M.Yu.Emelin, M.Yu.Ryabikin, and A.M.Sergeev

Institute of Applied Physics, Russian Academy of Sciences, 46 Ul'yanov st, Nizhny Novgorod, 603950 Russia

Key points on the way of bringing attosecond sources into practice are to enhance the efficiency of HHG and to control spectral and temporal characteristics of attosecond pulses. These enhancement and control can be based in many respects on the concept of electron wave-packet engineering, which implies the control of the ionization process and electron propagation in the continuum by means of optimal preparation of the initial state of atom or molecule and laser pulse shaping. Possible ways to provide this control on microscopic level are the use of symmetry of molecular valence orbital by appropriate choice of molecular species, molecular alignment, and optimization of the internuclear separation. The control can be implemented by using, for example, molecular vibrational or rotational wave packets. In addition, an essential enhancement of attosecond SXR pulse production can be achieved for atoms and molecules prepared initially in excited electronic states to provide much slower diffusion of the released electronic wave packet. The control over above-mentioned stages of HHG using the phenomenon of quantum mechanical interference of coherent electron wavepackets detached from the molecule allows tuning the yield of harmonics in the desired spectral range. Constructive and destructive interferences caused by coherent superposition of the contributions to X-ray radiation from scattering of different parts of the electron wave packet by different centers in a molecule is also exploited for this purpose.

In this report we will demonstrate that combining all the above ideas can result in increase by several orders of magnitude the efficiency of laser pulse energy conversion to the attosecond range, tuning the spectrum of attosecond bursts over a whole SXR wavelength band, and pulse shortening down to 10 attosecond duration.