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Study of two-photon near resonant four-wave difference mixing in atomic sodium in a CH4/air flame

J. SINGH, Fang Yu Yueh

Published Jul 6, 1992• 0 citations

Abstract

Study of a two-photon near resonant four wave difference mixing (FWDM) in Na in a CH4/air flame has been performed to understand the FWDM process reported by Singh and Yueh (1992). The FWDM signal was measured with reduced flame length and with the focal point at the center and also at one end of the flame to pinpoint the region in which the signal is generated. A partial contribution in the third peak was from the nonresonant background. A FWDM signal is generated slightly away from the focal point and nonresonant background, at the focal point in the present experimental conditions. An absorption spectrum of the Na D-lines at different dye laser energy was recorded to study the depletion of Na ground state at higher dye laser intensity. Comparative study of the FWDM signal with a narrow and broad bandwidth dye laser shows that a lower Na concentration detection limit could be achieved with a narrow linewidth probe laser tuned to about 9/cm lower frequency from the center of the D1 line. The two minima in the FWDM spectrum corresponding to Na D lines have been explained, qualitatively, with the saturation effect in the Na D-lines transition. 27 refs.

DOI: 10.2514/6.1992-2963

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Study of a two-photon near resonant four wave difference mixing (FWDM) in Na in a CH4/air flame has been performed to understand the FWDM process reported by Singh and Yueh (1992). The FWDM signal was measured with reduced flame length and with the focal point at the center and also at one end of the flame to pinpoint the region in which the signal is generated. A partial contribution in the third peak was from the nonresonant background. A FWDM signal is generated slightly away from the focal point and nonresonant background, at the focal point in the present experimental conditions. An absorption spectrum of the Na D-lines at different dye laser energy was recorded to study the depletion of Na ground state at higher dye laser intensity. Comparative study of the FWDM signal with a narrow and broad bandwidth dye laser shows that a lower Na concentration detection limit could be achieved with a narrow linewidth probe laser tuned to about 9/cm lower frequency from the center of the D1 line. The two minima in the FWDM spectrum corresponding to Na D lines have been explained, qualitatively, with the saturation effect in the Na D-lines transition. 27 refs.

Four-wave mixing is studied in a Na-seeded flame in the regime of strong saturation of the Na D-line transitions. The output frequency differs from that of the input lasers so that spectral discrimination can distinguish the mixing signal. Scans of the D2 resonance with another laser set at the D1-line resonance reveals two resonances corresponding to the D2 resonance and the frequency difference of the D levels. The mixing efficiency at the D2 resonance increases with the detuning of the D1 resonance. The frequency difference resonance is weak because of interference between multiple coherent pathways.

Degenerate four-wave mixing (DFWM) line shapes are investigated in sodium vapor near the D(2) resonance line using nearly Fourier-transform-limited pulses. At low pump intensities sub-Doppler resolution is obtained. When the Rabi frequency associated with the pump intensity becomes comparable with the ground-state hyperfine frequency separation of sodium, each component of the double-peaked DFWM spectrum splits further into two components. An adiabatic-following model explains the near-resonant intensity dependence of the DFWM signal. A qualitative explanation of the observed splitting is also given.

Two-photon resonant parametric four-wave mixing and a newly developed variant called seeded parametric four-wave mixing are used to detect trace quantities of sodium in a flame. Both techniques are simple, requiring only a single laser to generate a signal beam at a different wavelength which propagates collinearly with the pump beam, allowing efficient signal recovery. A comparison of the two techniques reveals that seeded parametric four-wave mixing is more than two orders of magnitude more sensitive than parametric four-wave mixing, with an estimated detection sensitivity of $5\ifmmode\times\else\texttimes\fi{}{10}^{9}$ atoms/

{\mathrm{cm}}^{3}

. Seeded parametric four-wave mixing is achieved by cascading two parametric four-wave mixing media such that one of the parametric fields generated in the first high-density medium is then used to ``seed'' the same four-wave mixing process in a second medium in order to increase the four-wave mixing gain. The behavior of this seeded parametric four-wave mixing is described using semiclassical perturbation theory. A simplified small-signal theory is found to model most of the data satisfactorily. However, an anomalous saturationlike behavior is observed in the large signal regime. The full perturbation treatment, which includes the competition between two different four-wave mixing processes coupled via the signal field, accounts for this apparently anomalous behavior.

Accurate measurements were made of lasing and 4-wave mixing processes in Na-vapor inside a double heatpipe oven. Scanning Nd:YAG pumped pulsed dye laser through the two-photon resonance 3s{sup 2}S{sub 1/2} {yields} 5s{sup 2}S{sub 1/2}, and a high resolution double-monochromator near the 5s - 4p{sup 2}P{sub 1/2,3/2} single photon resonance, we easily distinguished the two lasing multiplets due to J = {1/2} and 3/2 populations emitting blue 330 nm light. We also observed two other components. By varying the laser polarization, intensity and Na density these were determined to be parametric 4-wave mixing lines with maximum intensity for slight phase mismatch. As the dye laser was scanned, the wavelengths of the lasing and 4-wave mixing peaks shifted, each pair with its own slope, and crossed - a process providing a spectroscopic pump mechanism to populate the {vert_bar}M{sub J} {>=}{vert_bar} + {1/2} > magnetic substate of the ground state. Similar experiments are underway for other 4 wave mixing processes.

The nonlinear response of sodium vapor with a laser tuned near the 4D two-photon resonance is investigated over a large range of sodium concentrations and pump intensities with emphasis on the influence of coherent cancellation effects on the induced polarizations and the primary mechanism for the generation of the new frequencies. Over a wide range of circumstances, the response of the medium is dominated by parametric four-wave mixing as compared to amplified spontaneous emission or stimulated electronic hyper-Raman scattering. Secondary four-wave-mixing fields are generated by stimulated Raman scattering of one parametric four-wave-mixing field and a coupling of these fields with the driving laser field. A sharp onset of saturation of the parametric four-wave-mixing signals with sodium number density and a transition from a quadratic to a linear power dependence is observed. Both effects are explained in terms of the two-photon cancellation effect.