**1. Introduction**

332 Modern Metrology Concerns

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Time resolved investigations of electron dynamics in a variety of sample systems as atoms, molecules and solids establish the spearhead of modern ultrafast science. The technology that enables scientists to track and control these processes is referred to as "attosecond metrology" and is associated in symbiosis with ultrashort laser source development.

The origin of attosecond metrology is linked to the advent of ultrashort mode locked laser sources - femtosecond lasers, chirped pulse amplification, carrier-envelope-phase stabilization - and the technological aspects of this development that rendered experiments with a temporal resolution of tens of attoseconds possible. The high-order harmonicgeneration process yields laser pulses in the wavelength region around 10 nm with durations below 100 attoseconds, at the same time being the shortest controllable signals. We introduce the different spectroscopic techniques that make use of this unprecedented temporal resolution. While the "attosecond transient recorder" constitutes an oscilloscope for electromagnetic fields oscillating at visible frequencies, "attosecond tunneling spectroscopy" as well as "attosecond transient absorption" provides real-time insight into the dynamic processes of electronic rearrangements in matter.

Looking at the evolution of the development of ultrashort pulses, it is remarkable that optical communication stood behind RF communication and electronics for more than a century. Light did not have to be "invented", it was there – due to our ability to see electromagnetic radiation at wavelengths between 380 and 480 nm. Especially since man was able to control fire, light was used as a signal for information transport over wide distances: lighthouses, smoke signals, nautic flags; using optical telegraphy with movable signal arms, the 225 kilometers between Metz and Mainz could be covered in a few minutes. Just after the invention of the light bulb by Thomas Alva Edison and Heinrich Göbel, in 1886 Heinrich Hertz proved the radio waves which had been postulated by James Clerk Maxwell in 1864. Since switching rates for light bulbs were considerably smaller than the ones for radio waves, and since there were no transmission systems for light with low losses, within shor time signal communication via electric cables and RF signals was established. The spectacular advances in the development of optical fibers with extremely low losses (Nobel prize 2009 for Charles Kao) as well as the development of semiconductor-based incoherent and coherent light sources (LEDs and diode lasers) lead to the comeback and triumph of optical communication at the end of the laser century. These sources made possible to generate sufficiently short pulses to achieve extremely high switching and transmission rates.

The needs in optical communications have motivated the development of light sources with short and ultrashort pulse duration significantly. Scientists from atomic, molecular and optical physics made significant contributions and profited from this development. After breaking the picosecond border in pulse duration, an own filed, ultrafast physics evolved. Light pulses of ultrashort duration are a key tool to investigate processes in microcosm. Ahmed Zewail was awarded the Nobel prize 1999 for femtosecond chemistry (Anon 1994). The laws of quantum optics dictate the duration at which processes in atoms, molecules and solids take place (Ferenc Krausz 2009). Electronic transitions in atoms, for example, can happen on a timescale of femtoseconds or attoseconds, where an attosecond (10-18 s) compares to one second approximately like a second to the age of the universe.

From photography we know that an image of a moving object can only be unblurred if the exposure time is significantly shorter than the duration of the movement. The timeresolution of the electronic processes mentioned above therefore needs pulse durations of less than one femtosecond. That´s not so easy, since nature does not give us anything for free: Electromagnetic pulses, hence light pulses consist of many waves of different wavelengths which add up in a way that a temporarily confined signal – the pulse – is formed. The envelope within which the electric field oscillates with the carrier frequency defines the duration of the pulse. In order that a pulse can propagate in space, the carrier has to undergo at least one oscillation cycle, which sets a lower limit for the pulse duration. At 750 nm, which is a common wavelength for ultrafast lasers, one oscillation cycle lasts about 2.5 fs. Amplified laser pulses at this central wavelength can be made as short as 3.3 fs, today (A. L. Cavalieri, E. Goulielmakis, et al. 2007). There is not much to be improved in terms of pulse duration at this wavelength. To achieve even shorter pulse durations, shorter wavelengths in the extreme ultraviolet (XUV) or x-ray regime are needed. Unfortunately, nature doesn´t help us here, again. At these wavelengths no materials exist that could be used as laser media, directly. Besides demanding and expensive methods like free-electronlasers (FEL), which are big facilities with hundreds of meters extension, coherent light can be produced by a non-linear wavelength conversion from laser pulses in the visible range. The generation of high-order harmonics (McPherson et al. 1987) (Ferray et al. 1988) provides the possibility to produce attosecond pulses (Paul et al. 2001) (Christov et al. 1997). Before we concentrate on this method, let´s have a look at the formation of ultrashort pulses comprising barely more than one oscillation cycle of the driver wave.
