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AIP conference proceedings

The saturation effect of the parametric emission in potassium atoms under two-photon excitation

A. Armyras, Dionysios Pentaris, Nikolaos Merlemis, A. Lyras, Thomas Efthimiopoulos, Nikolaos A. Vainos, Stelios Couris, Emmanuel Paspalakis, Ioannis B. Koutselas, Stavros Pissadakis

Published Jan 1, 2010• 4 citations
Publisher

Abstract

We show that there are three distinct regions corresponding to the internally produced axial parametric emissions. Namely an exponentially grown region, of the 5 P3/2‐4 S1/2 emission, as a function of the laser peak intensity, a linear region due to mainly to the destructive quantum interference of the pump excitation and the produced emissions, and a region of complete saturation due to the considerable transfer of population to the excited states and the activation of an alternative available emission path.

DOI
10.1063/1.3521378

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Journal of Physics B Atomic Molecular and Optical Physics•May 25, 2017•7 citations
We show that there are three distinct regions corresponding to the internally produced axial parametric emissions. Namely an exponentially grown region, of the 5 P3/2‐4 S1/2 emission, as a function of the laser peak intensity, a linear region due to mainly to the destructive quantum interference of the pump excitation and the produced emissions, and a region of complete saturation due to the considerable transfer of population to the excited states and the activation of an alternative available emission path.
A theoretical study of the nonlinearity reduction in a four-level system of a potassium atom in the presence of a strong nanosecond laser field, which excites the transition |4S 1/2 ⟩↔|6S 1/2 ⟩ with two photons, is presented. It is shown that the destructive quantum interference between the laser field and the internally generated radiations results in a linear response of the atomic path-1 (|4S 1/2 ⟩↔|6S 1/2 ⟩↔|5P 3/2 ⟩↔|4S 1/2 ⟩) emitted parametric fields. For sufficiently high laser intensities and/or atomic densities, the path-1 emitted fields are driven into saturation, a regime accompanied by a substantial population redistribution among the states. Reasonable agreement between earlier experimental results and the theoretical ones is obtained. It is also shown that upon saturation of path-1, for low atomic density, the path-2 (|4S 1/2 ⟩↔|6S 1/2 ⟩↔|4P 3/2 ⟩↔|4S 1/2 ⟩) is activated. A subtle interplay between laser intensity and atomic density may determine the activation of path-2. Also, it is shown that the path-2 emission |6S 1/2 ⟩↔|4P 3/2 ⟩ is an amplified spontaneous emission process which induces a |4P 3/2 ⟩↔|4S 1/2 ⟩ emission, without population inversion, in a cascade scheme.
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.
A numerical study of the emissions of a four-level potassium atom excited either by a single pulse or by a pair of temporally separated pulses is presented. For a low-intensity single-pulse two-photon excitation (|4S1/2〉↔|6S1/2〉), parametric emissions along path-1 (|6S1/2〉↔|5P3/2〉↔|4S1/2〉) are generated while along path-2 (|6S1/2〉↔|4P3/2〉↔|4S1/2〉) are essentially inactive. But for high single-pulse excitation, the path-1 emissions saturate and delayed path-2 amplified spontaneous emissions appear. It is shown that in the case of a double-pulse excitation scheme, proper temporal separation of the pulses and saturation of path-1 emissions caused by the first pulse can be applied to selectively transfer the excitation energy from path-1 to path-2 emissions.
Abstract\nWe investigate the emissions of atomic potassium excited by two photons near the 4S1/2-6S1/2 transition. Radiation near the 6S1/2-4P3/2, 1/2 and 4P3/2, 1/2-4S1/2 transitions are studied for the unidirectional and the bidirectional cases respectively. It is shown that the total output intensity increases considerably for the bidirectional propagation. This is attributed to the effective increase of the Doppler free excitation and the cancellation of the quantum interference effect that is associated with the multiple paths of excitation of certain states. Keywords. Counter-propagating beams, amplified spontaneous emission (ASE), stimulated hyper Raman scattering (SHRS), parametric four-wave mixing (PFWM).