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The Journal of Chemical Physics

The enhancement of intense laser induced multiphoton processes by a static electric field

Gerald F. Thomas

Published Nov 15, 1983• 11 citations
Publisher

Abstract

The multiphoton absorption processes induced in model two-level systems through their interaction with intense laser and static electric fields are investigated. An analytical treatment of the degenerate system enables one to anticipate the effects of the static field on the multiphoton spectrum, including the influences of collisional damping. Using Floquet theory in the numerical treatment of a nondegenerate system confirms the enhancement of the multiphoton spectrum whose resonances will be more power broadened and will appear at higher frequencies than they would in the absence of the static field.

DOI
10.1063/1.445583

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The multiphoton absorption processes induced in model two-level systems through their interaction with intense laser and static electric fields are investigated. An analytical treatment of the degenerate system enables one to anticipate the effects of the static field on the multiphoton spectrum, including the influences of collisional damping. Using Floquet theory in the numerical treatment of a nondegenerate system confirms the enhancement of the multiphoton spectrum whose resonances will be more power broadened and will appear at higher frequencies than they would in the absence of the static field.
The non linear interaction between an intense laser radiation and atoms leads to ionization through the absorption of N photons from the laser radiation via laser-induced virtual states. The multiphoton ionization rate varies as a function of the laser intensity I as IN. We discuss the two most important effects which govern multiphoton ionization processes : resonance effects and laser-coherence effects. In a moderate laser intensity range (107 - 109 W cm-2) corresponding to the two, three or four-photon ionization of atoms, resonance effects play a dominant role while the photon statistics of the laser radiation are relatively unimportant. On the contrary, in the very high intensity range (1012 - 1013 W cm-2) required to observe large Nth - order ionization processes, coherence effects play a dominant role through N! enhancement in the N-photon ionization rate, while resonance effects are completely damped due to the high laser intensity. We conclude by reviewing the possible applications of multiphoton absorption processes in other fields.
The authors present an exact treatment of two- and multiquantum transitions in three-level systems driven by two intense linearly polarised monochromatic fields based on the semiclassical many-mode Floquet theory developed recently. Further they extend the almost degenerate perturbation theory of Salwen (1955-6) to the two-mode Floquet Hamiltonian and obtain approximate analytical formulae for multiphoton transition probabilities, resonance bichromatic shifts and widths, and absorption lineshapes beyond the conventional rotating-wave approximation (RWA). Detailed comparison of the analytical, the RWA, and the exact results is given. Several novel features of multiphoton lineshape characteristics are pointed out.
Within the rotating-wave and Magnus approximations, the electrical anharmonicity induced in an atom or molecule aligned by a static electric field is shown to lead to the absorption of many photons delivered by a Gaussian-modulated laser pulse of finite intensity and duration. In the presence of the static field the effective area of the near-resonant pulse vanishes at prescribed intensity-dependent frequencies, resulting in the appearance of satellite fringes flanking the usual Gaussian line shape. The transition probability for excitation of the oriented system by a train of phase-coherent Gaussian pulses is calculated.
We extend the many-mode Floquet theorem (MMFT) for the investigation of multiphoton resonance dynamics driven by intense frequency-comb laser fields. The frequency comb structure generated by a train of short laser pulses can be exactly represented by a combination of the main frequency and the repetition frequency. MMFT allows non-perturbative and exact treatment of the interaction of a quantum system with the frequency-comb laser fields. We observe simultaneous multiphoton resonance processes between a two-level system and frequency-comb laser. The multiphoton processes can be coherently controlled by tuning the laser parameters such as the carrier-envelope phase (CEP) shift. In particular, high-order harmonic generation shows immense enhancement by tuning the CEP shift, due to simultaneous multiphoton resonances.
For pt.I see Chem. Phys. Lett. vol.96, p.464 (1983). The non-linear multiphoton excitation dynamics of two-level systems under the influence of two strong linearly polarised monochromatic fields is studied exactly for the first time along the semiclassical many-mode Floquet theory. The present approach relaxes the restrictions of the conventional generalised rotating-wave approximation (GRWA) and allows correct treatment of various non-linear features such as power broadening, resonance shift, asymmetry in absorption lineshape, etc. By extending the nearly degenerate perturbative method of Salwen (1955), approximate analytical expressions for transition probabilities are obtained for multiphoton resonance processes for which (n+1) photons of one field are absorbed, whereas n photons of the other field are emitted, where n is an arbitrary integer. Detailed comparison of the analytical, the GRWA and the numerically exact results is given. Also presented is a case study of the time evolution of spin 1/2 systems in multi-quantum NMR transitions driven by pulsed bichromatic radiofrequency fields.