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Annual Meeting Optical Society of America

D4WM in a mercury discharge

Wayne H. Richardson, Lute Maleki, Greg Bearman, E. Garmire

Published Jan 1, 1985• 0 citations
Publisher

Abstract

The 73S1 → 63P2 (λ = 546.1-nm) transition of atomic mercury has received considerable spectroscopic attention. In this paper we examine phase conjugation reflectivities and line shapes for various pump powers on this line. The geometry is that of backward D4WM: counterpropagating pump beams and a weak probe intersect at an angle of 0.750° in a quartz mercury discharge cell. The inductively coupled rf discharge provides a density difference of 109–1011 atoms/cm3 between the levels of interest, which yields a small signal absorption α in the 0.3–3-cm−1 range. The best fit to the measured α(l) on line center is obtained using α(l) = α0/[1 + (l/l s )] with l s = 30 mW/cm2. This form of α(l) is different from what one would expect for a low pressure gas with Doppler broadening. Phase conjugation has been observed with pump powers as low as 3 mW. Spectra obtained by observing the conjugate beam as the laser frequency is scanned through resonance exhibit line broadening of 64–490 MHz depending on pump power. Additionally, line splitting is observed with increasing pump power. These splittings were 72 MHz at 134 mW of pump power. With an estimated Rabi frequency (Ω) of ~60 MHz, the predicted splitting is 3/2 Ω = 90 MHz.

DOI
10.1364/oam.1985.thj5

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The 73S1 → 63P2 (λ = 546.1-nm) transition of atomic mercury has received considerable spectroscopic attention. In this paper we examine phase conjugation reflectivities and line shapes for various pump powers on this line. The geometry is that of backward D4WM: counterpropagating pump beams and a weak probe intersect at an angle of 0.750° in a quartz mercury discharge cell. The inductively coupled rf discharge provides a density difference of 109–1011 atoms/cm3 between the levels of interest, which yields a small signal absorption α in the 0.3–3-cm−1 range. The best fit to the measured α(l) on line center is obtained using α(l) = α0/[1 + (l/l s )] with l s = 30 mW/cm2. This form of α(l) is different from what one would expect for a low pressure gas with Doppler broadening. Phase conjugation has been observed with pump powers as low as 3 mW. Spectra obtained by observing the conjugate beam as the laser frequency is scanned through resonance exhibit line broadening of 64–490 MHz depending on pump power. Additionally, line splitting is observed with increasing pump power. These splittings were 72 MHz at 134 mW of pump power. With an estimated Rabi frequency (Ω) of ~60 MHz, the predicted splitting is 3/2 Ω = 90 MHz.
Phase conjugation of a cw hydrogen fluoride (HF) laser beam has been investigated experimentally and theoretically using resonant degenerate four-wave mixing (DFWM) in an HF gas absorption cell. A single-line single-mode HF laser, operating on P1(8), P1(9),or P1(10) transitions, provided total (forward plus backward) pump beam intensities of 130, 600, and 400 W/sq. cm., respectively, in the HF absorption cell. A probe beam intensity of the order of 10 W/sq. cm. was also provided. Conjugate reflection of the probe beam was investigated as a function of gas cell pressure and as a function of frequency detuning from line center. The variation of reflectivity with gas cell pressure at fixed laser beam intensity indicated a peak reflectivity of the order of .0001 at pressures from 2 to 4 Torr. Measurement of conjugate beam intensity, as a function of frequency detuning from the line center,provided a signal with a full width at half maximum (FWHM) of about 30 MHz. The latter is of the order of the homogeneous width, for HF at the test pressure, and may be compared to the Doppler width delta nu sub d = 300 MHz for the medium. An Analytical model is presented which provides theoretical estimatesmore » for the variation of reflectivity with pressure. Effects of diffusion, thermal conduction, pump depletion, and a nonuniform (Gaussian) profile are considered. Theoretical estimates for peak reflectivity agree with experiments to within a factor of about 2.« less
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The phase-conjugate signal observed in resonant degenerate four-wave mixing on the 6/sup 3/P/sub 2/ to 7/sup 3/S/sub 1/ transition of atomic mercury in a Hg-Ar discharge is investigated. At a fixed argon pressure the variation of the signal with pump powers is explained by a model that includes the effects of velocity-changing collisions (VCCs). As the argon pressure was varied from 0 to 1 torr an increase in the phase conjugate signal was observed and is ascribed to a change in the discharge dynamics with argon pressure and to the influence of VCCs. To further clarify the role of collisions and optical pumping, degenerate four-wave mixing spectra are examined as a function of pump power. Line shapes are briefly discussed.>
Continuous wave (cw) phase conjugation of a HF laser beam has been observed using resonant degenerate four-wave mixing in a HF gas absorption cell. A HF laser was operated in the P1(8) transition to provide total (forward plus backward) pump beam and probe beam intensities of about 400 and 30 W/cm2, respectively, in the cell. Conjugate reflectivities were measured as a function of HF pressure and frequency detuning from line center. Reflectivities of the order of 10−4 were measured at HF gas pressures of the order of 2 Torr when the laser frequency was tuned near line center.
We examine theoretically the degenerate four-wave mixing (DFWM) signal intensities and line shapes obtained with the forward phase-matched geometry in which all beams propagate in the same direction and compare the results to those of the phase-conjugate geometry with counterpropagating pump beams. To examine the forward phase-matched geometry, we modify a theoretical approach used previously to calculate phase-conjugate DFWM signal intensities. This theoretical approach, which involves numerical integration of the time-dependent density-matrix equations, is validated for the forward phase-matched geometry by comparison of our calculated line shapes to both a perturbative solution and to experimental data. This methodology is then used to compare the signal intensities and line shapes obtained with the forward phase-matched geometry and the phase-conjugate geometry in the perturbative (low laser power) and saturated (high laser power) regimes. In the perturbative regime the forward phase-matched signal exhibits less sensitivity to the Doppler linewidth. At pump laser intensities approximately equal to the saturation intensity the signal for the forward phase-matched geometry is stronger than that for the phase-conjugate geometry for primarily Doppler-broadened resonances, assuming the same probe volume for both geometries. These advantages warrant further investigations employing the forward phase-matched configuration for DFWM measurements of gas-phase species.