Simulated laser-pulse evolution for high-order harmonic generation in a semi-infinite gas cell

Area of Science:

• Nonlinear optics
• Quantum optics
• Atomic physics

Background:

• High-harmonic generation (HHG) in gas cells is a key method for producing extreme ultraviolet and X-ray radiation.
• Understanding nonlinear effects is crucial for optimizing HHG efficiency and beam characteristics.
• Previous studies suggested significant nonlinear effects, like the Kerr effect, were necessary for observed phenomena.

Purpose of the Study:

• To numerically investigate the propagation of high-intensity laser pulses in helium.
• To determine the role of nonlinear effects in gas-cell high-harmonics experiments.
• To analyze the influence of an aperture on laser pulse propagation and harmonic generation.

Main Methods:

• Numerical simulation of high-intensity laser pulse propagation in helium.
• Inclusion of an aperture before the focusing lens in the simulation model.
• Analysis of radial fluence profiles, spectral shifts, ionization levels, and phase matching.

Main Results:

• Simulated radial fluence profiles, spectral shifts, and ionization levels match experimental observations.
• Confirmed that a significant Kerr effect is not required for the observed double focus.
• Identified that the latter portion of the laser pulse causes incidental double laser focusing.
• Harmonic energy primarily originates from the forward pulse portion.

Conclusions:

• The Kerr effect is not the primary driver for double focusing in this experimental setup.
• Good phase matching for harmonics results from a balance between atomic linear phase delay and the Gouy shift.
• Partial closure of the aperture elongates and linearizes the Gouy shift, improving phase matching.