High harmonics
High harmonics are a phenomenon of high intensity laser physics or atomic physics in strong electromagnetic fields . By focusing an intense femtosecond laser pulses in - usually - a gas under vacuum numerous be higher harmonics of the laser frequency was observed. This light with odd multiples (three times, five times, seven times etc.) of the original laser frequency usually extends into the ultraviolet or the soft X-ray range . It is characteristic that a large number of these harmonic orders are generated with similar intensity before the efficiency of the process decreases.
For the range below 100 nm wavelength, high harmonics represent a simple method of generating coherent light .
The process, which is based on the generation of high harmonics, is also applied to attosecond laser pulse to produce.
generation
The generation of high harmonics is based on the "three-step model" (see graphic):
- By bundling a laser pulse with a duration of 50 fs and shorter and a pulse energy of a few mJ (shorter pulses require less energy), light intensities of approx. 10 14 W / cm 2 are achieved. At these intensities, the electric field of light reaches the strength of the electric field in the individual atom . This is disturbed by the laser field so that electrons have a finite chance of the atom to the continuum to tunnel (the laser light "bends" the atomic potential ).
- In the continuum, the electrons in the vector potential of the laser field are initially accelerated away from the atomic nucleus until they return to the nucleus due to the changing sign .
- On returning to the nucleus consists of the electron a finite probability beaming with the atom recombine , thereby being kinetic energy of the electron (several electron volts ) emitted as light.
Since this process can happen after each maximum of the laser field of a laser pulse of longer duration and two of these maxima occur per laser cycle, one can conclude from the Fourier transformation of the emission that odd multiples of the laser frequency are generated, whereas the even multiples are omitted for reasons of symmetry .
Since the availability of laser pulse durations in the range of a few femtoseconds and thus laser cycles, the generation of high harmonics can be controlled in such a way that recombination is only possible at a single point in time (during the course of the laser pulse). This leads to the emission of a UV / XUV continuum, the discrete orders disappear.
Applications
High harmonics enable the generation of laser light in previously almost impossible spectral ranges, with a spectral bandwidth that enables the generation of light pulses whose duration is in the attosecond range. These light pulses enable time-resolved experiments on the dynamics of the electrons in the atom. Ultimately, the attosecond pulses are the continuation of ultra-short time physics , which began with the availability of femtosecond pulses.
Furthermore, high harmonics are used for X-ray holography and for " seeding " free-electron lasers .
Previous applications may include a. the radiography and spectroscopy of various objects.
literature
- ↑ Hentschel, M., et al. "Attosecond metrology." Nature 414.6863 (2001): 509-513.
- ↑ M. Lewenstein, Ph. Balcou, M. Yu. Ivanov, Anne L'Huillier and PB Corkum, Theory of high-harmonic generation by low frequency laser fields, Phys. Rev. A 49, 2117, 1994
- ^ T. Brabec & F. Krausz: Intense few-cycle laser fields: Frontiers of nonlinear optics . Rev. Mod. Phys. 72 (No. 2, April 2000), 545-591
- ^ PM Paul, ES Toma, P. Breger, G. Mullot, F. Auge, Ph. Balcou, HG Muller, P. Agostini: Observation of a Train of Attosecond Pulses from High Harmonic Generation . Science 292 (5522): 1689-1692, 2001.
- ↑ M. Hentschel, R. Kienberger, Ch. Spielmann, GA Reider, N. Milosevic, T. Brabec, P. Corkum, U. Heinzmann, M. Drescher and F. Krausz: Attosecond metrology . Nature 414 (2001) 509-513