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AIP conference proceedings

Mapping of the Optical Frequency Comb to the Atom Velocity Comb

Goran Pichler, Damir Aumiler, N. Vujičić, Silvije Vdović, Ticijana Ban, Hrvoje Skenderović

Published Jan 1, 2006• 6 citations
Publisher

Abstract

We present the experimental and theoretical study of the resonant excitation of rubidium and cesium atoms with fs pulse train in the conditions when the pulse repetition period is shorter than the atomic relaxation time. Velocity selective optical pumping of the ground state hyperfine levels and velocity comb‐like excited state hyperfine level populations is demonstrated. Both effects are a direct consequence of the fs pulse train excitation considered in the frequency domain. A simple experimental apparatus was employed to develop a modified direct frequency comb spectroscopy which uses a fixed frequency comb for the 85,87Rb 5s 2S1/2 → 5s 2P1/2,3/2 and 133Cs 6s 2S1/2 → 6p 2P1/2,3/2 excitation, and a weak cw scanning probe laser at 780 and 852 nm for Rb and Cs ground levels population monitoring.

DOI
10.1063/1.2400901

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We present the experimental and theoretical study of the resonant excitation of rubidium and cesium atoms with fs pulse train in the conditions when the pulse repetition period is shorter than the atomic relaxation time. Velocity selective optical pumping of the ground state hyperfine levels and velocity comb‐like excited state hyperfine level populations is demonstrated. Both effects are a direct consequence of the fs pulse train excitation considered in the frequency domain. A simple experimental apparatus was employed to develop a modified direct frequency comb spectroscopy which uses a fixed frequency comb for the 85,87Rb 5s 2S1/2 → 5s 2P1/2,3/2 and 133Cs 6s 2S1/2 → 6p 2P1/2,3/2 excitation, and a weak cw scanning probe laser at 780 and 852 nm for Rb and Cs ground levels population monitoring.
We present direct observation of the velocity-selective optical pumping of the Rb ground state hyperfine levels induced by 5S(1/2) --> 5P(1/2) femtosecond pulse-train excitation. A modified direct frequency comb spectroscopy based on the fixed frequency comb and a weak cw scanning probe laser was developed. The femtosecond pulse-train excitation of a Doppler-broadened Rb four-level atomic vapor is investigated theoretically in the context of the density matrix formalism and the results are compared with the experiment.
We investigate the time dynamics of a four-level rubidium atomic system excited by a train of $0\ensuremath{\pi}$ pulses. Resonant $0\ensuremath{\pi}$ pulse shaping is achieved by propagation of weak femtosecond pulses through rubidium vapor and characterized using frequency-resolved optical gating and within the linear dispersion theory. The excitation of the Rb atoms by $0\ensuremath{\pi}$ pulse train is studied experimentally by modified direct frequency comb spectroscopy and theoretically within the density-matrix approach. We show that accumulation of populations and coherences, as typical pulse train excitation effects, strongly depend on the strength of the resonant $0\ensuremath{\pi}$ pulse shaping. Stronger pulse shaping reduces the coherent accumulation effects, eventually leading to the disappearance of the velocity selective optical pumping observed in the frequency domain.
A mode-locked femtosecond laser is used to physically map the laser frequency comb into the velocity comb of the excited Rb atoms at room temperature. Upon resonant excitation by discrete optical frequencies the velocity distribution of the excited Rb atoms shows comblike structure. Simultaneously, velocity-selective population transfer occurs between ground-state hyperfine levels. Both facts are observed by modified direct frequency comb spectroscopy and verified by a detailed density matrix treatment of the multilevel system in resonant field.
The excitation of room-temperature four-level 87Rb^{87}\mathrm{Rb}87Rb atoms by a train of ultrashort pulses is investigated theoretically in the conditions when the pulse repetition period is shorter than the characteristic atomic relaxation times. It is shown that coherent accumulation of excitation leads to coherent population trapping and electromagnetically induced transparency of the excitation pulses when the pulse repetition rate is a subharmonic of the ground-state hyperfine splitting. It is illustrated how the judicious choice for the frequency comb parameters can provide a means to effectively control the degree of coherence between the ground-state hyperfine levels for selective atomic groups, and even transfer the whole atomic distribution to the dark state with up to 9595%95 efficiency.
We investigated the coherence accumulation in Rb atoms excited by a train of femtosecond pulses. The coherence accumulation results in the velocity selective optical pumping of Rb hyperfine levels, observed by a cw probe laser. The effects of the pulse train excitation on the cw probe laser transmission were investigated, depending on the probe laser power. We observed the probe laser absorption increase in the strong probe case, followed by the reduction of the modulations in the probe laser absorption. Results show that accumulation of population and coherence can be effectively reduced and eventually destroyed by increasing the cw laser intensity. A strong cw laser can therefore serve as a switch from the pulse-train to pulse-by-pulse type of interaction of Rb atoms with the fs laser. We developed a density-matrix based theoretical model of the eight-level Rb atoms interacting with the fs and cw laser fields, and present agreement with the experimental results.
We present an experimental and theoretical investigation of two-photon direct frequency-comb spectroscopy performed through velocity-selective excitation. In particular, we explore the effect of repetition rate on the $5{S}_{1/2}\ensuremath{\rightarrow}5{D}_{3/2,5/2}$ two-photon transitions excited in a rubidium atomic vapor cell. The transitions occur via stepwise excitation through the 5P1/2,3/25{P}_{1/2,3/2}5P1/2,3/2​ states by use of the direct output of an optical frequency comb. Experiments were performed with two different frequency combs, one with a repetition rate of $\ensuremath{\approx}925$ MHz and one with a repetition rate of $\ensuremath{\approx}250$ MHz. The experimental spectra are compared to each other and to a theoretical model.