Multiharmonic Spanning Nonlinear X-wave: An Asymptotic State of Extreme Long Wave Infrared Dispersive Shock Regularization.

We predict the emergence of novel X-waves emitted as a consequence of extreme dispersive shock regularization of an intense long wave few cycle pulse propagating through a weakly dispersive medium. This robust propagation-invariant solution to Maxwell's equations appears as the asymptotic state in the high harmonic conversion when the pump propagates in a strongly nonlinear weakly dispersive regime, while the weakly nonlinear conical emission is dominated by chromatic dispersion.

Assessment of tight-binding models for high-harmonic generation in zinc blende materials.

Using a simulator for semiconductor Bloch equations (SBEs) accounting for the entire Brillouin zone, we examine the tight-binding (TB) description for zinc blende structure as a model for high-harmonic generation (HHG). We demonstrate that TB models of GaAs and ZnSe exhibit second-order nonlinear coefficients that compare favorably with measurements. For the higher-order portion of the spectrum, we use the results published by Xia et al. in Opt. Express26, 29393 (2018)10.1364/OE.26.029393 and show that the HHG spectra measured in reflection can be closely reproduced by our simulations free of adjustable parameters. We conclude that despite their relative simplicity, the TB models of GaAs and ZnSe represent useful tools to study both the low- and higher-order harmonic response in realistic simulations.

Nonlinear localization of high energy long wave laser pulses in fully correlated 3D turbulence.

We study the interplay between three-dimensional (3D) fully correlated optical turbulence and nonlinearity in time and 3D space resolved long-wavelength infrared pulsed beam propagation. Here the average self-trapped beam waist exceeds the inner scale in contrast to near-infrared filaments, and we find that their nonlinear self-channeling remains robust even in the presence of strong turbulence. More surprisingly, our simulation results invite a conjecture that in regimes where diffraction and nonlinearity are roughly balanced, turbulence can result in a tighter localization of the nonlinear beam core.

Generation of long homogeneous plasma channels with high power long-wave IR pulsed Bessel beams.

Long-wave multi-joule ultrashort laser pulses are predicted to confine highly uniform electromagnetic energy and field intensities while sustaining high density uniform plasmas within nonlinear Bessel zones under extreme driving conditions in contrast to near-IR sources. This opens up novel applications in laser wakefield generation, radiofrequency/microwave guiding, and lightning control.

Random quasi-phase-matching in polycrystalline media and its effects on pulse coherence properties.

Polycrystalline materials can mediate efficient frequency up-conversion for mid-infrared light. Motivated by the need to understand the properties of the harmonic and supercontinuum radiation from such media, we utilize realistic numerical simulations to reveal its complex temporal and spatial structure. We show that the generated radiation propagates in the form of long-duration pulse trains that can be difficult to compress and that optical filamentation in high-energy pulses gives rise to fine-structured beam profiles. We identify trends concerning pulse energy, sample length, and the microstructure of the material that can inform optimization for different applications.

Nonlinear polarization and ionization in O2: metastable electronic state model.

We present a computational model for the nonlinear response of molecular oxygen exposed to strong mid-wavelength and long-wavelength infrared optical fields. Based on a non-Hermitian approach utilizing metastable electronic states, the nonlinear polarization and strong-field ionization are described as intimately connected properties. Good agreement with the measured nonlinear index and ionization rates is shown, and parameterized response functions are provided to facilitate applications in large-scale simulations of infrared optical pulses interacting with gaseous media.

Propagation Induced Dephasing in Semiconductor High-Harmonic Generation.

The influence of propagation on the nonperturbative high-harmonic features in long-wavelength strong pulse excited semiconductors is studied using a fully microscopic approach. For sample lengths exceeding the wavelength of the exciting light, it is shown that the propagation effectively acts as a very strong additional dephasing that reduces the relative height of the emission plateau up to six orders of magnitude. This propagation induced dephasing clarifies the need to use extremely short polarization decay times for the quantitative analysis of experimental observations.

In-line Spectral Interferometry in Shortwave-Infrared Laser Filaments in Air.

