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 1$0^3$ 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.