When a powerful femtosecond laser pulse propagates in an optical medium, self-focusing occurs. Normally, it is the most powerful part (slice) of the pulse that self-focuses first during its propagation. Self-focusing is balanced by the creation of plasma in the self-focal volume, which defocuses the pulse. This balance leads to a limitation of the peak intensity (intensity clamping). The series of self-foci from different slices of the front part of the pulse give rise to the perception of a so-called filament. The back part of the pulse undergoes self-phase modulation and self-steepening resulting in a strong spectral broadening. The final pulse is a white-light laser pulse (supercontinuum). The physics of such (long distance) filamentation and the self-transformation process are reviewed both in air and in condensed matters. The self-transformation leads to a shorter pulse and is currently being studied for efficient pulse compression to the single and (or) few-cycle level. The efficient generation of a third harmonic in the filament is due to a new phenomenon called self-phase locking. The potential applications in atmospheric sensing and lightning control will be briefly discussed. The capability of melting glass leading to index change will be underlined. The paper will end with an outlook into the future of the field. PACS Nos.: 42.65, 42.65Jx, 42.25, 42.79Qx

The terahertz region is a special region of the electromagnetic spectrum that incorporates the advantages of both microwaves and infrared light waves. In the past decade, metamaterials with effective medium parameters or gradient phases have been studied to control terahertz waves and realize functional devices. Here, we present a new approach to manipulate terahertz waves by using coding metasurfaces that are composed of digital coding elements. We propose a general coding unit based on a Minkowski closed-loop particle that is capable of generating 1-bit coding (with two phase states of 0 and 180°), 2-bit coding (with four phase states of 0, 90°, 180°, and 270°), and multi-bit coding elements in the terahertz frequencies by using different geometric scales. We show that multi-bit coding metasurfaces have strong abilities to control terahertz waves by designing-specific coding sequences. As an application, we demonstrate a new scattering strategy of terahertz waves—broadband and wide-angle diffusion—using a 2-bit coding metasurface with a special coding design and verify it by both numerical simulations and experiments. The presented method opens a new route to reducing the scattering of terahertz waves.

Our experiment shows that external focusing strongly influences the plasma density and the diameter of femtosecond Ti-sapphire laser filaments generated in air. The control of plasma filament parameters is suitable for many applications such as remote spectroscopy, laser induced electrical discharge, and femtosecond laser material interactions. The measurements of the filament showed the plasma density increases from ${10}^{15}{\mathrm{cm}}^{-3}\text{to}2\times {10}^{18}{\mathrm{cm}}^{-3}$ when the focal length decreases from $380\mathrm{cm}\text{to}10\mathrm{cm}$ while the diameter of the plasma column varies from $30\mu \mathrm{m}\text{to}90\mu \mathrm{m}$. The experimental results are in good qualitative agreement with the results of numerical simulations.

This is a review of some recent development in femtosecond filamentation science with emphasis on our collective work. Previously reviewed work in the field will not be discussed. We thus start with...