We investigate the nonlinear propagation of intense, two-cycle, carrier-envelope phase (CEP) stable laser pulses at 1.7 µm center wavelength in air. We observe CEP-dependent spectral interference in the visible part of the forward-propagating white light generated on propagation. The effect is robust against large fluctuations of the input pulse energy. This robustness is enabled by rigid clamping of both the peak optical field and the phase of the propagating waveform, which has been revealed by numerical simulations. The CEP locking can enhance the yield of the CEP-dependent strong-field processes in gaseous media with long-wavelength drivers, while the observed spectral interference enables single-shot, stand-off CEP metrology in the atmosphere.

Two-stage filamentation of 10 µm pulses as a broadband infrared backlighter in the atmosphere.

We identify a two-stage filamentation regime for high-power 10 µm multipicosecond pulses propagating in the atmosphere. The first low-intensity stage is mainly regularized by ionization through excitation induced dephasing, which can lead to strong pulse shortening downstream. This shortening in turn causes a significant reduction of the many-body induced plasma, which changes the dynamics drastically. As a result, a distinct second stage is predicted where peak intensities are clamped at 1 order of magnitude higher than in the first stage. The complex dynamics found in the second stage can result in the spatial and temporal breakup of the wavepacket, reduction of ionization losses, and extreme spectral broadening.

Ultrafast mid-infrared high harmonic and supercontinuum generation with n2 characterization in zinc selenide.

Polycrystalline ZnSe is an exciting source of broadband supercontinuum and high-harmonic generation via random quasi phase matching, exhibiting broad transparency in the mid-infrared (0.5-20 µm). In this work, the effects of wavelength, pulse power, intensity, propagation length, and crystallinity on supercontinuum and high harmonic generation are investigated experimentally using ultrafast mid-infrared pulses. Observed harmonic conversion efficiency scales linearly in propagation length, reaching as high as 36%. For the first time to our knowledge, n2 is measured for mid-infrared wavelengths in ZnSe: n2(λ=3.9 µm)=(1.2±0.3)×10-14 cm2/W. Measured n2 is applied to simulations modeling high-harmonic generation in polycrystalline ZnSe as an effective medium.

Multi-terawatt 10 µm pulse atmospheric delivery over multiple Rayleigh ranges.

We predict that long wavelength self-trapped multi-terawatt pulses can be sustained over multiple kilometers in the atmosphere. Unlike filaments, these pulses exhibit low loss propagation and retain most of their launch power at range. A novel mechanism involving an aggregation of weakly linear and nonlinear cumulative optical responses is shown to be responsible and is dominated by an ultrafast dynamical lensing resulting from a field intensity driven many-body Coulomb mediated free electron polarization associated with spatially separated species in the gas. An initial few picosecond pulse can compress down to 140 fs over multiple kilometers.

Nonlinear optical response in molecular nitrogen: from ab-initio calculations to optical pulse simulations.

Using first-principle multi-electron calculations via the hybrid anti-symmetrized coupled channels method, we create a model to describe both the nonlinear polarization and ionization of the nitrogen molecule. Based on the metastable electronic state approach, it is designed for space-and-time-resolved simulations in nonlinear optics that require modeling of optical pulses that exhibit rich spectral dynamics and propagate over long distances. As a demonstration of the model's utility, we study low-order harmonic generation in mid-infrared optical filaments.

Experimental tests of the new paradigm for laser filamentation in gases.

Since their discovery in the mid-1990s, ultrafast laser filaments in gases have been described as products of a dynamic balance between Kerr self-focusing and defocusing by free electric charges that are generated via multiphoton ionization on the beam axis. This established paradigm has been recently challenged by a suggestion that the Kerr effect saturates and even changes sign at high intensity of light and that this sign reversal, not free-charge defocusing, is the dominant mechanism responsible for the extended propagation of laser filaments. We report qualitative tests of the new theory based on electrical and optical measurements of plasma density in femtosecond laser filaments. Our results consistently support the established paradigm.

Filamentation of femtosecond laser Airy beams in water.

We report experiments on the propagation of intense, femtosecond, self-bending Airy laser beams in water. The supercontinuum radiation generated along the curved beam path is angularly resolved in the far field. Spectral maps of this radiation reveal the changing character of the laser-pulse evolution on propagation.

Curved plasma channel generation using ultraintense Airy beams.

