1. JILA and Department of Physics, University of Colorado and National Institute of Standards and Technology, Campus Box 440, Boulder, Colorado 80309-0440 , USA
2. Delft University of Technology, 2600 GA Delft, The Netherlands
3. Department of Physics, Sofia University , Sofia, Bulgaria

Correspondence to: H. C. Kapteyn¹ Correspondence and requests for materials should be addressed to H. C. K. (e-mail: Email: kapteyn@jila.colorado.edu).

Abstract

When an intense laser pulse is focused into a gas, the light–atom interaction that occurs as atoms are ionized results in an extremely nonlinear optical process^{1, 2, 3}—the generation of high harmonics of the driving laser frequency. 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 and technology; these include probing the dynamics in chemical and materials systems⁶ and imaging⁷. Here we report that by carefully tailoring the shape⁸ of intense light pulses, we can control^{9, 10} the interaction of light with an atom during ionization, improving the efficiency of X-ray generation by an order of magnitude. We demonstrate that it is possible to tune the spectral characteristics of the emitted radiation, and to steer the interaction between different orders of nonlinear processes.