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Physical Review A

Optical second-harmonic generation in atomic vapors with focused beams

Donald S. Bethune

Published Jun 1, 1981• 93 citations
Publisher

Abstract

Processes leading to second-harmonic generation when a single polarized beam is focused into neutral or partially ionized atomic vapor are considered, including electric-field-induced third-order mixing, and quadrupole and magnetic dipole processes. Estimates of the relative strengths of these processes are given. In addition, the polarization properties given by the tensor susceptibilities are discussed in detail.

DOI
10.1103/physreva.23.3139

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Soviet Journal of Quantum Electronics•May 31, 1981•3 citations
Processes leading to second-harmonic generation when a single polarized beam is focused into neutral or partially ionized atomic vapor are considered, including electric-field-induced third-order mixing, and quadrupole and magnetic dipole processes. Estimates of the relative strengths of these processes are given. In addition, the polarization properties given by the tensor susceptibilities are discussed in detail.
It is shown that optical second-harmonic generation is possible in dispersive atomic vapors since a static electric field is spontaneously created by the fundamental laser beam. When multiphoton ionization of atoms occurs simultaneously in the interaction, the spontaneous field and second-harmonic generation are significantly enhanced. Our experimental results for atomic sodium vapor irradiated by picosecond Nd-doped yttrium-aluminum-garnet laser pulses agree well with the theoretical predictions.
The formalism of Maker, Terhune, and Savage [Phys. Rev. Lett. 12, 507 (1964)] for third-order nonlinear susceptibilities is extended to account for nonlocal response. In the case of counterpropagating beams, we show that five constants instead of two are required in general to describe the nonlinearity. In the particular case of optical pumping nonlinearities, which we study extensively, we show that four constants, which describe the magnetization and electric-quadrupole moment of the medium induced by the forward and backward beams, are sufficient. We evaluate these constants for various values of the angular momenta of the atomic levels connected by the incident field. We finally show how this formalism can be applied to several problems such as induced focusing, four-wave-mixing generation, optical instability, and polarization properties of phase-conjugate and phase-contrast mirrors.
Second harmonic generation (SHG) in atomic Li vapor is studied when the 2 2 S 1/2 -3 2 S 1/2 two-photon transition is resonantly excited. This is an SHG in a medium with inversion symmetry, with no externally apptied electric or magnetic field, and with no apparent multipole moments associated with the resonant excitation. The SH polarization is found to be parallel to the intensity gradient of the incident laser beam when a cylindrical lens focusing is applied. The mechanism of this highly forbidden SHG is explained by the radial electric field induced by free expansion of photo-electrons around the laser beam axis.