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Physical Review A

Efficient generation of moderately high harmonics by multiphoton resonant excitation of atoms

H. K. Avetissian, B. R. Avchyan, G. F. Mkrtchian

Published Feb 11, 2008• 19 citations
Publisher

Abstract

The coherent scattering spectrum of a three-level atom driven by an intense laser field with multiphoton resonant excitation is considered. The spectrum corresponding to harmonic generation is investigated both analytically and numerically for different initial atomic states. The analytical calculations are based on the generalized rotating wave approximation, which allows one to explain obtained spectra containing in general both even and odd harmonics. The results applied to the hydrogen atom show that one can achieve efficient generation of moderately high harmonics using multiphoton resonant excitation by appropriate laser pulses.

DOI
10.1103/physreva.77.023409

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The coherent scattering spectrum of a three-level atom driven by an intense laser field with multiphoton resonant excitation is considered. The spectrum corresponding to harmonic generation is investigated both analytically and numerically for different initial atomic states. The analytical calculations are based on the generalized rotating wave approximation, which allows one to explain obtained spectra containing in general both even and odd harmonics. The results applied to the hydrogen atom show that one can achieve efficient generation of moderately high harmonics using multiphoton resonant excitation by appropriate laser pulses.
We consider harmonics generation and wave mixing by two-color multiphoton resonant excitation of three-level atoms and molecules in strong laser fields. The coherent part of the spectra corresponding to multicolor harmonics generation is investigated. The obtained analytical results on the basis of a generalized rotating wave approximation are in a good agreement with numerical calculations. The results applied to the hydrogen atoms and homonuclear diatomic molecular ions show that one can achieve efficient generation of moderately high multicolor harmonics via multiphoton resonant excitation by appropriate laser pulses.
An analytic study is presented of the efficient multiphoton excitation and strong harmonic generation in three-level systems specified by a pair of nearly degenerate, strongly dipole-coupled excited states. Such systems are physically formed by the three lowest states in, e.g., the hydrogen atom or evenly charged homonuclear diatomic molecular ions under reasonably chosen laser intensities. As a detailed analytic result, we found that the laser pulse of photon energy 2.05eV2.05\text{eV}2.05eV, duration 0.23ps0.23\text{ps}0.23ps, and intensity $5\ifmmode\times\else\texttimes\fi{}{10}^{13}\frac{\mathrm{W}}{{\mathrm{cm}}^{2}}$ is able to produce complete inversion of the initial population in the hydrogen atom through the five-photon excitation. At the same photon energy, the pulse of duration 0.41ps0.41\text{ps}0.41ps and intensity $3.44\ifmmode\times\else\texttimes\fi{}{10}^{14}\frac{\mathrm{W}}{{\mathrm{cm}}^{2}}$ was found to produce the same effect in the molecular ion but through the nine-photon excitation. We show that the accompanying scattering of light has very rich spectrum differing substantially from that of the two-level system.
The multiphoton resonant excitation of three-level atoms in a strong laser field is investigated. The creation of various superposition states is shown. For implementation of these effects either the mean dipole moment in the excited states of an atom or, in three-level systems, a transition dipole moment between excited states (with energies close enough to each other) must be nonzero. The time evolution of the system is found using a nonperturbative resonant approach. Our calculations for the hydrogen atom suggest that by the appropriate optical pulses with moderate strong intensities one can achieve various superposition states by multiphoton resonant excitation.
The dynamics of high-intense laser fields at coherent Raman self-scattering are investigated by the use of numerical simulation. The processes under investigation take place at the Raman scattering on electronic states in atoms with transition to be forbidden in electro-dipole approximation, on vibrational or rotational states in molecules, on states connected with the spin flip and other. It is found that the threshold intensities exist provided that the pulse duration is smaller than dephasing time of the Raman transition in the medium. Under this, the following processes occur: an effective generation of the high-order harmonics comes up; the plateau forms in the harmonic spectrum. The maximum which arises on this plateau shifts to the higher-harmonic numbers in case of the higher threshold intensities of the pulse. It is shown how the transformation of the initial pulse intensity into high-order harmonics intensity can be considerably increased with the different initial parameters of the pulse and the medium.
We develop a Floquet based formalism which describes a process of the generation harmonics of in an atom driven by a laser field. Using this formalism, we show that several harmonics are strongly enhanced in the spectrum when the laser frequency is such that a multiphoton resonance occurs between the initial state and a quasienergy state. We illustrate these findings by a numerical calculation of the harmonics generation in the potassium and rubidium atoms driven by a laser in the near-infrared energy range. The present formalism is also applicable when the initial state is a coherent superposition of several field-free atomic states.