Third-order harmonic generation in a bi-chromatic elliptical laser field

Area of Science:

• * Atomic, Molecular, and Optical Physics
• * Quantum Optics
• * Strong Field Physics

Background:

• * Strong laser fields drive nonlinear processes, producing broadband supercontinuum radiation.
• * Low-order harmonic generation is crucial for understanding light-matter interactions.
• * Bichromatic laser fields offer control over harmonic emission properties.

Purpose of the Study:

• * To experimentally and theoretically investigate third harmonic generation (THG) using bichromatic laser fields.
• * To elucidate the underlying mechanisms of low-order harmonic generation.
• * To compare quantum mechanical and semi-classical models for THG.

Main Methods:

• * Experimental control of phase-delay and ellipticity in bichromatic laser fields.
• * Theoretical analysis using extended strong field approximation (SFA).
• * Comparison of quantum-mechanical continuum-continuum transition model with semi-classical photocurrent model.

Main Results:

• * Third harmonic generation is significantly suppressed in counter-rotating bichromatic fields due to selection rules.
• * The continuum-continuum transition model accurately predicts THG dependence on phase delay and ellipticity.
• * Quantum mechanical treatment shows superior agreement with experimental data compared to the semi-classical model.

Conclusions:

• * Quantum mechanical continuum-continuum transitions provide a robust explanation for THG under elliptical bichromatic fields.
• * The overlapping of continuum states via distinct quantum paths of single electrons influences low-order harmonic generation.
• * Understanding these quantum pathways is key to controlling and optimizing harmonic emission.