Four-wave Mixing In Sodium Vapor Research Articles

FOUR WAVE MIXING OBSERVATION OF A SATELLITE IN THE Na-Xe NON-IMPACT COLLISIONAL LINESHAPE

Degenerate four-wave mixing in sodium vapor in the presence of a noble gas is measured and calculated for large detunings from resonance, where the collisional impact approximation is no longer applicable. The measurement thus probes the non-impact collisional potential of the Na-noble gas system. When the noble gas is Xe, and the detuning is to the red of the Na D 1 resonance, an avoided crossing between the quasi-molecular potentials results in a satellite in the absorption lineshape, which can be detected in the four wave mixing signal. We calculate collisional lineshapes with several available sets of Na-Xe interaction potentials using the quasi-static and semi-classic line broadening theories, and find that the available potentials do not yield line shapes which agree with the measurements. The lineshape derived from the measured degenerate four wave mixing signal shows good agreement with theoretical calculations, only if the non-adiabatic nature of the level crossing is considered. Based on our experimental lineshape and a non-adiabatic level crossing theory, we derive corrections to the Na-Xe potentials, near the level crossing, that is for interatomic separations in the range of 0.55–0.75 nm.

Satellite structure in the NaXe collisional lineshape observed in four wave mixing

Degenerate four-wave mixing in sodium vapor in the presence of a noble gas is measured and calculated for large detunings from resonance, where the collisional impact approximation is no longer applicable. The measurement thus probes the collisional potential of the Na-noble gas system. When the noble gas is Xe, and the detuning is to the red of the Na D 1 resonance, an avoided crossing between the quasi-molecular potentials results in a satellite in the absorption line shape, which can be detected in the four wave mixing signal. We calculate collisional line shapes with several available sets of NaXe interaction potentials using the quasi-static and semi-classic line broadening theories, and find that the available potentials do not yield line shapes which agree with the measurements. The lineshape derived from the measured degenerate four wave mixing signal shows good agreement with theoretical calculations, only if the non-adiabatic nature of the level crossing is considered. Based on our experimental lineshape and a non-adiabatic level crossing theory, we derive corrections to the NaXe potentials, near the level crossing, that is for interatomic separations in the range of 0.55–0.75 nm.

Suppression effects in stimulated hyper-Raman emissions and parametric four-wave mixing in sodium vapor.

Experimental investigations of two-photon enhanced stimulated hyper-Raman and parametric four-wave-mixing emissions are described for tuning unfocused laser beams near two-photon resonance with 3D and 4D states in sodium vapor. Over wide ranges of vapor pressures, path lengths, and laser power densities, the gains for stimulated hyper-Raman (SHR) scattering and for parametric four-wave mixing (PFWM) can become very greatly reduced from values expected from simple considerations. The suppression of gain can arise from interferences produced at two-photon and three-photon resonances, and from ac Stark shifts produced by internally generated near-resonant fields. Quantitative comparisons are made between observed and theoretically predicted values for SHR and PFWM production. Of 12 predicted consequences of the interferences and Stark shifting on the nonlinear behavior, ten are demonstrated through the experiments.

Optical phase conjugation of weak signals by using degenerate four-wave mixing

We have measured the conjugation fidelity produced by four-wave mixing in sodium vapor using weak probe beams that were aberrated by simulated atmospheric turbulence. Near-diffraction-limited (1.3 times the diffraction limit) return beams were obtained for both moderate (D/r(0) approximately 10) and strong (D/r(0) approximately 25) simulated turbulence, down to probe-beam pulse energies of 200 pJ. We believe this to be the first systematic study of conjugation fidelity as a function of probe energy.

Single-beam squeezed-state generation in sodium vapor and its self-focusing limitations

We describe an experiment that generates squeezed states by means of forward four-wave mixing in sodium vapor with a single optical beam. The single-beam arrangement maximizes the pump-probe spatial overlap in the nonlinear medium. Self-focusing (or self-defocusing) is found to be the major limiting factor in achieving optimal squeezing.

