Abstract

This feature issue presents recent advances in all fronts involving laser filamentation research.

© 2019 Optical Society of America

Since the observation of filamentation by Braun and Mourou in 1995, researchers spanning the fields of physics, mathematics, and engineering have set out to explain, predict, and use this highly nonlinear phenomenon from its initiation to the plasma induced by its high intensity and the broadband ancillary radiation it produces. With the rapid progress in laser technologies, including sources in the near-, mid- and long-infrared regimes, the field continues to grow, uncovering new territories that require novel diagnostic techniques and theoretical models.

The interaction of a high peak power laser pulse with transparent media induces nonlinear self-focusing. At a critical pulse power, the self-focusing can cause the pulse to nearly collapse, leading to ionization of the medium. The high-intensity core of the pulse, or filament, can then propagate for distances far greater than linear diffraction would allow due to a dynamic balance between self-focusing and plasma-refraction.

This original paradigm for filamentation has repeatedly been questioned and expanded upon as laser technologies and applications push into new parameter regimes, advanced diagnostics refine existing or reveal new phenomena, and theoretical models become more sophisticated. Not without controversy, many physical characteristics have been introduced, such as high-order Kerr effect coefficients, the role of polarization in ionization, and the post-filamentation behavior of the medium (e.g., molecular orientation, plasma dynamics, and hydrodynamic expansion). These new paradigms have been spurred by significant progress in theoretical models, such as the UPPE (Unidirectional Pulse Propagation Equations) model and the inclusion of polarization-dependent terms in the Nonlinear Schrodinger Equation, which has led to the more accurate prediction and reproduction of experimental results.

Laser filaments have opened the door to a variety of applications, such as remote sensing, EM wave guiding, and directed energy, based on their ability to project high intensities at a distance and to create a broad range of spectral emission.

This feature issue will present recent advances in all fronts involving laser filamentation research.

Self-focusing, known as the initiation process for filaments, can be suppressed for few-cycle pulses by the presence of significant normal dispersion. Supercontinuum generation and the effect of the leading and trailing edges of flat-top pulses are studied in solids, focusing on the spectral intensity, extension into the blue side of the spectrum as well as the dimensions of the filament. Properties of post-filamentation channels, “intense spatially localized light structures observed in the laser beam at the plasma-free stage of pulse propagation,” are experimentally studied to extend the community’s knowledge on this “ionization free propagation regime.”

The secondary filament radiations such as THz have been the focus of many studies due to the ability to create and control a spectral source at a remote location using filaments. The interaction between filaments of different central wavelengths (two-color filaments), widely used to enhance filament THz generation, is theoretically studied, revealing “different scenarios of the initial evolution of the pulses,” including the role of dispersion. It is also shown that the effect of introducing an external DC field is twofold: it will enhance the THz signal and can easily control its polarization by applying two crossing fields. The third harmonic generated from the filament, “a simple method to convert near-infrared laser pulses in the UV range,” can be optimized by controlling the medium’s pressure and the numerical aperture used to initiate the filamentation process. “Air lasing” is addressed as well, studying the effect of the NIR pump polarization on $\text{N}_{2}^{+}$ emission at 391 and 428 nm in an effort to explain this controversial phenomenon.

Applications involving the filament plasma channel rely heavily on its conductivity and lifetime. A new regime in filamentation “burst mode filamentation” is introduced with the ability to induce guiding structures with higher conductivity and significantly longer lifetime, ideal for long distance guiding applications. One of the many interesting filament plasma-based applications is the change in the DC air breakdown in the presence of a filament. This is studied for the case of a sequence of four concatenated plasma filaments, independently produced.

Guided by theoretical models, recent experimental results shed light on fundamental aspects of self-induced waveguides in different parts of the spectrum (NIR-LWIR). LWIR filaments are shown to have a considerably larger core, allowing for the projection of much higher energies over long distances. It is also shown that using “designer pulse trains” in this spectral region, controlling the nonlinear dynamics, improves the spatio-temporal profile of the wave packets propagating long distances.

Engineered filament structures are also presented in this issue as they are a game changer in filament applications. The propagation of multiple engineered low-ionized filaments produced by terawatt-power picosecond UV pulses is investigated along a 100 m propagation in air.

The editors hope that this special issue reflects the broad range of research activity and exciting breakthroughs that continue to drive filamentation science. As its silver jubilee approaches in 2020, the filamentation community continues to grow around the world, strengthening its understanding in the pursuit of fundamental science and applications.

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