Visible Harmonic Emission as a Way of Measuring Profile Steepening

Carman, R. L.; Forslund, D. W.; Kindel, J. M.

doi:10.1103/physrevlett.46.29

Cited by 163 publications

(65 citation statements)

References 9 publications

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“…Therefore, the field cannot be thought of as a small perturbation to the crystal. The non-perturbative HHG from bulk crystals has been considered theoretically 12-14 but has never been observed experimentally until now.Previously, experimental observation of non-perturbative HHG in solids was only in reflection geometry [15][16][17][18] perturbative harmonics below the band edge up to the seventh order were produced by exciting semiconducting ZnSe with a maximum field strength of ∼0.08 V Å −1 at a central wavelength of 3.9 µm (ref. 19).…”

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confidence: 99%

Observation of high-order harmonic generation in a bulk crystal

et al. 2010

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Harmonic generation is a general feature of driven nonlinear systems. In particular high-order harmonic generation (HHG) in atomic gases 1 is the basis for producing attosecond pulses 2,3 . In molecules and clusters, the existence of multiple ionization and recombination sites makes for richer dynamics allowing imaging of molecular orbitals 4,5 , higher conversion efficiency 6 and the possibility of extending the high-energy cutoff 7 . In the strong-field limit, HHG in bulk crystals is fundamentally different from that in the atomic case owing to the high density and periodic structure. Here we present the first observation of HHG in a bulk crystalline solid using a long-wavelength few-cycle laser. The harmonics spectra extend well beyond the band edge of the ZnO crystal, show a clear non-perturbative character and exhibit a cutoff that scales linearly with the electric field of the drive laser. Our results have important implications for the understanding of attosecond electron dynamics and other non-equilibrium band-structure-related phenomena in strongly driven bulk solids.The HHG spectrum from a gas typically comprises a region of rapidly decreasing low orders that scale perturbatively followed by a slowly varying succession of higher orders that scale nonperturbatively with the strength of the drive laser 1 . At the tunnelling limit of strong-field ionization 8 , the non-perturbative harmonic generation process has been described semiclassically in a recollision model 9,10 consisting of three steps: tunnel ionization of an electron, its acceleration in the laser field, and its recombination to the parent ion with an energy release in the form of higher-energy photons-a coherent process that occurs on successive half-cycles of the laser pulse, leading to emission of odd harmonics. The single-atom maximum photon energy is 9,11h ω max = I p + 3.2U p , where I p is the ionization potential and U p = e 2 E 2 λ 2 /16π 2 mc 2 is the ponderomotive energy of the electron in the laser field. The amplitude of the maximum excursion of a recolliding electron is r max = eEλ 2 /4π 2 mc 2 . For the conditions of our experiments, E = 0.6 V Å −1 at λ = 3.25 µm, the atomic case would be U p = 5 eV and r max = 32 Å. The latter is many times the lattice constant of a typical crystal. Thus, we expect the possibility of ionization from one site and recombination on another; however, because of the lattice periodicity the process would still be coherent. We note that at this field strength, the potential across a lattice constant is comparable to the bandgap of a typical insulator. Therefore, the field cannot be thought of as a small perturbation to the crystal. The non-perturbative HHG from bulk crystals has been considered theoretically 12-14 but has never been observed experimentally until now.Previously, experimental observation of non-perturbative HHG in solids was only in reflection geometry [15][16][17][18] perturbative harmonics below the band edge up to the seventh order were produced by exciting semiconducting ZnSe with...

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confidence: 99%

Observation of high-order harmonic generation in a bulk crystal

et al. 2010

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“…From the point of view of harmonic production a competing process of harmonic production by so-called coherent wake emission (CWE) [26] requires a brief mention. Briefly, these harmonics result from the periodic pumping of the plasma waves at the steep plasma-vacuum interface and decay by radiating harmonics of the laser frequency and were first investigated using high power CO 2 lasers in the late 1970s and early 1980s [12]. Consequently the highest harmonic that can be generated by this process must have a frequency that is no higher than the maximum plasma frequency ω p max in the density ramp, i.e.…”

Section: Competing Mechanisms Of Harmonic Generationmentioning

confidence: 99%

“…HOHG [12][13][14][15] essentially results from an oscillatory extension to Einstein's prediction for the frequency up shift of light reflected off a perfect mirror moving at relativistic velocitiesthe relativistic Doppler effect [16]. In this theory a pulse of duration t at frequency ω 0 is shifted to a frequency of ω = 4γ 2 ω 0 (with the Lorentz factor γ 1).…”

Section: Introductionmentioning

confidence: 99%

High harmonics from relativistically oscillating plasma surfaces—a high brightness attosecond source at keV photon energies

Zepf¹,

Dromey²,

Kar³

et al. 2007

Plasma Phys. Control. Fusion

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An intense laser pulse interacting with a near discontinuous plasma vacuum interface causes the plasma surface to perform relativistic oscillations. The reflected laser radiation then contains very high order harmonics of fundamental frequency and-according to current theory-must be bunched in radiation bursts of a few attoseconds duration. Recent experimental results have demonstrated x-ray harmonic radiation extending to 3.3 Å (3.8 keV, order n > 3200) with the harmonic conversion efficiency scaling as η(n) n −2.5 over the entire observed spectrum ranging from 17 nm to 3.3 Å. This scaling holds up to a maximum order, n RO 8 1/2 γ 3 , where γ is the peak value of the Lorentz factor, above which the harmonic efficiency decreases more rapidly. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy hν > 1 keV is found to be into a cone angle ∼4 • , significantly less than that of the incident laser cone (20 • ).

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“…After the invention of the laser, many new important physical phenomena were observed, such as above-threshold ionization [1], Freeman resonances [2][3][4][5][6][7], high-order harmonic generation [8,9], etc. At the beginning of the last century, Einstein proposed a two-level model [10] to study the transitions between energy levels of an atom that interacts with light.…”

Section: Introductionmentioning

confidence: 99%