the fundamental parameters that underlie the extreme nonlinear optics phenomenon of ‘femtosecond filamentation’ in gases. Femtosecond filamentation occurs when a sufficiently intense short pulse propagating in a medium generates its own lens and self-focuses, whereupon the medium’s atoms/molecules are ionized by the intense focused laser field. The resulting plasma acts as a negative lens, causes the beam to refract outwards so that the self-focusing/plasma defocusing cycle can begin again. This dynamic interplay between self-focusing and defocusing operates over every time slice of the beam, giving rise to an intense optical core that propagates over distances much longer than the normal diffraction length of the beam. Understanding the nonlinearities behind filamentation is an important, if not the most important consideration, from both the filamentation and fundamental perspectives. Even though femtosecond filamentation has been a very active field since the mid-1990s, neither the atomic/molecular nonlinearity leading to the self-lens, nor the ultrafast ionization process leading to the negative lens, had ever been measured. A French/Swiss collaboration even claimed that plasma was unnecessary for negative lens formation. In this grant we definitively measured the atomic and molecular nonlinearities as well as the plasma generation using home-invented versions of spectral and spatial interferometry, in the process providing new understanding of extreme nonlinear optics and settling any debate about the mechanisms of filament generation. In addition, we used our fundamental understanding of extreme laser field nonlinearities to perform experiments optimizing filamentation by controlling those nonlinearities with sequences of laser pulses.
