APS Medal: High-Harmonic Generation -- A Universal Response of Matter to Intense Light
ORAL · Invited
Abstract
When atoms are illuminated by intense infrared light the electron can tunnel from the atom. Tunnelling is most important near the field crest and the burst of current that tunneling implies leads to low harmonic emission [1]. However, the field that is required for electron tunneling is sufficient to pull the electron far from the atom over a subsequent quarter laser period. When the direction of the field reverses, the electron can return to its birthplace, now with substantial energy where it beats with its former self, emitting radiation at the beat frequency. In a low-density gas, nothing impedes the electron motion and so harmonics with a wavelength as short as the driving wavelength divided by 4000 can be produced! This mechanism of harmonic generation is known as recollision. Since the electron recollision is controlled by conventional laser light, it is possible to engineer a pulse to have only one chance at recollision, creating an isolated attosecond pulse [2].
Transparent dielectrics and semiconductors are like molecular gases, except for having a higher atomic density and more closely spaced excited states. Electrons in a solid are therefore, more confined by the lattice through which they must move, and correspondingly the harmonics contain information on the band structure of the solid [3]. In solids there is an additional source of harmonics in the non-harmonic motion of the electron in non-parabolic bands. (Often this is called the Bloch oscillation harmonics). However, in most cases recollision dominates [4].
Most of the incident electric field does not penetrate a metal and we might have expected that we will see no high harmonics, but that is not the case. Harmonics are found in semimetals such as TiN [5] and in more conventional metals such as silver[6]. The role of recollision is not clear yet, but Vannier basis simulations suggests that even below multi-shot damage in silver, the highest harmonics that we measure have their origin in recollision.
With the laser field (which we can control) and material fields sharing the force on electrons in condensed matter systems, the coherent emission of high-harmonic generation is a new tool for measuring the band structure of materials [7] at intensities near optical damage.
1. F Brunel, JOSA B 7 521 (1990)
2. PB Corkum, NH Burnett, MY Ivanov, Opt Lett 19, 1870 (1994)
3. G Vampa, et
Transparent dielectrics and semiconductors are like molecular gases, except for having a higher atomic density and more closely spaced excited states. Electrons in a solid are therefore, more confined by the lattice through which they must move, and correspondingly the harmonics contain information on the band structure of the solid [3]. In solids there is an additional source of harmonics in the non-harmonic motion of the electron in non-parabolic bands. (Often this is called the Bloch oscillation harmonics). However, in most cases recollision dominates [4].
Most of the incident electric field does not penetrate a metal and we might have expected that we will see no high harmonics, but that is not the case. Harmonics are found in semimetals such as TiN [5] and in more conventional metals such as silver[6]. The role of recollision is not clear yet, but Vannier basis simulations suggests that even below multi-shot damage in silver, the highest harmonics that we measure have their origin in recollision.
With the laser field (which we can control) and material fields sharing the force on electrons in condensed matter systems, the coherent emission of high-harmonic generation is a new tool for measuring the band structure of materials [7] at intensities near optical damage.
1. F Brunel, JOSA B 7 521 (1990)
2. PB Corkum, NH Burnett, MY Ivanov, Opt Lett 19, 1870 (1994)
3. G Vampa, et
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Presenters
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Paul B Corkum
Natl Res Council
Authors
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Paul B Corkum
Natl Res Council