Filamentation

Twenty years after its first observation in air, the filamentation of ultrashort optical pulses remains a topic of high fundamental and practical interest. The filamentation process touches on many fields: nonlinear optics, plasma physics, molecular and atomic physics, ultrafast dynamics, hydrodynamics, acoustics, and atmospheric sciences. Filamentation has promising applications, many of them related to the fact that high laser intensities may be transported large distances in the atmosphere or that extended atmospheric modification may be possible.

This special issue of J. Phys. B: At. Mol. Opt. Phys. is a snapshot of recent work in the filamentation field, some of which was presented at the recent 5th International Symposium on Filamentation (COFIL 2014) held in Shanghai in September 2014. The work presented here is a partial sample of the topics covered at the conference.

In gases, transparent solids or liquids, filamentation shows striking similarities. Because of self-focusing induced by the atomic or molecular bound electron nonlinear response to the laser field, a short laser pulse self-contracts when propagating in a transparent medium until high field ionization of the medium occurs and self-focusing is arrested. Beyond the onset of ionization, a high intensity core of the beam maintains a small diameter over multiple Rayleigh ranges, as if diffraction were switched off, owing to the dynamic competition between self-focusing and plasma defocusing. This first region of quasi-continuous plasma is followed by a region of quasi-periodic cycles of focusing/defocusing. Instances of ionization collapse then become more and more sporadic. Eventually, an ionization-free non-linear propagation regime settles in, characterized by a near balance between optical Kerr effect and diffraction, with a pulse with peak intensity barely below ionization slowly expanding. On the theoretical front, increasingly sophisticated propagation codes are able to reproduce experimental results faithfully. The situation is still not completely satisfactory, however, because material parameters are often introduced in an ad hoc manner to fit experiments. It is now imperative for experimentalists to provide reliable data so that theorists can further refine propagation models.

One of the more intriguing fundamental aspects of filamentation is that it is a process of extreme nonlinear optics, where substantial temporal slices of a filamenting pulse envelope experience propagation just below and just above the ionization threshold. Thus, its accurate modeling requires understanding atomic and molecular nonlinear response under highly nonperturbative laser fields. This has given rise to a fascinating debate in the filamentation and nonlinear optics community about exactly how a negative nonlinear response arises to arrest the self-focusing process.

As an example of recent research, attention is turning progressively to the interactions between filaments. Binary interaction between closely spaced parallel filaments born at the same time can lead to their mutual attraction, repulsion or a spiral motion, depending on the relative phase of the filaments. Mutual attraction or repulsion of filaments can also occur with delayed filaments, by exploiting rotational revivals. A delayed change of the refractive index due to the revival of a rotational wave-packet can lead to attraction or deflection of a retarded filament. Another class of interaction occurs when two filamentary pulses cross, setting up gratings in the nonlinear response that can redistribute laser energy. Nonlinear propagation of Airy and Bessel beams is also an area of increasing research activity.

Recent exciting applications of filamentation include high power THz generation with filaments generated with fundamental and second harmonic laser wavelengths, filament-induced high power electrical discharges, generation of air lasing through filament pumping of gain in atmospheric molecular constituents, and generation of refractive index structures in air. These index structures are produced from the plasma left in the wake of a filament, from the molecular rotational revivals, or, with the very longest lifetimes, from nonlinear filament heating of the air, and are capable of guiding very high power secondary laser beams. All of these applications strikingly demonstrate the broad and interdisciplinary nature of the filamentation field.

We hope you enjoy the articles in this special issue. They illustrate that the field of ultrashort pulse optical filamentation occupies a 'sweet spot', where fundamental physics overlaps with promising applications.