Beaming of high-order harmonics generated from laser-plasma interactions

Area of Science:

• High-intensity laser-matter interactions
• Plasma physics
• Extreme ultraviolet (XUV) generation

Background:

• Understanding beam divergences is crucial for applications of high-order harmonic generation (HHG).
• Previous studies attributed HHG divergences to ponderomotive surface denting, particularly for the relativistically oscillating mirror (ROM) mechanism.
• The coherent wake emission (CWE) mechanism's divergence characteristics require further investigation.

Purpose of the Study:

• To experimentally investigate and theoretically analyze the beam divergences of high-order XUV harmonics.
• To differentiate and explain the divergence mechanisms of ROM and CWE emission.
• To explore the temporal dynamics and phase dependencies influencing harmonic beam divergence.

Main Methods:

• Experimental setup for generating high-order XUV harmonics from intense laser interactions with steep plasma density gradients.
• Fourier analysis of the harmonic spatial phase to understand emission characteristics.
• Theoretical modeling to explain observed beam divergences.

Main Results:

• Relativistically oscillating mirror (ROM) emission divergence is consistent with ponderomotive surface denting.
• Coherent wake emission (CWE) divergence is influenced by both ponderomotive surface denting and an intrinsic emission phase.
• Temporal dependence of divergences for both ROM and CWE mechanisms was observed.
• CWE divergence can be minimized when phase terms effectively compensate each other.

Conclusions:

• The study clarifies the distinct divergence mechanisms of ROM and CWE in HHG.
• Accurate modeling of CWE requires considering multiple phase contributions.
• Optimizing phase compensation offers a pathway to control XUV beam divergence for potential applications.