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High Harmonic Generation

Author: the photonics expert Dr. Rüdiger Paschotta

Acronym: HHG

Definition: the phenomenon that very high harmonics of an intense input laser beam are generated in a gas

More general term: nonlinear frequency conversion

Categories: article belongs to category nonlinear optics nonlinear optics, article belongs to category light pulses light pulses

DOI: 10.61835/xb7 Cite the article: BibTex plain text HTML

When a very intense light pulse is focused into a gas (usually at reduced pressure), strong nonlinear interactions can lead to the generation of very high odd harmonics of the optical frequency of the pulse, i.e., to an extreme form of nonlinear frequency conversion. This typically occurs at optical intensities of the order of 10¹⁴ W/cm² or higher. Although only a tiny fraction of the laser power can be converted into higher harmonics, the frequency-upconverted output can still be useful for measurements down to wavelengths in the hard ultraviolet or even the X-ray spectral region. Such high harmonics may be used instead of synchrotron radiation. They are also used for generating ultrashort pulses with attosecond durations in the extreme ultraviolet spectral region [4, 10, 11, 13, 12, 22]. Such attosecond pulses are now used for various fundamental studies e.g. of electronic motion in various kinds of materials. Even zeptosecond pulses (i.e., with durations well below one attosecond) might be possible [25].

In most cases, the pump source used contains a passively mode-locked laser and a regenerative amplifier based on titanium–sapphire crystals as the gain media. The repetition rate is then between a few hertz and a few kilohertz. Recently, however, a resonant cavity (enhancement resonator) has been used instead of an amplifier to increase the pulse energy to the level required for high harmonic generation [15]. This allowed for a much higher repetition rate of more than 100 MHz.

For the detection of weak high harmonics, one may use a microchannel plate followed by a phosphor layer and a CCD image sensor.

Although a detailed description of the physical processes behind high harmonic generation is complicated (and often relies on computationally intensive numerical quantum simulations), a number of basic aspects can be grasped with the “simple man's model” [3], describing how an electron under the influence of a strong electromagnetic field can leave its atom, be accelerated and later collide with the atom, thereby emitting harmonic radiation. More sophisticated models describe the quantum dynamics of the involved electrons.

For further dealing with the generated ultraviolet light, special ultraviolet optics are required. For the shorter wavelengths, the choice of available components is very restricted, and one can essentially use only reflective optics.

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Bibliography

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