Conical-wave nonlinear optics: From Raman conversion to extreme UV generation

Conical waves (CW) are a particular class of wavepackets that exhibit the notable feature of being stationary solutions for both linear and nonlinear propagation equations. The possibility of independently tuning the phase and group velocities of a CW is a unique feature that opens the possibility of a number of applications, in particular, we shall consider frequency conversion processes and divide the discussion into two parts: group velocity (GV)-matched and phase-matched nonlinear processes. As a GV-matched process, we consider stimulated raman scattering within optical filaments and we highlight the role of cross-phase modulation in the formation of GV-matched X-wave pulses, leading to high Raman conversion efficiencies and the formation of 15-fs compressed pulses. Conical waves, in particular, pulsed-Bessel beams, are then considered for phase-matching higher-order harmonic generation in the extreme UV region.