Introduction. Theory and Experiment of High-Order Harmonic Generation in Narrow and Extended Media
High-order harmonic generation is a technique for producing spatially and temporally coherent extreme ultraviolet light. This process in isotropic media occurs when an intense laser fields are focused into a gas jet or plasma. Such fields are able to deta
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Introduction. Theory and Experiment of High-Order Harmonic Generation in Narrow and Extended Media
High-order harmonic generation (HHG) is a technique for producing spatially and temporally coherent extreme ultraviolet (XUV) light. HHG in isotropic media occurs when an intense pulsed laser beam is focused into a gas jet or plasma. The intensity of the laser light is chosen such that its electric field amplitude is comparable to the electric field in atoms and ions. Such fields are able to detach electrons from above species by tunnel ionization, as opposed to photo-ionization by a weak field with high enough photon energy. The detached electron is accelerated in the field and under certain conditions has significant probability to hit the ion left behind upon return. The “collision” results in the emission of high energy photons. This description is called three-step model. The details of this process will be discussed below. The use of extended media may offer new opportunities in application of XUV radiation for various needs. In Introduction, we discuss the use of extended gaseous media for HHG and show the advantages of this approach, which has prompted the use of long medium for the studies of the high-order nonlinear optical properties of various materials through the plasma harmonic approach.
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High-Order Harmonic Generation in Isotropic Medium: Three-Step Model and Macroscopic Consideration of Frequency Conversion
When a high-intensity laser pulse passes through a gaseous or plasma medium, its atoms and ions emit odd harmonics. For a laser radiation wavelength λ, a superposition of the components λ, λ/3, λ/5, λ/7, etc. is observed at the output of the nonlinear medium. The harmonics of laser radiation result from a three-stage process [1–3] that comprises the ionization of an atom (or ion), the electron acceleration in the electromagnetic field, and the subsequent recombination with parent particle and emission of harmonics. This process is periodically repeated every half cycle of the electromagnetic wave. The highest-order harmonics are due to the electron acceleration at the instant of ionization at the peak intensity of the © Springer Science+Business Media Singapore 2016 R.A. Ganeev, Frequency Conversion of Ultrashort Pulses in Extended Laser-Produced Plasmas, Springer Series on Atomic, Optical, and Plasma Physics 89, DOI 10.1007/978-981-10-0194-9_1
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Introduction. Theory and Experiment …
laser pulse. Therefore, the generation of highest-order harmonics in isotropic media results from the interaction of a high-intensity light field with atoms [4–6], atomic clusters [7, 8], molecules [9, 10], and ions [11–16]. A characteristic feature of the three-stage HHG is a rapid decrease in the intensity of low-order harmonics followed by a long plateau where the intensities of high-order harmonics differ only slightly from one another, and an abrupt decrease in the intensity of the highest-order harmonics generated (the so-called harmonic cut-off Hc). The position of Hc is determined by the ionization pot
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