High-order harmonic generation in atom clusters

We report the generation of short-wavelength, high-order harmonics of intense laser radiation from atom clusters. Clusters containing about 10{sup 3} atoms are produced in a high-pressure gas jet. We show them to be a unique nonlinear medium. Compared with monomer gases they yield a higher appearance intensity for a given harmonic order, stronger nonlinear dependence of harmonic signal on laser intensity, higher-order harmonics, and reduced saturation of the harmonic signal at high laser intensity. {copyright} {ital 1996 The American Physical Society.}