The electronic nonlinear refractive index in Cs vapor at 1.06 μ is shown to be negative and to consist of two distinct contributions. One of these contributions arises from a two-photon resonance in the susceptibility and can be made comparable in magnitude to the positive n2 in laser glass. The other term arises from population redistributions between the ground and first excited state. We present measurements of self-defocusing of a single pulse from a Nd: YAG mode-locked laser that are in agreement with the theoretical predictions.
We outline a complete theory of the nonlinear susceptibility of cesium around 1.06 \ensuremath{\mu}m, and present the first measurements of the negative nonlinear refractive index
n2 primarily responsible for the self-defocusing that is observed. For linearly polarized light, our measured value of
n2 is $\ensuremath{-}(1.4\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}N$ esu, where
N is the atomic density. This is in reasonable agreement with our calculated value of $\ensuremath{-}2.62\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}N$. The main portion of
n2 comes from a two-photon resonance between the
6s and
7s levels, and an additional negative term arises from induced population changes between
6s and
6p. For circular polarization,
n2 arises mainly from the induced population changes, giving the measured and calculated values of $\ensuremath{-}(0.26\ifmmode\pm\else\textpm\fi{}0.03)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}N$ and $\ensuremath{-}0.525\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}N$, respectively. In our experiments where the 35-psec pulses were shorter than the $6s\ensuremath{-}6p$ inverse linewidth, the nonlinear susceptibility depends mainly on the instantaneous intensity; however, for longer pulses, one would obtain additional contributions proportional to time integrals over the intensity. Since the useful output power from large Nd laser systems is limited by self-focusing due to the laser glass, our results suggest the possibility of increasing this power by using Cs vapor for compensation.
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.
Atomic vapors are systems well suited for nonlinear optics studies but very few direct measurements of their nonlinear refractive index have been reported. Here we use the
z-scan technique to measure the Kerr coefficient,
n2, for a Cs vapor. Our results are analyzed through a four-level model, and we show that coherence between excited levels as well as cross-population effects contribute to the Kerr nonlinearity.
We present measurements of the nonlinear refraction coefficients for the D1 line of cesium vapor. A variant of the z-scan technique is applied for which we scan the laser frequency across resonance for different sample positions and detect the transmission after an aperture in the far field. From the spectra for different positions we reconstruct the position-dependent transmission z-scan curve for a given laser detuning. This technique allows obtaining the vapor Kerr coefficient over a large and continuous range of frequencies. From the saturation of the Kerr coefficient, n2, with beam power we obtain n4, the next term in the refraction index expansion. A theoretical model is used to calculate n2 and n4 giving values consistent with experimental ones.