Plasma channel generation (or filamentation) using ultraintense laser pulses in dielectric media has a wide spectrum of applications, ranging from remote sensing to terahertz generation to lightning control. So far, laser filamentation has been triggered with the use of ultrafast pulses with axially symmetric spatial beam profiles, thereby generating straight filaments. We report the experimental observation of curved plasma channels generated in air using femtosecond Airy beams. In this unusual propagation regime, the tightly confined main intensity feature of the axially nonsymmetric laser beam propagates along a bent trajectory, leaving a curved plasma channel behind. Secondary channels bifurcate from the primary bent channel at several locations along the beam path. The broadband radiation emanating from different longitudinal sections of the curved filament propagates along angularly resolved trajectories.

Extended filamentation with temporally chirped femtosecond Bessel-Gauss beams in air.

We report experimental results on ultrafast filamentation with temporally chirped femtosecond Bessel-Gauss beams. We find that by chirping the pulses, the longitudinal range of the generated plasma channels can be extended relative to filaments generated by fully compressed, transform-limited femtosecond pulses. We find a clear correlation between the extent of filamentation and the intensity of the on-axis emission by the femtosecond Bessel-Gauss beam. The on-axis emission is negligible for fully compressed pulses, but it can become quite substantial (up to 10% of the input pulse energy) when chirped pulses are used. Under certain conditions, the on-axis emission becomes sufficient for generating its own plasma channel thus resulting in extended filamentation. This effect may offer means of remote control over filament formation with femtosecond Bessel-Gauss beams.We identify a four-wave mixing process, enhancement of which is likely to result in a maximum of the on-axis emission, and derive a simple expression for estimating the duration of the chirped pulse that is required for such enhancement. Our estimations are in good agreement with the experiment.

Generation of extended plasma channels in air using femtosecond Bessel beams.

Extending the longitudinal range of plasma channels created by ultrashort laser pulses in atmosphere is important in practical applications of laser-induced plasma such as remote spectroscopy and lightning control. Weakly focused femtosecond Gaussian beams that are commonly used for generating plasma channels offer only a limited control of filamentation. Increasing the pulse energy in this case typically results in creation of multiple filaments and does not appreciably extend the longitudinal range of filamentation. Bessel beams with their extended linear foci intuitively appear to be better suited for generation of long plasma channels. We report experimental results on creating extended filaments in air using femtosecond Bessel beams. By probing the linear plasma density along the filament, we show that apertured Bessel beams produce stable single plasma channels that span the entire extent of the linear focus of the beam. We further show that by temporally chirping the pulse, the plasma channel can be longitudinally shifted beyond the linear-focus zone, an important effect that may potentially offer additional means of controlling filament formation.

Mode shaping in multicore fibers.

We propose a novel multicore fiber design strategy for obtaining a flat in-phase supermode that optimizes utilization of the active medium inversion in the multiple cores. The spatially flat supermode is achieved by engineering the fiber so that the total mutual coupling between neighboring active cores is equal. Different designs suitable for different fabrication processes, such as stack-and-draw and drilling, are proposed. An important improvement over previous methods is the design simplicity and better tolerance to perturbations.

Ultra-flattened-dispersion selectively liquid-filled photonic crystal fibers.

We propose a method to control the chromatic dispersion properties of photonic crystal fibers using the selective hole filling technique. The method is based on a single hole-size fiber geometry, and uses an appropriate index-matching liquid to modify the effective size of the filled holes. The dependence of dispersion properties of the fiber on the design parameters such as the refractive index of the liquid, lattice constant and hole diameter are studied numerically. It is shown that very small dispersion values between 0+/-0.5ps/nm-km can be achieved over a bandwidth of 430-510nm in the communication wavelength region of 1300-1900nm. Three such designs are proposed with air hole diameters in the range 1.5-2.0microm.

Stability and transient effects in nanosecond ultraviolet light filaments in air.

We investigate the transient behavior and stability of nanosecond duration ultraviolet pulses propagating in air. Both the transient behavior arising from the finite pulse duration and the modulational instability, are found to cause pulses to fragment over lengths on the scale of meters. We discuss the theoretical and experimental implications of the instability and transient effects for long duration pulse propagating in air and generating filaments. In particular, our results imply that continuous-wave models are very limited when used to predict dynamical properties of pulse propagation.