Since a single oscillation of the electric field takes a little more than a femtosecond for visible light , shorter periods can only be generated in the ultraviolet or X-ray part of the electromagnetic spectrum . Attosecond radiation is created when a pulse from a femtosecond laser interacts with a suitably selected material. Electrons can be released and accelerated by the electrical field of the femtosecond pulse. As soon as the electric field is reversed, the electrons are driven back to their atom, where they recombine , emitting a high-energy photon can. The frequency of the emitted light is a multiple of the carrier frequency of the femtosecond laser. This process is therefore also known as the generation of high harmonics .
The efficiency of the process and the exact spectrum of the resulting light depend sensitively on the shape of the wave packet , in particular on the carrier-envelope phase . The coherence of the resulting radiation, which leads to the (slightly misleading) term attosecond laser, is a consequence of the perfect synchronization with the original femtosecond light field. Attosecond lasers of this type are still relatively far removed from practical applications due to the low radiation intensity that can be achieved; synchrotron radiation or, in the future, free-electron lasers are alternativesat. In basic research, however, they have already opened up completely new possibilities for investigating processes in the atomic shell or for making individual vibrations of a light field directly visible.
The shortest attosecond pulse generated so far had a duration of about 43 attoseconds. 
- Record: ETH created the world's shortest laser pulse. October 31, 2017, accessed October 31, 2017 .