Attosecond Pulses

Abstract

Attosecond Pulses E. Constant and E. M´evel With 17 figures 12.1 Introduction It is now possible to generate optical pulses with a duration corresponding to only a few optical cycles of the electromagnetic radiation in a wide spectral range. The generation of such short pulses is usually performed in two ways. Low-energy pulses can be obtained directly from femtosecond oscillator and the minimum pulse duration supported by broadband gain media, such as the titanium-doped sapphire (Ti:Sa), is currently around 10 fs. Careful intracavity gain shaping allows ultimate pulse duration below 5 fs in Ti:Sa oscillators [12.1,2]. However amplification of these pulses results in a spectral narrowing, and the shortest pulses obtained until now above the mJ level are 17.5 fs long [12.3]. Those amplified pulses can then be further shortened by postcompression techniques and the shortest high-energy visible pulses obtained by these techniques have a duration of ∼ 5 fs [12.4,5], which corresponds to only 2 optical cycles of the central wavelength and could not be significantly shortened in this wavelength range. The possibility to generate and use such short pulses allowed many fascinating applications in the time domain through pump-probe techniques. However, many events occur on a subfemtosecond time scale, and their study requires shorter pulses. This includes, for instance, most of the electronic motion, the evolution of molecules in highly excited states or even the evolution of molecular hydrogen, which is one of the best-known systems in theory but still cannot be experimentally studied in the time domain. Obtaining pulses shorter than 1 fs and entering the attosecond domain (1 as = 10−18 s) will therefore open new applications in physics. As for femtosecond pulses generated in mode-locked oscillators, attosecond pulses require phase-locked frequencies over an even larger bandwidth. There are two known processes that provide a coherent spectra over a bandwidth broad enough to support attosecond emission. A first one is based on stimulated Raman scattering [12.6–8]. A subsequent spectral broadening of a UV