Spectral investigation of higher-order Kerr effects in a tight-focusing geometry

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.

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.