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Journal of Physics B Atomic Molecular and Optical Physics

Two-photon resonant optical second harmonic generation from s, p and d states of K atoms

Stoyan Dinev

Published May 14, 1988• 14 citations
Publisher

Abstract

Second harmonic generation in potassium atoms has been studied under two-photon resonant excitation. High-lying s, p and d states (n=7-34) are involved. Intensity, density and polarisation characteristics of the second harmonic have been measured as well as the influence of a foreign gas. Comparison with two theoretical models (multiphoton ionisation DC-induced field and collisional l-mixing of states) shows a reasonable agreement.

DOI
10.1088/0953-4075/21/9/025

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Second harmonic generation in potassium atoms has been studied under two-photon resonant excitation. High-lying s, p and d states (n=7-34) are involved. Intensity, density and polarisation characteristics of the second harmonic have been measured as well as the influence of a foreign gas. Comparison with two theoretical models (multiphoton ionisation DC-induced field and collisional l-mixing of states) shows a reasonable agreement.
Optical second harmonic generation has been observed on or near the 4s 2 S 1/2 -6s 2 S 1/2 , 4s 2 S 1/2 -np 2 P 1/2, 3/2 (n=5, 6) and 4s 2 S 1/2 -nd 2 D 5/2, 3/2 (n=3,4) two-photon resonances in potassium vapour, as laser scanning in the 660-940 nm wavelength range. The second harmonic intensities and ionisation currents have been measured as a function of pumping laser detuning from the two-photon resonances. On or near the two-photon resonances, the intensity and vapour-density dependence of second harmonic generation have also been investigated. The saturation of second harmonic generation has been observed. Some calculations and analyses have been made with the chi (3) model. The results have been compared with the measurements. Based on these analyses, the mechanisms for second harmonic generation in isotropic vapours are discussed.
Picosecond laser pulses were used to study the highly forbidden resonant second-harmonic generation (SHG) in potassium vapor. The input intensity dependence, vapor density dependence, buffer-gas pressure dependence, and spatial profile of the SHG were measured. A pump–probe experiment was conducted to probe the time dependence of the SHG signal. The experimental results can be understood from an ionization-initiated dc-field-induced SHG model. A theory of a dc-field-induced SHG model is developed that takes into account the time development of the dc electric field in detail. This temporal buildup of the dc field along with transient coherent excitation between two-photon-allowed transitions can explain the experimental results quantitatively, including the previous vapor SHG results with nanosecond laser pulses.
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.