Attosecond pulses

Because electrons are much lighter than nuclei, they move much faster. The intrinsic temporal regime for valence bond electron dynamics is the few femtosecond to several hundred attosecond timescale. Therefore, efficient and accurate control of electron dynamics requires extreme precision regarding the control field. Commonly attosecond techniques are considered to be the appropriate tools for efficient manipulation of electron motions [61-63, 111, 112]. However, attosecond pulses in the XUV region are not suited for efficient valence bond excitation (see Section 6.1). Here we demonstrate that ultrafast electron dynamics are controlled efficiently on the sub-10 as timescale employing a pair of femtosecond laser pulses with a temporal separation controllable down to zeptosecond precision [8]. 

S. Gilbertson, M. Chini, X. Feng, S. Khan, Y. Wu, Z. Chang, Monitoring and controlling the electron dynamics in helium with isolated attosecond pulses, Phys. Rev. Lett. 105 (26) (2010) 263003. 

In the last few years, first measurements have been performed determining the length of the attosecond soft X-ray bunches. Paul et al. measured 250 attosecond pulses in a two-color photoionization experiment [5]. Making use of the fact that in a train of soft X-ray bunches the frequency spectrum consists of odd multiples ( harmonics ) of the frequency of the driver laser, they studied the ionization of Ar atoms by simultaneous photo-absorption from the soft X-ray bunch and the fundamental color of the femtosecond laser. Measured... 

In the second part, we discuss possible applications of attosecond laser pulses to future studies of time-resolved electron dynamics in strongly driven systems. We discuss our current understanding of the time-dependent behaviour of non-perturbatively driven electrons in atoms, molecules and clusters. In Sect. 3.4 we discuss differences that arise when the generation of attosecond pulses is performed in different atomic media. This is followed in Sect. 3.5 by a description of the role of electron dynamics in dynamical alignment and enhanced ionization of molecules. Finally, in Sect. 3.6 the role of electron dynamics in laser heating of large clusters is discussed. 

There is also a great need for broadband reflection, i.e., the capability for a number of harmonic components to be reflected and/or focused all together. In particular, the XUV supercontinuum (25-40 nm) that has recently been demonstrated using a half-cycle polarization gating method requires broadband optics in its applications toward attosecond pulse char-... 

The research areas treated in this series will be (i) atoms, molecules, and clusters in intense laser fields, (ii) control of molecules and clusters in intense laser fields, (iii) attosecond pulse generation, metrology, and applications, (iv) wavepacket control for high-order harmonics, (v) generation,... 

Note that the IR pulse generated high harmonics that were used to make the attosecond XUV pulses. As a consequence, in the case of the 35 fs IR field, one has a train of a finite number of attosecond pulses serving as the XUV pump, rather than a single one. We will ignore this distinction in the following. 

Significant advances in the production and use of attosecond pulses have been made during fhe pasf few years [152]. 

The optical phase of the carrier wave in a linearly polarized femtosecond pulse can be measured by the photoelectron rate (Fig. 6.59). If the electrons are produced by the nth harmonic of the visible femtosecond pulse, the rate is proportional to the 2nth power of the visible field amplimde. The amplitude depends strongly on the phase of the optical wave relative to the envelope maximum of the pulse. Measurements of this photo-electron rate as a function of the phase shift of the field amplitude against the pulse maximum allows the determination of the phase and the pulse width of the high-harmonic attosecond pulse [754]. There are many more applications of attosecond pulses these can be found, for example, in the publications of the groups of P. Corkum at the NRC in Ottawa [753] and F. Krausz at the MPI for Quantum Optics in Garching, who have pioneered this field [754]. 

For more experimental details and special experimental setups for the generation of femtosecond and attosecond pulses, the reader is referred to the literature [749-751, 753-755],... 

For sub-femtosecond pulses a new techniques has been developed which is called CRAB complete reconstruction of attosecond bursts). Instead of using a nonlinear interaction for generating the signal, a weak femtosecond pulse is used to probe the attosecond pulse by measuring the energy spectrum of photo-electrons produced by photo-ionization of atoms by the attosecond pulse (see Fig. 6.59 and Sect. 6.2.5). 

S. Cormier, LA. Walmsley, E.M. Kosik, A.S. Wyatt, L. Comer, Spectral phase interferometry for complete reconstmction of attosecond pulses. Laser Phys. 15,909 (2005)... 

I. Walmsley, Attosecond pulse measurement techniques, http //www.attosecond.oig/ theproject/metrology/xuvspider.asp... 

This positive development is partly based on new experimental techniques, such as improvements of existing lasers and the invention of new laser types, the realization of optical parametric oscillators and amplifiers in the femtosecond range, the generation of attosecond pulses, the revolution in the measurements of absolute optical frequencies and phases of optical waves using the optical Ifequency comb, or the different methods developed for the generation of Bose-Einstein condensates of atoms and molecules and the demonstration of atom lasers as a particle equivalent to photon lasers. 

Further possibilities that are already emerging include the use of lasers with attosecond pulse dura-... 

When an external field ionizes an electron and subsequently drives it back to the parent hole-state for recollision, high-order harmonics are generated. This high-harmonic generation, as detailed in recent reviews and tutorials [101, 212, 222], has been the focus of current intense studies, both as a source of ultrashort, attosecond pulses [76] and as a means of extracting... 

Though we focused on a femtosecond means to monitor electron-nucleus simultaneous dynamics, we do never claim that attosecond investigation of electron dynamics is unnecessary in chemical applications. On the contrary, it would be extremely exciting to know how the electronic wavepacket is driven by the spike-like pulses of subfemtoseconds or shorter width. In addition, it would be interesting to apply additional (independent) attosecond pulses during the refractory period in a pulse train. 

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