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Near-resonant femtosecond laser induced cone emission from rubidium vapor

Silvije Vdović, Hrvoje Skenderović, Gordana Školnik, Ticijana Ban, Nataša Vujičić, Damir Aumiler, Goran Pichler

Published Apr 6, 2009• 0 citations
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Abstract

In this work we report the CE from the dense rubidium vapor excited by the output from the mode-locked, 100 fs laser oscillator. The cone emission was observed only when the laser beam propagated through the high-density vapor (N>5x1016 cm− 3) and for the smaller densities no CE was observed. The laser central wavelength was changed in the wide interval, from the far blue wing of the D2 line to the far red wing of the D1 line. The observed CE in the far blue wing of the D2 line is interpreted as a consequence of the self-focusing and self-phase modulation according to the adiabatic following limit. The emission in the red wing of the D2 line shows defocusing of the propagating beam. In contrast to the CE reported so far, the bright central spot was not observed in the far blue wing. In the near blue wing the central spot appears due to the two- photon resonance at 778 nm. The temporal shape of the CE, measured by cross-correlation in the SHG crystal shows pulse breaking and extreme lengthening.

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We studied the emission from dense rubidium vapor under 100-fs pulsed laser excitation in the 730--820 nm range. Self-focusing along with a conical emission (CE) was observed for the excitation in the blue wing of $D2$ atomic resonance. Self-defocusing occurred for the red-wing excitation. This is in accordance with the theory of the intensity dependent nonlinear refraction index. In the case of the CE observation, our experimental results are attributed to the self-phase modulation of the laser beam interacting with the near-resonant atomic medium.

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In our experiment conical emission (CE) has no wavelength shift and the same polarization as the original beam. Varying laser power, detuning, and atomic density, the images of CE were captured and analyzed. Typical far-field images are presented. Under the blue detuning condition, manifest CE patterns were observed, and under red detuning, original beam blurs due to not only to self-defocusing but also the conical pattern were observed. By comparing laser power and atomic density dependence of cone angle and theoretical curve, we could estimate spectroscopic data such as nonlinear refractive index. From the density dependence data, we get the fact that cone angle is proportional to atomic density which supports the self-phase-modulation (SPM) model. In conclusion, we observed and analyzed CE from near-resonant Sm vapor. With this simple setup, we could measure spectroscopic data.

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arXiv:1605.00312•Journal of Physics B Atomic Molecular and Optical Physics•Feb 28, 2017•6 citations
In this work we report the CE from the dense rubidium vapor excited by the output from the mode-locked, 100 fs laser oscillator. The cone emission was observed only when the laser beam propagated through the high-density vapor (N>5x1016 cm− 3) and for the smaller densities no CE was observed. The laser central wavelength was changed in the wide interval, from the far blue wing of the D2 line to the far red wing of the D1 line. The observed CE in the far blue wing of the D2 line is interpreted as a consequence of the self-focusing and self-phase modulation according to the adiabatic following limit. The emission in the red wing of the D2 line shows defocusing of the propagating beam. In contrast to the CE reported so far, the bright central spot was not observed in the far blue wing. In the near blue wing the central spot appears due to the two- photon resonance at 778 nm. The temporal shape of the CE, measured by cross-correlation in the SHG crystal shows pulse breaking and extreme lengthening.
We studied the emission from dense rubidium vapor under 100-fs pulsed laser excitation in the 730--820 nm range. Self-focusing along with a conical emission (CE) was observed for the excitation in the blue wing of D2D2D2 atomic resonance. Self-defocusing occurred for the red-wing excitation. This is in accordance with the theory of the intensity dependent nonlinear refraction index. In the case of the CE observation, our experimental results are attributed to the self-phase modulation of the laser beam interacting with the near-resonant atomic medium.
In our experiment conical emission (CE) has no wavelength shift and the same polarization as the original beam. Varying laser power, detuning, and atomic density, the images of CE were captured and analyzed. Typical far-field images are presented. Under the blue detuning condition, manifest CE patterns were observed, and under red detuning, original beam blurs due to not only to self-defocusing but also the conical pattern were observed. By comparing laser power and atomic density dependence of cone angle and theoretical curve, we could estimate spectroscopic data such as nonlinear refractive index. From the density dependence data, we get the fact that cone angle is proportional to atomic density which supports the self-phase-modulation (SPM) model. In conclusion, we observed and analyzed CE from near-resonant Sm vapor. With this simple setup, we could measure spectroscopic data.
We report the observation of cone emission generated when $\ensuremath{\sim}10\text{\ensuremath{-}}\mathrm{nJ}$, 100-fs laser pulses in the 730--770 nm wavelength range are transmitted through a dense cesium vapor. The spatial and spectral characteristics of the observed cone emission were studied experimentally. Cone angle dependence on the laser wavelength, laser average power, and Cs atomic and molecular concentrations were investigated. The cone emission (CE) was only observed when the laser beam self-focused after passing through the medium. We identify the self-phase modulation as a dominant mechanism for CE generation in our experiment.
Femtosecond (fs) laser propagation and fluorescence of dense potassium vapor was studied, and the spectral region around the first and the second doublets of the principal series lines of potassium atoms was investigated. In our search we did not observe the conical emission in the far field, although it was previously observed in the case of rubidium. We discuss the possible reason of this unexpected result. The fluorescence spectrum revealed Rb impurity resonance lines in emission due to the collisional redistribution from the K(4p) levels into the Rb(5p) levels. In the forward propagation of 400 nm femtosecond light we observed the molecular band red shifted from potassium second doublet. However, no molecular spectrum was observed when the mode-locked fs laser light was discretely tuned within the wings of the first resonance lines, at 770 nm.
Conical emission, redshifted from the atomic resonance, is observed when a ∼2-ps laser beam, blueshifted from resonance, propagates through potassium vapor. In contrast, no cone is visible when a 150-fs pulse is used. The 2-ps-pulse-induced cone has a unique, small, angular spread of Δθ/θ≈0.1, compared with Δθ/θ≈0.3 from other pulsed experiments, which used 25-ps–5-ns-excitation pulses. In addition, the fraction of the 2-ps-pulse power emitted in the cone reached 7.5%, far exceeding this fraction from the longer-pulse experiments. An additional, unexpected result is that the spectral width of the laser pulse is narrower, without significant intensity loss, after propagation through the vapor. The vapor densities and laser detunings that yield cones in the present experiment are similar to those from the longer-pulse experiments. Although the 2-ps-pulse spectrum partially overlaps the resonance, the cone spectrum and cone angles versus density and laser detuning are similar to those previously reported for nanosecond pulses. The fourth (blue-detuned) wave required for satisfying four-wave mixing is absent, again equivalent to what is observed in the nanosecond experiments. The physical origin of cone emission by pulsed excitation of atomic vapors is still largely unknown, and this high efficiency and narrow angular spread of a spectrally broad cone further contradicts existing models, perhaps ultimately providing a clue to a correct explanation.
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.