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Soviet Journal of Quantum Electronics

Determination of atomic level populations by generation of third-harmonic ultrashort optical pulses

Yu. P. Malakyan, David G. Sarkisyan, A. S. Sarkisyan

Published Aug 31, 1991• 0 citations
Publisher

Abstract

A simple method of measuring atomic level populations was implemented by comparing the energies of the third harmonic generated by two identical ultrashort pulses. When the first ultrashort pulse induces a process that removes some of the atoms from the ground (initial) level for third harmonic generation, the energy of the second third-harmonic pulse Wth2 becomes lower than Wth1 as a result of a reduction in the number of atoms. In the experiments K = Wth1 / Wth2 increased as the ultrashort pulse intensity increased. Experimental investigations and theoretical estimates showed that the most probable process causing a reduction in the population of the 62S1/2 level of cesium atoms was multiphoton ionization. It was found that the degree of multiphoton ionization could be estimated from K.

DOI
10.1070/qe1991v021n08abeh003970

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A simple method of measuring atomic level populations was implemented by comparing the energies of the third harmonic generated by two identical ultrashort pulses. When the first ultrashort pulse induces a process that removes some of the atoms from the ground (initial) level for third harmonic generation, the energy of the second third-harmonic pulse Wth2 becomes lower than Wth1 as a result of a reduction in the number of atoms. In the experiments K = Wth1 / Wth2 increased as the ultrashort pulse intensity increased. Experimental investigations and theoretical estimates showed that the most probable process causing a reduction in the population of the 62S1/2 level of cesium atoms was multiphoton ionization. It was found that the degree of multiphoton ionization could be estimated from K.
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Resonant enhancement of optical third-harmonic generation in cesium vapor has been observed when twice the energy of the fundamental photon becomes equal to the energy difference between the 9d2D329d^{2}D_{\frac{3}{2}}9d2D23​​ level and the ground state of the cesium atom. The measured, effective coefficient is ${10}^{\ensuremath{-}30}$ esu/atom, which is more than a factor 108{10}^{8}108 larger than the corresponding coefficient for the helium atom. An effective coefficient is synthesized by first calculating the coefficient for a single cesium atom at rest and then including the effects of Doppler broadening and the fundamental laser spectrum. The calculated value is in satisfactory agreement with experiment.
Abstract Experiments involving resonantly enhanced multiphoton ionization (REMPI) of alkali atoms contained in a heat pipe are discussed with reference to the quantum defect theory modification of the hydrogen atom energy levels. Quantum defects for the ns and nd levels are extracted from the data. This experiment is an excellent introduction to atomic spectroscopy and the selection rules involved. Ionization potentials are extracted from the Rydberg spectra. In the second part of this chapter a Raman shifter is employed to produce IR photons for multiphoton excitation of the low-lying p states of cesium. Three-photon excitation also produces third harmonic generation. The use of heat pipes for the production of alkali metal atom vapor is discussed as well as the use of a simple pick-up wire for ion detection.
We investigate the third order harmonic generation for model atoms interacting with intense short laser pulses. We focus on the cases where the ponderomotive energy is not big enough to generate high-order harmonics, and where resonant transitions between bound states are also as important as multi-photon ionization and low-order harmonic generation. We find that the third order harmonic intensity from a model hydrogen atom can decrease as the laser frequency draws near a resonance between bound states. We explain this result by looking into the ionization and Rabi oscillation from the numerical calculation of the time-dependent 1-D Schrödinger equation. In order to support our explanation we also investigate the populations of bound states that play an important role in this atomic process.
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