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Journal of Optics B Quantum and Semiclassical Optics

Third-harmonic generation and laser-induced continuum structure in sodium

K. G. H. Baldwin, Mario Bott, Hans‐A. Bachor, Philip B. Chapple

Published Jun 21, 2000• 3 citations
Publisher

Abstract

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.

DOI
10.1088/1464-4266/2/4/303

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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).
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.
We report the observation of a structure in the ionization spectrum of sodium, induced by a strong radiation field embedding the 5s 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​ state into the continuum. The structure, probed by single-photon ionization, shows a marked asymmetry, typical of a laser-induced continuum structure.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
Laser induced continuum structures (LICS) can be produced by a strong laser embedding an excited atomic bound state into a flat atomic continuum, leading to a tunable resonance of adjustable width that can be probed by a second laser. If this second laser is of similar intensity to the embedding laser then the distinction between the two becomes artificial and saturation effects become important. In this paper the effect on LICS of transitions caused by both lasers between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods and allowing for the ionization of each atomic state by both laser fields. Formal results correct to all orders are also given in terms of a T-matrix approach. Numerical calculations are presented showing the effects on the LICS resonant structures of continuum-continuum coupling processes. Significant changes to the Fano profiles (second laser weak) and to coherence holes (second laser strong) occur. Analytical results are also given.