6 - Strong Field Nonlinear Optics

Summary

Atoms subject to intense laser light will witness their bound electrons undergo nonlinear oscillatory motion and subsequent field-driven ionization. This nonlinear electron motion will reradiate electromagnatic waves at harmonic multiples of the laser frequency, often to high nonlinear orders. This chapter explores this high-order harmonic generation, considering the single-atom response combined with the coherent addition from many atoms leading to spatially coherent, short-wavelength radiation. After a phenomenological justification of high harmonics in which a quasi-classical three-step model is employed to describe harmonic generation from field-ionizing atoms, the chapter launches into a quantum description of the single-atom nonlinear dipole reponse using the strong field approximation. The following sections then address the effects of propagation and phase-matching through an extended media of nonlinearly driven atoms, assessing the effects of the geometric, intensity-dependent, and plasma-induced phase on the spatial coherence of the generated harmonics. The concluding section looks at the effect of attosecond pulse generation that accompanies harmonic production.