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Physical Review A

Laser-induced continuum structure and third-harmonic generation in sodium: Theory and calculations

Jian Zhang, P. Lambropoulos

Published Jan 1, 1992• 23 citations
Publisher

Abstract

We present a theoretical study of the influence of laser-induced continuum structure on the third-harmonic generation in atomic gases. We employ the density-matrix formalism including the spatiotemporal structure of laser pulses. The theoretical model is then applied to third-harmonic generation in sodium experiments. Extensive numerical calculations using realistic atomic parameters in combination with the parameters in actual experimental situations demonstrate the subtle interplay of various effects and pinpoint causes of possible complications. We obtain good agreement with the few existing experimental data.

DOI
10.1103/physreva.45.489

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Optics Letters•Sep 20, 2013•4 citations
We present a theoretical study of the influence of laser-induced continuum structure on the third-harmonic generation in atomic gases. We employ the density-matrix formalism including the spatiotemporal structure of laser pulses. The theoretical model is then applied to third-harmonic generation in sodium experiments. Extensive numerical calculations using realistic atomic parameters in combination with the parameters in actual experimental situations demonstrate the subtle interplay of various effects and pinpoint causes of possible complications. We obtain good agreement with the few existing experimental data.
Observation of laser-induced continuum structure (LICS) in sodium due to the coupling of a bound state to the continuum by a strong laser field has been reported elsewhere. Such experiments have mostly employed a single laser photon to probe the structured continuum from the ground state. Other experiments have used multiphoton (third harmonic) probe schemes, but only one of these has successfully observed LICS, in agreement with theoretical predictions. We report here a very similar experiment generating third-harmonic radiation in sodium to probe the anticipated LICS produced by coupling the 5s state to the p continuum. However, under no conditions was any continuum structure observed, in agreement with in a number of other experiments, but at variance with theoretical predictions.
Structure in the continuum of an atomic system attributed to the coupling of a bound state by a strong laser field has been reported elsewhere (1-3). Such experiments have employed a second laser to probe the continuum from the ground state by measuring polarization rotation 0), third harmonic generation (2) or three photon ionization (3). We report here an experiment similar to reference 2 using two photon resonant, third harmonic generation in sodium, perturbed by a second laser which couples the 5s state to the continuum. Under no conditions was any laser induced continuum structure (LICS) observed in these experiments, in contrast to the results reported elsewhere (2).
The effect of temporal coherent control on the two-photon resonant third-harmonic generation in the continuum of ionization of an atomic system is experimentally investigated by means of a pair of partially incoherent, identical laser pulses. The evidence of quantum interference in the third-harmonic generation, as a function of delay between pulses, is reported. The transition from a resonant to a nonresonant case is also shown. The results agree with the predictions of a recently published theoretical model showing that the onset of quantum interference appears to be related to the coherence time of the laser radiation that is much shorter than the pulse duration.
In femtosecond degenerate four-wave mixing experiments in sodium vapor, quantum beats are observed at the wavelength of the third harmonic when the fundamental beams are three-photon resonant. The period of beating, which corresponds to fine structure energy-level splitting, does not change with the atomic number density, buffer gas pressure, or laser intensity. Surprisingly, the third-harmonic signal is observed only at negative time delays for the transient grating pulse sequence. We relate this to the well-known interference effect between excitation pathways involving the fundamental and the third-harmonic fields, which leaves atoms virtually unexcited and prevents formation of population grating.