Optimization of conversion efficiency and spatial quality of high-order harmonic generation

We studied high-order harmonic generation with a 30 fs laser at an intensity of <2×10¹⁴ W/cm² in a quasistatic gas cell (<~10 mm) filled with argon (<~90 torr). To our knowledge, this is the first systematic study on the effect of propagation length and gas density on the conversion efficiency and spatial quality of high-order harmonics. As a result of phase-matched propagation, both the spatial quality and conversion efficiency of the 29th harmonic are successfully optimized to a Gaussian-like profile with a divergence of 4.5 mrad (full width at 1/e² maximum) and a saturated efficiency. A quasi-two-dimensional simulation confirms the phase-matching effect.