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Journal of Physics B Atomic Molecular and Optical Physics

Optical second harmonic generation from low-lying excited states in potassium vapour

Michael S.-C. Lu, J Tsai

Published Mar 14, 1990• 4 citations

Abstract

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.

DOI: 10.1088/0953-4075/23/5/014

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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.

Two-photon resonant optical processes, using potassium ns2S1/2 states (7 ≤ n ≤ 20), are investigated in detail by a pulsed dye laser. About 100 coherent lines between 288–422 nm are generated through the processes ωUV ± ωIR = 2ωL and [Formula: see text]. The strongest lines (near mP–4S transitions) are formed by parametric oscillation and are generally split into components that are frequency shifted relative to atomic transitions. About 70 lines are generated by six-wave mixing processes and 15 other lines by four-wave mixing processes. The wavelengths, relative intensities of the lines, and the components of the strongest lines are explained with a theoretical analysis based on the wave equation, atomic polarization, and phase-match condition for both wave-mixing processes and parametric oscillations.

The interaction of a laser with potassium atoms is investigated by monitoring the radiation of the 5P-4S transition under the two-photon 4S(1/2)-6S(1/2) excitation. For low number density and laser intensity the observed radiation is parametric axial or conical, depending on the laser detuning from the two-photon resonance, the number density and the laser intensity. With increasing laser intensities and number densities, an additional delayed component is observed corresponding to a two-step four-wave mixing with the participation of internally generated photons. A two-photon quantum interference effect is observed for the total (axial and conical) parametric four-wave mixing emission. A deviation from the proposed model under certain experimental parameters is attributed to a two-step four-wave mixing and the phase mismatch of the parametric processes.