This presentation enhances the understanding of the underlying mechanism behind the generation of higher harmonics from electronic self-phase modulation which is driven by the Electric Field E(t) of the optical, NIR, and MIR femtosecond light pulses to produce attosecond laser pulses and providing the direction to produce zeptosecond laser pulses. The HHG arises from driving the phase of the electric field, E(t) by itself to create HHG in form of odd Bessel functions in time of odd harmonics driven by laser pulse from the nonlinear refractive index of Kerr media, n2 (χ3) and extending to even HHG from n1 (χ2).The three characteristic features of HHG spectrum are initial decreasing harmonics, a plateau region of HHG, and cutoff frequency. We show theoretically in the EM Kerr electronic self-phase modulation (ESPM) model that the distinctive features of HHG arise mainly from the changes in the phase of the E(t) wave driven by the envelope of the laser pulse causing the cosine of the cosine squared for χ3 and the cosine of the cosine for χ2 in time. The outcomes are Bessel function expansions producing odd harmonics for n2 (χ3) and even harmonics for n1 (χ2).
A universal model is described for Higher Harmonic Generation (HHG) in different states of matter. Based on an electromagnetic model (EM), the generation of Odd Higher Harmonic (HHG) and Supercontinuum (SC) from intense fs and ps pulses for visible, NIR, and MIR lasers is simulated for the different Kerr material response times τ from the ultrafast on the order of 100 attoseconds for electronic cloud distortion to fast ~10 femtoseconds from plasma and molecular redistribution and to the slower picoseconds rotational and vibrational molecular processes. The number of odd HHG generated is shown to depend critically on the fastest Kerr response time on the order of ~ a femtosecond from electronic self-phase modulation (ESPM). The EM model is universal for the production of HHG and SC in different states of matter.
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