Fundamental processes in atoms, molecules, as well as condensed matter are triggered or mediated by the motion of electrons inside or between atoms. Electronic dynamics on atomic length scales tends to unfold within tens to thousands of attoseconds (1 attosecond [as] = 10-18 s). Recent breakthroughs in laser science are now opening the door to watching and controlling these hitherto inaccessible microscopic dynamics.

The key to accessing the attosecond time domain is the control of the electric field of (visible) light, which varies its strength and direction within less than a femtosecond (1 femtosecond = 1000 attoseconds). Atoms exposed to a few oscillations cycles of intense laser light are able to emit a single extreme ultraviolet (xuv) burst lasting less than one femtosecond [1,2]. Full control of the evolution of the electromagnetic field in laser pulses comprising a few wave cycles [3] have recently allowed the reproducible generation and measurement of isolated 250-attosecond xuv pulses [4], constituting the shortest reproducible events and fastest measurement to date. These tools have enabled us to visualize the oscillating electric field of visible light with an attosecond "oscilloscope" [5] as well as steering and real-time observation of the motion of electrons in atoms [6] and molecules [7]. Recent experiments [8] hold promise for the development of an attosecond x-ray source, which may pave the way towards 4D electron imaging with sub-atomic resolution in space and time.

[1] M. Hentschel et al., Nature 414, 509 (2001);
[2] R. Kienberger et al., Science 291, 1923 (2002);
[3] A. Baltuska et al., Nature 421, 611 (2003);
[4] R. Kienberger et al., Nature 427, 817 (2004);
[5] E. Goulielmakis et al., Science 305, 1267 (2004);
[6] M. Drescher et al., Nature 419, 803 (2002).
[7] J. Seres et al, Nature 433, 596 (2005).
[8] M. Kling et al., Science 312, 246 (2006).