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arXiv:1207.3998 · Optics Letters

Two-photon transitions driven by a combination of diode and femtosecond lasers

Marco Polo Moreno de Souza, Giovana Trevisan Nogueira, D. Felinto, S. S. Vianna

Published Oct 15, 2012• 8 citations
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Abstract

We report on the combined action of a cw diode laser and a train of ultrashort pulses when each of them drives one step of the 5S-5P-5D two-photon transition in rubidium vapor. The fluorescence from the 6P(3/2) state is detected for a fixed repetition rate of the femtosecond laser while the cw-laser frequency is scanned over the rubidium D(2) lines. This scheme allows for a velocity selective spectroscopy in a large spectral range including the 5D(3/2) and 5D(5/2) states. The results are well described in a simplified frequency domain picture, considering the interaction of each velocity group with the cw laser and a single mode of the frequency comb.

DOI
10.1364/ol.37.004344

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We report on the combined action of a cw diode laser and a train of ultrashort pulses when each of them drives one step of the 5S-5P-5D two-photon transition in rubidium vapor. The fluorescence from the 6P(3/2) state is detected for a fixed repetition rate of the femtosecond laser while the cw-laser frequency is scanned over the rubidium D(2) lines. This scheme allows for a velocity selective spectroscopy in a large spectral range including the 5D(3/2) and 5D(5/2) states. The results are well described in a simplified frequency domain picture, considering the interaction of each velocity group with the cw laser and a single mode of the frequency comb.
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.
The two-photon transition $5S\ensuremath{-}5P\ensuremath{-}5D$ in rubidium vapor is investigated by detecting the fluorescence from the 6P3/26{P}_{3/2}6P3/2​ state when the atomic system is excited by the combined action of a cw diode laser and a frequency comb. The cw laser plays a role as a velocity-selective filter and allows for sub-Doppler spectroscopy over a large spectral range including the 5D3/25{D}_{3/2}5D3/2​ and 5D5/25{D}_{5/2}5D5/2​ states. For a counterpropagating beam configuration, the response of each atomic velocity group is well characterized within the Doppler profile and the excited hyperfine levels are clearly resolved. The contribution of the optical pumping to the direct two-photon process is also revealed. The results are well described in a frequency domain picture by considering the interaction of each velocity group with the cw laser and the modes of the frequency comb.
Abstract The coherent blue light generated in rubidium vapor due to the combined action of an ultrashort pulse train and a continuous wave diode laser is investigated. Each step of the two-photon transition 5S–5P 3 / 2 –5D is excited by one of the lasers, and the induced coherence between the 5S and 6P 3 / 2 states is responsible for generating the blue beam. Measurements of the excitation spectrum reveal the frequency comb structure and allow us to identify the resonant modes responsible for inducing the nonlinear process. Further, each resonant mode excites a different group of atoms, making the process selective in atomic velocity. The signal dependency on the atomic density is characterized by a sharp growth and a rapid saturation. We also show that for high intensity of the diode laser, the Stark shift at resonance causes the signal suppression observed at low atomic density.
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.
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.
We use a 1 GHz femtosecond laser as a tool to perform coherent spectroscopy in an atomic vapor. The action of the ultrashort pulse train over the various velocity groups or over a selective group of rubidium atoms is probed by a diode laser using velocity-selective or repetition rate spectroscopies. In particular, we show that the 1 GHz frequency separation of the modes in the frequency comb allows distinguishing of the different hyperfine levels within the Doppler broadened profile. The data are in good agreement with numerical and analytical results.