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Journal of Modern Optics

Temporal dynamics of the internally generated radiations in a two-photon excited four-level potassium atom

Dionysios Pentaris, Thomas Efthimiopoulos, Nikolaos Merlemis, A. Lyras

Published Dec 1, 2011• 7 citations
Publisher

Abstract

A numerical study, stimulated by existing experimental results concerning the temporal dynamics of internally generated radiations, is presented for a four-level system of the potassium atom. An intense nanosecond duration laser pulse excites the two-photon transition and initiates the generation of internal radiations from quantum noise. It is shown that the temporal profiles of the generated radiations along the atomic path-1, ( ) and path-2, ( ), evolve differently as a function of certain system parameters. It is also shown that the excitation laser intensity affects the parametric emissions along path-1 and the population of the states. The path-2 emissions, which also depend on the excitation laser intensity, are most probably due to amplified spontaneous emission (ASE), without population inversion, in the case of strong excitation. We finally examine the radiation dynamics under the effect of the elastic dephasing collisions between the potassium atoms with the buffer gas atoms.

DOI
10.1080/09500340.2011.640950

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A numerical study, stimulated by existing experimental results concerning the temporal dynamics of internally generated radiations, is presented for a four-level system of the potassium atom. An intense nanosecond duration laser pulse excites the two-photon transition and initiates the generation of internal radiations from quantum noise. It is shown that the temporal profiles of the generated radiations along the atomic path-1, ( ) and path-2, ( ), evolve differently as a function of certain system parameters. It is also shown that the excitation laser intensity affects the parametric emissions along path-1 and the population of the states. The path-2 emissions, which also depend on the excitation laser intensity, are most probably due to amplified spontaneous emission (ASE), without population inversion, in the case of strong excitation. We finally examine the radiation dynamics under the effect of the elastic dephasing collisions between the potassium atoms with the buffer gas atoms.
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.
This work investigates the generation of amplified spontaneous emission (ASE) without population inversion in the 4P/sub 3/2/-4S/sub 1/2/ transition of the potassium atom under two-photon excitation of the atom from the 4S/sub 1/2/ to the 6S/sub 1/2/ state. The calculations show that the radiation observed in the 4P/sub 3/2/-4S/sub 1/2/ transition is generated without population inversion for a broad range of the parameters used. It seems that a V-type scheme due to the 5P/sub 3/2/-4S/sub 1/2/ emission and a cascade scheme due to the 6S/sub 1/2/-4P/sub 3/2/ emission are both important in the generation of the radiation in the 4P/sub 3/2/-4S/sub 1/2/ transition. Both schemes are examined and compared with the experimental results in order to assess their impact in the generation of ASE without population inversion.
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.
We report the experimental observation of the amplified spontaneous emission (ASE) generated at the 4P 3/2 –4S 1/2 transition of potassium atoms following the excitation of the 4S 1/2 –6S 1/2 two-photon transition. We propose that the amplification of the observed radiation is inversionless, due to driving fields on neighbouring transitions that are not imposed externally, but are internally generated in the medium. A theoretical model based on the density operator formalism is presented, which confirms that the radiation observed in the 4P 3/2 –4S 1/2 transition is generated without population inversion for a broad range of the parameters used. It seems that a self-generated ladder scheme is the dominant mechanism, but other schemes are also examined and compared with the experimental results in order to assess their impact on the generation of the inversionless 4P 3/2 –4S 1/2 emission.
The combined excitation of high density potassium (K) vapour by 100 fs pump-coupling pulses is experimentally studied. The intense pump pulse excites the two-photon 4S1/2↔6S1/2 transition and internally generated emissions are initiated along the atomic paths: 6S1/2↔5P3/2,1/2↔4S1/2 (path-1) and, 6S1/2↔4P3/2,1/2↔4S1/2 (path-2). The temporally delayed coupling pulse coherently drives the 6S1/2↔4P3/2,1/2 transitions, in a Λ-type excitation scheme. The competing axial and conical emission components of the well-resolved 4P3/2,1/2↔4S1/2 transitions (D2 and D1 lines of K) are substantially enhanced and controlled, for appropriate detunings and pump-coupling temporal delays. The coherence relaxation time (CRT) of the two-photon excited 6S1/2 state is determined by exploiting the temporal delay in the pulse sequence. The effect of the pulse delay and the fs pulse bandwidth on the system dynamics is discussed as well as the role of dephasing collisions between K and buffer gas atoms. The proposed scheme can be employed in radiative multi-level systems, for the direct estimation of coherence relaxation rates of various states.
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.