High harmonic generation

High harmonics are a phenomenon of high intensity laser physics or atomic physics in strong electromagnetic fields. By focusing an intense femtosecond laser pulse in - mostly - a gas under vacuum, numerous higher harmonics of the laser frequency can be observed. This light with odd multiples of the original laser frequency ranges usually down to the ultraviolet or soft X-ray range. It is characteristic that a plurality of harmonic orders is generated with a similar intensity, before the efficiency of the process decreases.

High harmonic ask for the range below 100 nm wavelength is an easy way to produce coherent light. The process underlying the generation of high harmonics, also finds application in the generation of attosecond laser pulses. (Paul and Krausz, 2001)

Generation

The light intensities, in which the generation High Harmonic normally occurs in the range of 1014 W/cm2. This can be achieved by concentrating a laser pulse with a duration of 50 fs and shorter, and a pulse energy of a few mJ. By convention, shorter pulses require less energy. At these intensities, the electric field of the light reaches the intensity of the electric field in the single atom. This is disturbed by the laser field so that electrons can tunnel into the continuum of the atom. This is step 1 of the so-called "three- step - model " (see chart):

• The laser light " bends " the atomic potential, electrons have a finite chance to escape into the continuum.
• In the continuum, the electrons are initially wegbeschleunigt in the vector potential of the laser field from the nucleus until they - again return to the core - due to the changing sign.
• Upon return to the nucleus, there is a finite probability, radiant recombine with the atom, it is the kinetic energy of the electron ( several electron volts ) emitted as light.

Since this process can happen after each peak of the laser field of a laser pulse of longer duration and two of these maxima occur each laser cycle, one can conclude from the Fourier transform of the emission that odd multiples of the laser frequency are generated, whereas the even multiples of symmetry reasons no longer apply. Since the availability of laser pulse durations in the range of a few femtosecond laser cycles and thus the generation of high harmonics can be so controlled that only at a single time ( in the course of the laser pulse ) recombination is possible. This leads to the emission of a UV / XUV continuum, the discrete systems disappear.

Applications

Allow the generation of high harmonics of the laser light in spectral regions far almost impossible, with a spectral range, which allows the production of light pulses whose duration is attosecond. These pulses allow time-resolved experiments on the dynamics of electrons in the atom ..

Previous applications include, among others, the radiography and spectroscopy of various objects. Furthermore, High harmonic for X-ray holography and the " seeding " of free-electron lasers are used.

Ultimately, the attosecond pulses are the continuation of ultrafast physics, which began with the availability of femtosecond pulses.