Shaped-pulse optimization of coherent emission of high-harmonic soft X-rays

When an intense laser pulse is focused into a gas, the light-atom interacti on that occurs as atoms are ionized results in an extremely nonlinear optic al process(1-3)-the generation of high harmonics of the driving laser frequ ency. Harmonics that extend up to orders of about 300 have been reported(4, 5), some corresponding to photon energies in excess of 500 eV. Because this technique is simple to implement and generates coherent, laser-like, soft X-ray beams, it is currently being developed for applications in science an d technology; these include probing the dynamics in chemical and materials systems(6) and imaging(7). Here we report that by carefully tailoring the s hape(8) of intense light pulses, we can control(9,10) the interaction of li ght with an atom during ionization, improving the efficiency of X-ray gener ation by an order of magnitude. We demonstrate that it is possible to tune the spectral characteristics of the emitted radiation, and to steer the int eraction between different orders of nonlinear processes.