Mathematical Modeling and Experimental Validation of Ultrafast Nonlinear Light-Matter Coupling associated with Filamentation in Transparent Media

A Coherent Short Pulse X-Ray Source

Team members from JILA in collaboration with a team at the University of Vienna have generated bright high-harmonic x-ray supercontinuam with photon energies spanning the EUV to 1.6 keV (<7.7 Å) by focusing 3.9-mm wavelength pulses from a tabletop femtosecond laser into a waveguide filled with He gas. This represents an extreme >5000-order nonlinear process while also demonstrating fully phase-matched frequency upconversion. They discovered that the multi-atmosphere pressures necessary for efficient x-ray generation also supports laser beam filamentation, enhancing the x-ray yield by another order of magnitude. They observe coherent, laser-like x-ray beams, despite the fact that ultrahigh harmonic generation occurs in a regime where the laser-driven electrons encounter many neighboring atoms before they re-encounter their parent ions. Preliminary calculations indicate that the kilo-electron volt-bandwidth coherent supercontinuum has a well-behaved chirp that, when compensated, could support a single-x-ray-cycle 2.5-attosecond pulse duration.

From Single Particle Photoionization to an Isotropic Plasma State

Recent breakthroughs in velocity imaging techniques make it possible to capture very intense field photo-ionized electron/ion angular distributions with femtosecond resolution. The resultant angular momentum distributions display a sensitive dependence on the exciting optical carrier frequency/wavelength and carrier-envelope phase.

The team has developed a hierarchy of full 3D quantum simulation models of localized intense few cycle pulse photo-ionization of atoms/molecules with the goal of quantitatively verifying these experimental observations and also of providing first principles light-matter coupling sources for propagation studies. These single particle angularly (momentum) resolved anisotropic photo-ionized electron distributions have been used as initial data for the first full 3D quantum Boltzmann study of the evolution of nonequilibrium electron/ion distributions towards a quasi-equilibrium isotropic plasma state. The complexity of this 3D many-body problem made it necessary to decouple the initial photo-ionized electron/ion distribution creation from the Coulomb mediated electron/electron and electron/ion scattering problem.

Remote Delivery of Conical Wave Beams

Another important accomplishment of the project has been the initial theoretical demonstration that apodized Bessel, Vortex or other beams can be reconstructed remotely at km range by using an optimization algorithm based on an extension of the Gerchberg-Saxton Fourier integral relation. The essence of this algorithm is to assume a magnified far-field ring-like amplitude profile near the launch point and run an iterative mapping where the phase profile is allowed to vary such as to yield the desired solution at target. Initial results indicate that one can stipulate an on-axis uniform extended intensity profile at the target and obtain a convergent solution at km range. For example, it has been explicitly shown that a Bessel beam at source can be imaged at 1km to give a near uniform on-axis intensity extending over 200m. Extension of this theory study to include an extended ground level turbulent path and explore possible adaptive optics solutions is underway.

AFRL Transitions

At Kirtland AFB, the MURI team is designing and testing custom hardware for generating TW-class pulses with robust transverse profiles. A fused silica phase mask to generate vortex transverse profiles has been delivered to AFRL for field testing at multi-km range. The MURI effort will subsequently transition the results of developed remote delivery methodologies following turbulence mitigation studies demonstrated at laboratory scale. Working with AFRL at Kirtland, the MURI team will explore pre-chirp to extend plasma propagation distances. Personnel at AFRL/Kirtland are interested in coupling radiofrequency and microwave signals to plasma channels to provide low probability of intercept communication capability and remote surface interrogation capabilities. Additionally, the plasma filament is a conductive path, and as such can assist in the highly local delivery of electromagnetic energy for the purpose of interacting with a target either for benign communication or destructive illumination.

2012 - JILA, University of Colorado: Sizzling Vibrations

Former research associate Antonio Picon, research associate Agnieszka Jaron-Becker, and Fellow Andreas Becker have discovered a way to make the hydrogen molecular ion (H2+) fall apart into its constituent atoms without exciting or ionizing the electron.

November 2012: JILA, University of Colorado

JILA University of Colorado student, Carlos Hernandez-Garcia, awarded "OSA Best Student Paper Award"; for MURI referenced paper "Temporal structure of ultra high-order harmonic generation in the keV regime driven by mid-infrared lasers". For more information please see page 4 of http://www.osa-opn.org/opn/media/Images/PDF/2012/1112/48-52-Backscatter-Nov.pdf?ext=.pdf

October 7 - 12, 2012: COFIL

The 4-th International Symposium on Filamentation(COFIL 2012), hosted in Tucson from Oct 7-12, 2012 was partially supported through the MURI program and was chaired by the PI of the MURI. This symposium, held biannually since 2006, was the first to be held in the U.S. and brought together world experts on filamentation science from North America, Europe, and Asia. Prior meetings were held in Quebec, Canada (2006), Paris, France (2008) and Crete, Greece (2010). The MURI was well represented through a plenary talk, 10 invited talks and 8 poster presentations.