Phase conjugation by nondegenerate four-wave mixing in sodium vapor

We studied the generate conjugate wave with a frequency different from that of the input signal in a three-level system where the contribution of both resonant one- and two-photon transitions are present. Using the nondegenerate four-wave mixing technique with high-intensity pulsed lasers in sodium vapor a conversion efficiency of 30% and the ac Stark effect were observed. The results are in good agreement with a theoretical calculation using the density matrix equations for a three-level system.

Interference between competing quantum-mechanical pathways for four-wave mixing

The interference between two different quantum-mechanical pathways for four-wave mixing in sodium vapor has been observed. The four-wave mixing was enhanced by two-photon resonance with the 5s state, for one pathway, and the 4d state, for the other. The phase of the interference could be varied continuously by varying laser frequencies, and constructive and destructive interference effects were studied. A model based on third-order perturbation theory gives a good description of the features observed.

Observation of squeezed noise produced by forward four-wave mixing in sodium vapor

We have generated squeezed-state light through forward four-wave mixing in sodium vapor as verified by homodyne detection. Optical phase-sensitive noise with a minimum falling 4% below the shot-noise limit was observed.

Degenerate four-wave mixing in sodium vapor in the Rabi regime

Degenerate four-wave mixing in sodium vapor is investigated using a flashlamp-pumped dye laser synchronized on a cw dye laser. At low atomic density, the reflectivity versus the pump intensity and the line shapes are in good agreement with theoretical predictions in the case of both plane waves and Gaussian beams. At high atomic density, reflectivities much higher than unity have been observed. The influence of the pump imbalance has been studied as well as the deformation of the wave front of the conjugated beam introduced by the pump beams. In the high-atomic-density case, the theoretical problem is more complex, and our experimental results exhibit only qualitative agreement with theory.

High-resolution studies of collision-induced population grating resonances in optical four-wave mixing in sodium vapor

Optical four-wave mixing resonances in sodium vapor induced by collisions with inert buffer gases (He, Ar, Xe, ${\mathrm{N}}_{2}$) are characterized at high resolution ( 5 MHz) using lasers tuned near the $D$ lines of Na. These resonances are interpreted in terms of coherent scattering from a population grating established by collisional absorption of the incident radiation. The roles of homogeneous and inhomogeneous contributions to the resonance line shapes are clarified. The line shapes exhibit structure sharper than the predicted linewidth, that characteristic of the $3P$-to-$3S$ transition rate. We show that this is caused by collision-induced optical pumping which introduces the ground-state lifetime into the grating relaxation physics. These long-lived gratings are exemplified dramatically by experiments in the presence of ${\mathrm{N}}_{2}$ which collisionally quenches the Na $3P$ states. Similar results are obtained in Xe where it is argued that excimer formation with the $3P$ states plays a role in the population dynamics. Model calculations of the effects of sodium level degeneracy on the grating dynamics are presented to support these conclusions. Finally we present data illustrating interference between the collision-induced population grating resonances detailed here and collision-induced coherent Raman resonances between Zeeman sublevels of the ground state.

Narrow-band optical filter through phase conjugation by nondegenerate four-wave mixing in sodium vapor

An ultrahigh-Q, tunable optical filter with a FWHM bandwidth of 41 MHz is demonstrated. The filtering is produced by nondegenerate phase conjugation through four-wave mixing in atomic-sodium vapor. The filter is observed to have a maximum quantum efficiency of 4 x 10(-3). However, degenerate phase-conjugation experiments in sodium suggest that a quantum efficiency of greater than unity can be attained on a cw basis.

Pressure-induced degenerate frequency resonance in four-wave light mixing

Phase-matched four-wave mixing in sodium vapor with a helium buffer gas is carried out in a noncoplanar geometry. The directions of the four light beams remain nondegenerate even if all frequencies are equal. A sharp zerodifference frequency resonance in the intensity of the parametrically generated fourth beam by three incident beams is observed that is proportional to the square of the pressure of the buffer gas. A theoretical interpretation of this effect caused by elimination of destructive interference between two coherent pathways is presented.