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Journal of Physics B Atomic Molecular and Optical Physics

Action of counter-propagating laser beams on two-photon excitation of potassium vapour

Thomas Efthimiopoulos, Nikolaos Merlemis, M. E. Movsesyan, Dionysios Pentaris, M. Katharakis

Published Feb 2, 2010• 6 citations
Publisher

Abstract

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).

DOI
10.1088/0953-4075/43/5/055401

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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).
The two‐photon excitation of 4S1/2–7S1/2 transition of potassium atoms is studied. Several coherent emissions and processes are possible, such as parametric four‐wave (PFWM), parametric six‐wave (PSWM) mixing and competition with the stimulated hyper Raman (SHRS) and the amplified spontaneous emission (ASE). The radiations at the transitions 6P3/2,1/2−4S1/2, 6S1/2−4P3/2,1/2 and 5P3/2,1/2−4S1/2 are emitted only in the forward direction (indicating a parametric process), while the radiation at the transition 4P3/2,1/2−4S1/2 is emitted in the forward and in the backward direction, indicating an ASE process.
Potassium 4S1/2 - 6S1/2 two-photon excitation initiates the emission of several internally generated photons. For the first time two emission lines one close and one below the potassium 4P3/2 level are reported. Radiation emitted below the 4P3/2 - 4S1/2 level is due to a Parametric Four Wave Mixing (PFWM) process initiated by the internally generated radiation from 5P3/2 - 4S1/2 level. The line emitted close to the 4P3/2 level is produced by a two-step Four Wave Mixing (FWM) process. Direct one photon driving of 4S3/2 - 4P3/2 transition produces similar to HEP radiation.
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.
It is shown that by proper use of the excitation spectra, the emission spectra of the transition and the far-field patterns it is possible to distinguish between competing processes involved in the two-photon excitation to the 6S state of potassium, namely: (a) resonant fluorescence emission associated with the cascading spontaneous emission (ASE) and (b) the four-wave mixing parametric process. The effect of the buffer gas on the intensities of the spectra of the radiations emitted is discussed. Conical emission and spatial instabilities of the excitation radiation and the emission were also observed for high excitation and high vapour pressure of potassium.
If an extended Na vapor column (heat pipe configuration) is subjected to strong laser pumping near two-photon resonance with 3D3/2,5/2 or 4D3/2,5/2 states at pressures above tens of millitorr, strong stimulated emissions are produced by amplified spontaneous emission (ASE) or stimulated electron in hyper-Raman scattering (SEHRS) and by phase matched parametric four-wave mixing (PFWM). These new electromagnetic fields that are generated within the near-resonant nonlinear medium can strongly influence the total atomic response and produce several saturation and total cancellation types of effect.
The stimulated hyper-Raman scattering (SHRS) emissions associated with the excitation of the 4P 3/2 state in sodium vapour have been observed in both the forward and backward directions as the pumping laser was tuned near the 3S-4D two-photon resonance. The results show that though the forward SHRS process is greatly reduced by a destructive interference between it and the four-wave mixing (FWM) process induced by itself, the forward SHRS emission has a positive gain and can still be observed under a certain condition.