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Physical Review A

Third-harmonic generation from a laser-induced autoionizing level

Burke Ritchie

Published Feb 1, 1985• 11 citations
Publisher

Abstract

A nonperturbative theory is presented for the gas polarization induced by two lasers. The first laser ionizes Li(2s) by three-photon absorption, with the first two photons in resonance with the 3d level. A second laser stimulates recombination into the 3s level, thereby inducing an autoionizing-like structure in the continuum. It is found that the polarization (and hence possibly third-harmonic generation) is enhanced near the induced resonance.

DOI
10.1103/physreva.31.823

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Physical Review Letters•May 17, 1993•76 citations
A nonperturbative theory is presented for the gas polarization induced by two lasers. The first laser ionizes Li(2s) by three-photon absorption, with the first two photons in resonance with the 3d level. A second laser stimulates recombination into the 3s level, thereby inducing an autoionizing-like structure in the continuum. It is found that the polarization (and hence possibly third-harmonic generation) is enhanced near the induced resonance.
Multiphoton ionization in the region near a three-photon resonance is treated for focused, plane-polarized Gaussian beams with diffraction-limited beam divergence. In this situation, a third-harmonic field is generated within the laser beam. At, and very near, three-photon resonance the driving rate for the upper-state probability amplitude due to one-photon absorption of third-harmonic light becomes nearly equal to the corresponding three-photon rate due to the laser field, but these effects are 180\ifmmode^\circ\else\textdegree\fi{} out of phase. As a consequence of this cancellation between two pumping terms, the three-photon resonance line essentially disappears at moderate concentrations and the observed ionization has a line shape that is close to the phase-matching curve for third-harmonic generation. The ionization signal, near but not on the resonance, is due almost entirely to absorption of third-harmonic photons plus other laser photons; three-photon resonantly enhanced multiphoton ionization by the laser is much weaker. This is particularly true on the blue side of the three-photon resonance at detunings where phase matching occurs. The problem is treated quite generally with predictions of the full line shape for nnn-photon ionization and third-harmonic light generation near three-photon resonance, including the rather strong influences of positively dispersive buffer gases. We also show that the cancellation between the one-photon and the three-photon process is partially spoiled in the presence of a counterpropagating beam at the same frequency.
A two-laser theory is formulated for the photoionization of Li atoms. The first laser is tuned above the ionization threshold of Li(2s)\mathrm{Li}(2s)Li(2s). The second laser probes the ionized target. Stimulated recombination into Li(ns)\mathrm{Li}(\mathrm{ns})Li(ns) occurs near the Raman-type resonance point ${\ensuremath{\epsilon}}_{\mathrm{ns}}\ensuremath{-}{\ensuremath{\epsilon}}_{2s}\ensuremath{-}{\ensuremath{\epsilon}}_{1}+{\ensuremath{\epsilon}}_{2}=0$ (where n>2n>2n>2 and ${\ensuremath{\epsilon}}_{q}$ is a photon energy). Near the resonance point the ionization probability is characterized by a line profile which resembles that for an autoionizing resonance.
Experimental investigations of the laser-induced continuum structure in the vicinity of autoionizing states (AIS's) of Ca are reported for both ionization and third-harmonic generation. The shape of the induced structures is shown to be strongly affected by the presence of the AIS's, as well as exhibiting a detuning- and laser-intensity-dependent transition from a window resonance dip to a peak. The observed modification of the continuum is discussed in terms of interfering excitation channels.
We theoretically study intensity-saturation effects in resonant harmonic generation and multiphoton ionization. A first laser excites a two-photon resonance which is coupled by a second laser to an autoionizing state. Starting from an effective Hamiltonian for the three resonant atomic states, we derive a set of equations for the density matrix of the gaseous medium and the electromagnetic field within a semiclassical framework. We present and discuss analytical and numerical solutions of these equations which show a variety of line profiles depending critically on the intensities of the incident laser pulses.