Transition from linear- to nonlinear-focusing regime in filamentation

Laser filamentation in gases is often carried out in the laboratory with focusing optics to better stabilize the filament, whereas real-world applications of filaments frequently involve collimated or near-collimated beams. It is well documented that geometrical focusing can alter the properties of laser filaments and, consequently, a transition between a collimated and a strongly focused filament is expected. Nevertheless, this transition point has not been identified. Here, we propose an analytical method to determine the transition, and show that it corresponds to an actual shift in the balance of physical mechanisms governing filamentation. In high-NA conditions, filamentation is primarily governed by geometrical focusing and plasma effects, while the Kerr nonlinearity plays a more significant role as NA decreases. We find the transition between the two regimes to be relatively insensitive to the intrinsic laser parameters, and our analysis agrees well with a wide range of parameters found in published literature

Highly nonlinear and ultrafast optical phenomena in metallic nanostructures

This Chapter presents recent findings on nonlinear ionization and photoemission processes at metallic nanostructures. A particular emphasis is placed on processes which—due to the localized excitation in optical near-fields—exhibit different properties and scalings from their counterparts in the gas phase or at planar surfaces. The Chapter is structured in two parts. The first part discusses various regimes in highly nonlinear photoelectron emission from metallic nanotips, including field-driven photoemission at near- and mid-infrared frequencies, and the control of localized photoemission using intense terahertz transients. In the second part, multiphoton and strong-field ionization of atomic gases in plasmonic antennas and waveguides is presented. It is demonstrated that local ionization is enabled by optical field enhancements in various types of structures. At the same time, fundamental physical limitations preventing efficient high-harmonic generation in nanostructures are discussed.This Chapter presents recent findings on nonlinear ionization and photoemission processes at metallic nanostructures. A particular emphasis is placed on processes which—due to the localized excitation in optical near-fields—exhibit different properties and scalings from their counterparts in the gas phase or at planar surfaces. The Chapter is structured in two parts. The first part discusses various regimes in highly nonlinear photoelectron emission from metallic nanotips, including field-driven photoemission at near- and mid-infrared frequencies, and the control of localized photoemission using intense terahertz transients. In the second part, multiphoton and strong-field ionization of atomic gases in plasmonic antennas and waveguides is presented. It is demonstrated that local ionization is enabled by optical field enhancements in various types of structures. At the same time, fundamental physical limitations preventing efficient high-harmonic generation in nanostructures are discussed