K. Schuh

Self-Channeling of High-Power Long-Wave Infrared Pulses in Atomic Gases

We simulate and elucidate the self-channeling of high-power 10  μm infrared pulses in atomic gases. The major new result is that the peak intensity can remain remarkably stable over many Rayleigh ranges. This arises from the balance between the self-focusing, diffraction, and defocusing caused by the excitation induced dephasing due to many-body Coulomb effects that enhance the low-intensity plasma densities. This new paradigm removes the Rayleigh range limit for sources in the 8-12  μm atmospheric transmission window and enables transport of individual multi-TW pulses over multiple kilometer ranges.

Simulations of 10 μm filaments in a realistically modeled atmosphere

We numerically study the formation and propagation dynamics of 10 μm filaments in the atmosphere. We investigate filament formation of multi-Joule 100 fs and 1 ps duration pulses over propagation lengths of 100 m and find that the carrier self-steepening regularization mechanism predicted for 4 μm wavelengths still holds. In our study we include multiple physical effects ignored in the past, such as rotational Raman and avalanche ionization. In addition we incorporate in our simulations the most detailed dispersive model available for atmospheric air, which includes multiple species, such as O2, N2, Ar, CO2, CO, and CH4, for a variety of humidity levels. Results presented here are expected to have a significant impact in the wider field of nonlinear optics where the use of mid-infrared lasers is rapidly growing.

Interaction-induced nonlinear refractive-index reduction of gases in the midinfrared regime.

The nonlinear optical response of a dilute atomic gas to ultrashort high-intensity midinfrared pulse excitation is calculated fully microscopically. The optically induced polarization dynamics is evaluated for the interacting many-electron system in a gas of hydrogen atoms. It is shown that the many-body effects during the excitation distinctly influence not only the atomic ionization dynamics, but also the nonlinear polarization response in the midinfrared regime. The delicate balance between the Kerr focusing and the ionization-induced defocusing is dramatically modified and a significant decrease of the nonlinear refractive index is predicted for increasing wavelength of the exciting pulse.

Similarities and differences between high-harmonic generation in atoms and solids [Invited]

The basic features of high-harmonic generation in dilute atomic gases and solid-state systems are analyzed using a microscopic many-body theory. Whereas it is sufficient for atomic gases to include the ground state and the ionization continuum, the semiconductor approach has to deal with the relevant interband polarizations and intraband currents. For both systems, a closed set of Bloch-type equations is derived, which has to be solved numerically. As common features, the emission of odd-order harmonics, the development of a plateau, and a high-frequency cutoff are observed if the solid system is modeled as a two-band system. However, already the addition of a third coupled band effectively breaks the inversion symmetry and leads to the appearance of even harmonic orders with comparable strength. In all cases, frequency- and time-resolved studies reveal a fundamentally different timing in the emission dynamics of the individual frequency components in atomic and solid-state systems, which can be attributed to the presence of energy bands in solids.

Influence of optical and interaction-induced dephasing effects on the short-pulse ionization of atomic gases

The ionization dynamics of dilute atomic gases induced by ultra-short high-intensity optical-pulse excitation is treated fully microscopically. The optical excitation is self-consistently coupled to the many-body interactions of the electrons, including their interactions with free electrons, ions, and neutral atoms. The theory is numerically evaluated for the example of a gas of hydrogen atoms for a broad range of pulses covering the tunnel ionization, multi-photon ionization, as well as the one-photon ionization regimes. It is shown that the many-body effects during the excitation distinctly influence the atomic ionization dynamics. The ionization degree after the pulse is dominated by the dephasing caused by the excitation-dependent interplay between the purely optical processes and the many-body interactions.

Simple model for the nonlinear optical response of gases in the transparency region.

We present a simple model for the nonlinear optical response of atomic gases for pulses with center wavelengths in the transparency region and peak fields for which ionization is not prevalent. By comparing with simulations based on the Schrödinger equation for a hydrogen atom we demonstrate that the model accurately captures the dispersion of the nonlinear polarization as well as noninstantaneous effects for a variety of photon energies and also a two-color pulse. Our approach should be of utility in simulating near- and mid-infrared pulse propagation in dielectric media for which extreme nonlinear effects can arise.

Simulation of LWIR TW ultrashort pulses over kilometer ranges in the atmosphere

We have identified major paradigm shifts relative to near-IR filamentation when high power multiple terawatt laser pulses are propagated at mid-IR and long-IR wavelengths within key atmospheric transmission windows. Individual filaments at near-IR (800 nm) wavelengths typically persist only over tens of centimeters, despite the whole beam supporting them being sustained over about a Rayleigh range. In the important mid-IR atmospheric window (3.2 - 4 μm) optical carrier wave self-steepening (carrier shocks) tend to dominate and modify the onset of long range filaments. These shocks generate bursts of higher harmonic dispersive waves that constrain the intensity growth of the filament to well below the traditional ionization limit, making long range low loss propagation possible. For long wavelength pulses in the 8-12 μm atmospheric transmission window, many-electron dephasing collisions from separate gas species act to dynamically suppress the traditional Kerr self-focusing lens and leads to a new type of whole beam self-trapping over multiple Rayleigh ranges. This prediction is key, since strong linear diffraction at these wavelengths are the major limitation and normally requires large launch beam apertures. We will present simulation results that predict multiple Rayleigh range propagation paths for whole beam self-trapping and will also discuss some recent efforts to extend the HITRAN linear atmospheric transmission/refractive index database to include nonlinear responses of important atmospheric molecular constituents.

Long range robust multi-terawatt MWIR and LWIR atmospheric light bullets

There is a strong push worldwide to develop multi-Joule femtosecond duration laser pulses at wavelengths around 3.5-4 and 9-11μm within important atmospheric transmission windows. We have shown that pulses with a 4 μm central wavelength are capable of delivering multi-TW powers at km range. This is in stark contrast to pulses at near-IR wavelengths which break up into hundreds of filaments with each carrying around 5 GW of power per filament over meter distances. We will show that nonlinear envelope propagators fail to capture the true physics. Instead a new optical carrier shock singularity emerges that can act to limit peak intensities below the ionization threshold leading to low loss long range propagation. At LWIR wavelengths many-body correlations of weakly-ionized electrons further suppress the Kerr focusing nonlinearity around 10μm and enable whole beam self-trapping without filaments.

Ultrafast light-matter coupling in condensed and gaseous nonlinear media

After a brief historical review, we describe recent research in the study of tera-Watt class femtosecond lasers propagating in air and condensed media. Here critical self-focusing of the light field reflects the presence of a famous singularity (blow-up in finite time) in the governing Nonlinear Schrö dinger equation (NLS) — this contribution deals with moving into a regime where NLSE fails and more exact optical carrier resolved pulse propagators need to be developed and secondly, addresses the failure of well-established phenomenological nonlinear optical susceptibilities and their replacement by more fundamental quantum models.

Fully Microscopic Studies of Strong-Field Atom Ionization

The interaction of a highly off-resonant light pulse with an atomic gas is modeled fully microscopically. The resulting equations are solved numerically for an atomic model system excited by a strong light pulse.