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Journal of Modern Optics

Coherently controlled emissions |4P3/2,1/2〉 ↔ |4S1/2〉 from a femtosecond Λ-type excitation scheme in potassium atom

Dionysios Pentaris, Dimitrios Damianos, G. Papademetriou, A. Lyras, K. Steponkevičius, Virgilijus Vaičaitis, Thomas Efthimiopoulos

Published Feb 17, 2016• 3 citations
Publisher

Abstract

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.

DOI
10.1080/09500340.2016.1142620

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AIP conference proceedings•Jan 1, 2010•2 citations
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 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.
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.
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.
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 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.