Quantum electrodynamic theory of optical high-order harmonic generation

A theoretical description of high order harmonic generation has been developed that draws upon quantum electrodynamic scattering theory and relationships developed previously for multiphoton process. In the full quantum description, both the fundamental optical field and the harmonic photon field are represented by quantized fields and harmonic photon generation processes are described by transition probabilities. Analytical expression of the transition matrix is obtained with defined energy and momentum relationships for both electrons and photons. Calculations using the obtained formula reproduce the salient characteristics of the experimental data with no adjustable parameters.; The most significant thing about the thesis is that the framework presented here offers a new perspective on the harmonic generation mechanism; it is demonstrated that the quantized field Volkov → Volkov states transition represents the dominant contribution to the harmonic photon generation. This suggests that at high intensity the electron-photon interaction is directly responsible for the secondary radiation process instead of the electron-Coulomb-field interaction.; The theory also demonstrates the clear link between the high order harmonic generation and the above threshold ionization (ATI) process. For the first time, a quantitative relationship between the two intense field phenomena is established and furthermore it is observed that the discrete nature of the photoelectron spectrum is preserved and mapped onto the harmonic generation process. Comprehensive comparing to the experimental data suggests that only the lowest few orders of ATI photoelectrons contribute to the harmonic generation in the experiments reported to date.; Based on the theoretical analysis, harmonies with higher orders are predicted to be generated with higher efficiency, if a means is devised to utilize more energetic photoelectrons to the harmonic generation process.