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Journal of Modern Optics

Emissions enhancement in a pump–coupling V-type coherently controlled four-level atomic system

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

Published Dec 1, 2013• 4 citations
Publisher

Abstract

The nonlinear interaction of a four-level potassium system with a strong pump and a weak coupling laser is investigated. A strong pump pulse excites the two-photon transition , causing prompt parametric emissions along the path-1 and delayed emissions in the second available de-excitation path-2 . A weak coupling pulse is introduced to coherently excite either the transition (path-1) or the one (path-2) in a V-type scheme. Results are presented for the emissions generated under the combined action of the pump–coupling pulses as a function of their delay for certain peak pulse intensities and coherence relaxation times (CRTs) related to dephasing collisions. We find that emissions in path-1 are considerably enhanced for negative delays (counterintuitive pulse sequence) of the order of the CRT. An approach is suggested for the estimation of CRTs. Emissions in path-2 are enhanced and temporally shifted, suggesting an approach to control this path by varying the pulse delay.

DOI
10.1080/09500340.2013.863985

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Journal of Modern Optics•Dec 1, 2011•7 citations
The nonlinear interaction of a four-level potassium system with a strong pump and a weak coupling laser is investigated. A strong pump pulse excites the two-photon transition , causing prompt parametric emissions along the path-1 and delayed emissions in the second available de-excitation path-2 . A weak coupling pulse is introduced to coherently excite either the transition (path-1) or the one (path-2) in a V-type scheme. Results are presented for the emissions generated under the combined action of the pump–coupling pulses as a function of their delay for certain peak pulse intensities and coherence relaxation times (CRTs) related to dephasing collisions. We find that emissions in path-1 are considerably enhanced for negative delays (counterintuitive pulse sequence) of the order of the CRT. An approach is suggested for the estimation of CRTs. Emissions in path-2 are enhanced and temporally shifted, suggesting an approach to control this path by varying the pulse delay.
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.
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.
Wave mixing in a three-level quantum system induced by two-photon resonant pump and one-photon resonant coupling at lower transition has been studied theoretically and experimentally. Resonantly enhanced difference-frequency generation efficiency was observed clearly dependent on the time delay and its sign between the femtosecond pump and probe. The role of the atomic coherences on the amplification without inversion during different sequences of interaction in a closed three-level system has been also stressed. These findings were confirmed experimentally in atomic potassium, where strong parametric amplification, induced by the pump delayed with respect to the probe, has been observed for the first time.
We report increased stimulated emission on some optically pumped laser transitions coupled to suppressed stimulated emission on other transitions when a weak Stokes laser beam and a strong pump laser beam interact with K 2 molecules in the counterintuitive sequence. The Stokes laser photons begin to interact with the K 2 molecules 3 to 4 ns before the pump laser photons can interact with the K 2 molecules. Both the Stokes and the pump laser pulses last for approximately 5 ns and the suppressed and enhanced stimulated emissions coincide with the pump laser pulse. The stimulated emissions described in this paper propagate parallel to the Stokes and pump laser beams.
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.