High harmonic generation
Perturbative Harmonic Generation
Perturbative Harmonic Generation is a process whereby lar light of frequency ω and photon energy ħω can be ud to generate new frequencies of light. The newly generated frequencies are integer multiples nω of the original light's frequency. This process was first discovered in 1961 by Franken et al.,[1] using a ruby lar, with crystalline quartz as the nonlinear medium.
Harmonic generation in dielectric solids is well understood and extensively ud in modern lar physics (e cond harmonic generation). In 1967 New et al. obrved the first third harmonic generation in a gas.[2] In monatomic gas it is only possible to produce odd numbered harmonics for reasons of symmetry. Harmonic generation in the perturbative (weak field) regime is characterid by rapidly decreasing efficiency with increasing harmonic order and harmonics up to the 11th order have been obrved under the conditions .[3] This behaviour can be understood by considering an atom absorbing n photons then emitting a single high energy photon. The probability of absorbing n photons decreas as n increas, explaining the rapid decrea in the initial harmonic intensities.
High Harmonic Generation (HHG)
The first High Harmonic Generation (HHG) was obrved in 1977 in interaction of inten CO
lar puls with
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plasma generated from solid targets.[4] HHG in gas, far more widespread in application today, was first obrved by McPherson and colleagues in 1987,[5] and later by Ferray et al. in 1988,[6] with surprising results: the high harmonics were found to decrea in intensity at low orders, as expected, but then were obrved to form a plateau, with the intensity of the harmonics remaining approximately constant over many orders.[7] Plateau harmonics spanning hundreds of eV have been measured which extend into the soft x-ray regime.[8] This plateau ends abruptly at a position called the High Harmonic Cut-off.
Properties of High Harmonics
向日葵少女High harmonics have a number of interesting properties. They are a tunable table-top source of XUV/Soft X-rays, synchronid with the driving lar and produced with the same repetition rate. The harmonic cut-off varies linearly
where harmonic generation stops.[9] The saturation with increasing lar intensity up until the saturation intensity I
sat
intensity can be incread by changing the atomic species to lighter noble gas but the have a lower conversion efficiency so there is a balance to be found depending on the photon energies required.
High harmonic generation strongly depends on the driving lar field and as a result the harmonics have similar temporal and spatial coherence properties.[10] High harmonics are often generated with pul durations shorter than that of the driving lar. This is due to pha matching and ionization. Often harmonics are only produced in a very small temporal window when the pha matching condition is met. Depletion of the generating media due to ionization also means that harmonic generation is mainly confined to the leading edge of the driving pul.[11]
High harmonics are emitted co-linearly with the driving lar and can have a very tight angular confinement, sometimes with less divergence than that of the fundamental field and near Gaussian beam profiles.[12]
Semi-classical approach to describe HHG
The maximum photon energy producible with high harmonic generation is given by the cut-off of the harmonic plateau. This can be calculated classically by examining the maximum energy the ionized electron can gain in the
生态林electric field of the lar. The cut-off energy is given by,
where U p is the ponderomotive energy from the lar field and I p is the ionization potential.
如何写起诉书This derivation of the cut-off energy is derived from a mi-classical calculation. The electron is initially treated quantum mechanically as it tunnel ionizes from the parent atom, but then its subquent dynamics are treated classically. The electron is assumed to be born into the vacuum with zero initial velocity, and to be subquently accelerated by the lar beam's electric field.
The three-step model.Half an optical cycle after ionization, the electron will rever direction我的动物朋友作文
as the electric field changes, and will accelerate back towards the
学习是为了什么parent nucleus. Upon returning to the parent nucleus it can then emit
bremsstrahlung-like radiation during a recombination process with the
atom as it returns to its ground state. This description has become
known as the recollisional model of high harmonic generation .[13]
新闻记者证Some interesting limits on the HHG process which are explained by
this model show that HHG will only occur if the driving lar field is
linearly polarid. Ellipticity on the lar beam caus the returning
electron to miss the parent nucleus. Quantum mechanically, the overlap of the returning electron wavepacket with the nuclear wavepacket is reduced. This has been obrved experimentally, where the intensity of harmonics decreas rapidly with increasing ellipticity.[14] Another effect which limits the intensity of the driving lar is the Lorentz force. At intensities above 1016 Wcm −2 the magnetic component of the lar pul, which is ignored in weak field optics, can become strong enough to deflect the returning electron. This will cau it to 'miss' the parent nucleus and hence prevent HHG.
References
[1]P. A. Franken, A. E. Hill, C. W. Peters, and G. Weinreich, Phys. Rev. Lett. 7, 118 (1961).
[2]G. H. C. New and J. F. Ward, Phys. Rev. Lett. 19, 556 (1967).
[3]J. Wildenauer, Journal of Applied Physics 62, 41 (1987).
[4]N. H. Burnett et al., Appl. Phys. Lett., vol. 31, pp. 172–174, 1977.
[5] A. McPherson et al, JOSA B 4, 595 (1987).
[6]M. Ferray et al., Journal of Physics B-Atomic Molecular and Optical Physics 21, L31 (1988).
[7]X. F. Li, A. L'Huillier, M. Ferray, L. A. Lompre, and G. Mainfray, Physical Review A 39, 5751 (1989).
[8]J. Seres et al., Nature 433, 596 (2005).
[9]T. Brabec and F. Krausz, Reviews of Modern Physics 72, 545 (2000).
[10] A. L'Huillier, K. J. Schafer, and K. C. Kulander, Journal of Physics B Atomic Molecular and Optical Physics 24, 3315 (1991).
[11]K. J. Schafer and K. C. Kulander, Physical Review Letters 78, 638 (1997).
[12]J. W. G. Tisch et al., Physical Review A 49, R28 (1994).
[13]P. B. Corkum, Physical Review Letters 71, 1994 (1993).
国营单位[14]P. Dietrich, N. H. Burnett, M. Ivanov, and P. B. Corkum, Physical Review A 50, R3585 (1994).
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