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AIP conference proceedings

Two-Photon Frequency Comb Excitation of Rubidium Atoms in External Magnetic Field

N. Vujičić, Ticijana Ban, Hrvoje Skenderović, Silvije Vdović, Goran Pichler, Marco Antonio Gigosos, Manuel Ángel González

Published Jan 1, 2008• 0 citations
Publisher

Abstract

In the present experiment the 5S‐5D two‐photon transitions in 85Rb and 87Rb atoms as a result of the interaction of the femtosecond frequency comb with atomic levels of both rubidium isotopes are investigated. The main problem in studying of two‐photon transitions is in optimization of the excitation efficiency of the desired state. There are two general cases: those transition with an intermediate resonance those in which the pulse spectrum is far detuned from an intermediate resonance. In order to investigate the dependence of the two‐photon fluorescence signal as a result of interaction of the frequency comb with perturbed energy‐level pattern an external magnetic field was applied.

DOI
10.1063/1.3026439

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AIP conference proceedings•Jan 1, 2006•6 citations
In the present experiment the 5S‐5D two‐photon transitions in 85Rb and 87Rb atoms as a result of the interaction of the femtosecond frequency comb with atomic levels of both rubidium isotopes are investigated. The main problem in studying of two‐photon transitions is in optimization of the excitation efficiency of the desired state. There are two general cases: those transition with an intermediate resonance those in which the pulse spectrum is far detuned from an intermediate resonance. In order to investigate the dependence of the two‐photon fluorescence signal as a result of interaction of the frequency comb with perturbed energy‐level pattern an external magnetic field was applied.
We study resonantly enhanced cascaded two-photon transitions in Doppler-broadened rubidium vapor, induced by an optical frequency comb. Cascaded two-step excitation of atoms from the ground state 5SSS1/2{}_{1/2}1/2​ to the final 5DDD5/2{}_{5/2}5/2​ state via an intermediate 5PPP3/2{}_{3/2}3/2​ state as well as the influence of the frequency comb repetition rate on the two-photon fluorescence signal have been investigated. It is shown that the careful choice of fR{f}_{R}fR​ leads to enhancement of the two-photon signal with considerable increase of the final 5D5/25{D}_{5/2}5D5/2​-state population due to velocity-selective double resonance. Since the double-resonance condition for two different rubidium isotopes is fulfilled for different fR{f}_{R}fR​, the possibility of isotope-selective two-photon excitation is explored. The dependence of the two-photon signal on external magnetic field values and different repetition rates of the optical frequency comb is also investigated.
A high-resolution two-photon spectrum of 5S 1/2 → 5P 3/2 → 5D 5/2 transitions in a thermal 85 Rb vapor cell is presented by using an optical frequency comb and a cw laser. The fluorescence of 6P 3/2 → 5S 1/2 spontaneous emission is detected when the cw laser frequency is scanned from the 5S 1/2 ground state to 5P 3/2 hyperfine levels and the optical frequency comb repetition rate is fixed. The hyperfine splittings (F f = 2–5) of the 5D 5/2 excited state are well resolved. The dependences of fluorescence intensities on the cw laser intensity and temperature of 85 Rb vapor cell are studied, respectively. The experimental results are in good agreement with the theoretical analyses.
Abstract We report on the direct observation of the frequency comb printed in the blue fluorescence excitation spectrum of rubidium vapor induced by the combined action of an ultrashort pulse train and a cw diode laser. Each laser drives one step of the 5S—5P—5D two-photon transition in a copropagating configuration and the excitation spectrum is obtained while the cw-laser frequency is scanned over the rubidium D 2 lines. Measurements of the fluorescence as a function of the diode intensity and atomic density allow us to investigate how the effect of power broadening and absorption of the diode laser blur the excitation process. The experimental results and the printing process of the comb on the Doppler profile are described well by numerical integration of the Bloch equations for a cascade three-level system.
We demonstrate temporal coherent control in two-photon transitions. A pair of broadband (Δλ∼20nm), ultrashort (Δt∼100fs), collinear pulses with a variable time delay excites the rubidium atoms into the 5D state from the 5S ground state where the two-photon transitions are enhanced by the intermediate level 5P. The excited atoms radiate 5μm (5D–6P) and 420nm (6P–5S) light. As a result of tuning the wavelength of the input laser, a superfluorescence at 420nm exhibits different temporal behaviors. A switching from a beating at the frequency given by the difference between the sequential atomic transitions that involve the 5P3/2 intermediate level, to a quantum beating due to two different two-photon excitation paths, 5S→5P1/2→5D and 5S→5P3/2→5D is observed. Based on the simple atom-field interaction theory, an analytic solution, which qualitatively elucidates experimental results, is obtained.
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.