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Optimization of high-order harmonic generation has been studied with a 30 fs laser at an intensity of < 2x10 14W/cm2in a quasi-static gas cell ([MATH]) filled with argon ([MATH]). The effect of propagation length and gas density on the conversion efficiency and spatial quality of high-order harmonics are investigated comprehensively. As a result of phase-matched propagation, both the spatial quality and conversion efficiency of the 29th harmonic are successfully optimized to Gaussian-like profile with a divergence of 4.5 mrad (full width at l/e2maximum) and a saturated efficiency. A quasi-two-dimensional simulation confirms the phasematching effect.

We report on the generation of 9.0 fs, 550 μJ, carrier-envelope phase (CEP)-stabilized optical pulses around 1.6 μm at 1 kHz. Few-cycle IR pulses are obtained from a BiB_3O_6 optical parametric chirped-pulse amplifier. The amplification of nearly octave-spanning ultrabroad pulses without spectral broadening results in good stability in output energy (0.85% rms) and CEP (160 mrad rms). We observed high harmonics in the water window from a neon cell that corresponds to a laser intensity of 4.1×10^14  W/cm^2.

We demonstrate a 1.2 mJ, 3.8 fs, carrier-envelope phase-controlled laser source by novel energy-scalable spectral broadening in an ionizing gas medium, which is widely applicable to sub-10-fs multimillijoule laser systems to obtain sub-2-cycle (<4 fs) multi-millijoule pulses.