We have observed that the polarizations and intensities of counterpropagating laser beams in sodium vapor are temporally unstable. The output from a stabilized dye laser tuned near to the 3S1/2 → 3P1/2 atomic transition was split to form the two counterpropagating beams. These beams were polarized with the same linear polarization and were weakly focused to a spot size of ~750 μm in sodium vapor having a density of ~3 × 1013 cm−3. We monitored the light generated in the polarization orthogonal to that of the pump beams. For a detuning of 1.01 GHz to the high-frequency side of the 3S1/2(F = 2) → 3P1/2(F = 2) transition and a power of 160 mW in one of the beams, we found that the threshold for the instability occurred at a power of 15 mW in the other beam. Slightly above threshold, the instability was found to be periodic in nature. As the intensity in the counterpropagating beam was increased far above the threshold value, wild fluctuations in the polarizations and intensities of the transmitted beams were observed. We have found that these fluctuations are chaotic and that the system evolves on a strange attractor whose dimension increases with increasing laser power. These observations are in qualitative agreement with the theoretical predictions of Gaeta et al.,1 which predict that the polarizations of counterpropagating beams in a Kerr medium are unstable to the growth of temporal fluctuations.
We observe amplification of a linearly polarized laser beam propagating through a Doppler-broadened atomic potassium vapor driven by an intense, linear and orthogonally polarized counterpropagating laser beam. The observed gain spectra are well explained by a model that incoporates only the effects of the coherent driving of the atomic dipole moment and not atomic recoil. Our results suggest that the recently reported observations of amplification and lasing using a laser-driven sodium vapor may not arise from the effects of collective atomic recoil.
Two independently tunable, linearly polarized dye lasers are counterpropagated through a heat pipe containing sodium vapor. The frequency of the first laser ${\ensuremath{\omega}}_{1}$ is incremented through both 3
p2P1/2,3/2 fine-structure levels while the second laser ${\ensuremath{\omega}}_{2}$ is scanned through the energy increment necessary to reach ns
2S1/2 and
nd2D3/2,5/2 levels (n=5--35). Following excitation, ionization occurs through chemi-ionizing collisions. The polarization (defined as the ionization intensity measured when the two planes of polarization of the laser beams are parallel minus the intensity when perpendicular divided by the sum of the intensities) P=(
I?-${I}_{\ensuremath{\perp}}$)/(
I?+${I}_{\ensuremath{\perp}}$) goes from +1 to -1 as ${\ensuremath{\omega}}_{1}$ is tuned between the 3
p2P1/2,3/2 fine-structure levels. This novel polarization effect is shown to be due to an interference between the two pathways to the ns
2S1/2 levels. Upon exciting the unresolved nd
2D3/2,5/2 levels, the polarization is very close to zero when ${\ensuremath{\omega}}_{1}$ is at or near the 3p
2P3/2,1/2 levels. A simple few-level atom theory reproduces the experimental observations except for the case when both lasers are in resonance.