Quasi-phase-matching high-harmonic radiation using chirped THz pulses

Area of Science:

• * Quantum optics
• * Ultrafast science
• * Laser-matter interactions

Background:

• * High-order harmonic generation (HHG) is a key process for producing extreme ultraviolet and X-ray radiation.
• * Phase matching is crucial for efficient HHG, but often limits the generated photon flux and cutoff energy.
• * Terahertz (THz) fields offer a unique tool to control and enhance HHG due to their long wavelengths and tunable properties.

Purpose of the Study:

• * To numerically investigate the effect of THz pulse assistance on high-order harmonic generation.
• * To explore the potential of using chirped THz pulses to further enhance HHG efficiency and extend the spectral cutoff.
• * To demonstrate quasi-phase-matching (QPM) via lateral THz illumination for improved macroscopic HHG.

Main Methods:

• * Employed a complete 3D nonadiabatic numerical model to simulate HHG.
• * Incorporated the influence of an assisting, laterally illuminating THz pulse (constant wavelength and chirped).
• * Analyzed the phase mismatch compensation and its effect on the harmonic spectrum and efficiency.

Main Results:

• * Achieved a two-order-of-magnitude increase in harmonic efficiency at the cutoff energy (≈170 eV) using a constant wavelength THz pulse.
• * Demonstrated further enhancement by a factor of 3 when employing a chirped THz pulse.
• * Showcased the ability to compensate phase mismatch and extend the macroscopic cutoff to the single-dipole cutoff.

Conclusions:

• * THz pulse assistance is a powerful technique to significantly enhance HHG efficiency and extend the cutoff.
• * Chirped THz pulses offer an additional route to further boost harmonic generation.
• * Lateral THz illumination provides an effective method for achieving quasi-phase-matching in HHG.