D. Pentaris

Probing electronic coherences by combined two- and one-photon excitation in atomic vapors

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.

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

We 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.

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

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

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.

Optical free induction memory in potassium vapor under a partially-truncated two-photon excitation

We numerically study the conditions under which a memory is induced in a sample of potassium vapor by a two-photon partially-truncated excitation field detuned from resonance. We assume a four-level atom and solve numerically the appropriate equations observing the time dependence of the coherence σ12 in order to determine the region of the proper parameters: two-photon detuning, the external field intensity, and the atomic density. Subsequently we investigate the stability and the ‘read-out’ process of the optical free induction memory.

Parametric Four‐Wave Mixing in Low Atomic Densities of Potassium Vapor

We study the generation of radiation at the 5P3/2‐4S1/2 transition of potassium vapor under excitation of the 4S1/2‐6S1/2 two‐photon resonance. The model consists of the coupled Maxwell‐Bloch equations for a four‐level configuration of Potassium atom including states 4S, 4P, 5P and 6S, which is numerically solved. Previous experimental studies have shown the emission of Parametric Four‐Wave Mixing (PFWM) radiation near the 5P3/2‐4S1/2 transition at low vapor pressures (P<1014 atoms/cm3) which is also shown in the case of the model studied.

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

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 transition and internally generated emissions are initiated along the atomic paths: (path-1) and, (path-2). The temporally delayed coupling pulse coherently drives the transitions, in a Λ-type excitation scheme. The competing axial and conical emission components of the well-resolved 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 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.