Beam-quality measurement of a focused high-order harmonic beam

Using a multilayer spherical mirror, we focus the high-order harmonic radiation produced near 55 nm by the nonlinear interaction of an intense femtosecond laser pulse and a xenon gas jet. The focused XUV beam is characterized by a knife-edge technique in the focal region and by far-field imaging. We show that good-quality beams, nearly two times diffraction limited, can be generated, a conclusion that is at variance with recent predictions of harmonic phase-front distortion. Spot sizes close to $10 \mathit{\mu }\mathrm{m}$ are obtained, resulting in a high XUV intensity. Increasing the gas density and the length of the generating medium results in a large increase in the divergence in and degradation of the beam quality.