Probe of high-order harmonic generation in a hollow waveguide geometry using counterpropagating light

Area of Science:

• Nonlinear Optics
• Quantum Optics
• Laser Physics

Background:

• High harmonic generation (HHG) is a key process for producing coherent extreme ultraviolet and X-ray light.
• Controlling the coherence and phase matching of HHG is crucial for many applications.
• Traditional phase matching methods are limited, especially for high photon energies.

Purpose of the Study:

• To directly observe and measure the coherent buildup of high harmonic generation.
• To investigate coherence lengths for high photon energies beyond conventional phase matching limits.
• To probe the transition through phase matching and its dependence on ionization and laser depletion.

Main Methods:

• Utilizing counterpropagating light within a hollow waveguide geometry.
• Directly observing the coherent buildup dynamics of high harmonic generation.
• Measuring coherence lengths across a range of harmonic orders and ionization levels.

Main Results:

• First measurement of coherence lengths for high photon energies unattainable with conventional phase matching.
• Detailed probing of the phase matching transition and its dependence on ionization.
• Observation of how driving laser depletion affects coherence length.
• Characterization of harmonic order generation linked to specific ionization levels.

Conclusions:

• The findings provide direct insights into the coherent dynamics of high harmonic generation.
• Results offer a pathway to designing optimal structures for quasiphase matching.
• This work enables improved control over high-energy coherent X-ray sources.