D. G. Steel

Polarization‐rotation and thermal‐motion studies via resonant degenerate four‐wave mixing

We report experimental observation of polarization rotation in resonant degenerate four‐wave mixing (DFWM) arising from the third‐order coupling of the electric fields. This effect allows investigation of molecular motion and its effect on the efficiency of the DFWM. By controlling the pulse width and therefore γ=T0/τp (where T0≡λ/vth) we show quantitative agreement with a third‐order density matrix perturbation calculation. In particular, using a CO2 laser at 10.6 μm and SF6 as the nonlinear medium, we show that for γ≫1 (stationary limit) we see no molecular motion and for γ≈1, we see a significant ’’washout’’ of one third–order polarization.

Phase conjugation at 10.6 μm by resonantly enhanced degenerate four‐wave mixing

A resonantly enhanced phase‐conjugate 10.6‐μm signal has been generated in the saturable absorber SF6. Direct evidence of aberration correction is shown through the use of far‐field photographs taken with a pyroelectric vidicon camera. Reflection coefficients of 7% have been achieved in a few Torr of SF6 for a 2‐cm interaction length.

Degenerate four‐wave mixing of 10.6‐μm radiation in Hg1−xCdxTe

We report observation of a large nonlinear optical susceptibility in HgCeTe for degenerate four‐wave mixing of 10.6‐μm radiation. Using pump pulses of 180 ns duration and 160 kW/cm2 peak intensity, nonlinear reflection coefficients of oiver 10% have been observed. The experimentally measured value of χ(3) is approximately 5.4×10−6 esu, in reasonable agreement with calculations based on a Drude model for the optically generated plasma. We have also observed nonlinear Fabry Perot effects in our samples.

Effects of unequal pump intensity in resonantly enhanced degenerate four-wave mixing

Experiments performed in SF 6 using a CO 2 laser show the effect of unequal pump intensity in resonantly enhanced degenerate four-wave mixing (DFWM). A two-level saturable absorber model was extended to include the effects of unequal pump intensity and the results show significant departure from typical nonsaturating behavior. The experimental results and theory show qualitative agreement.

Phase Conjugation By Resonantly Enhanced Degenerate Four-Wave Mixing

The use of resonantly enhanced four-wave mixing to generate high reflectivity phase-conjugate waves is discussed. A physical understanding of the mechanisms underlying the nonlinear response for a simple two-level system is presented followed by a discussion of the complicating effects of pump absorption, pump depletion, unbalanced pumps, and thermal motion. Modifications of the theory to describe four-level behavior are presented and compared with experiments. The application of nearly degenerate four-wave mixing to narrow-band filtering complicated by the A. C. Stark effect is dis-cussed along with examples of experimental observations.

Atomic coherence effects in resonant degenerate four‐wave mixing

We present a study of atomic coherence effects in resonant degenerate four‐wave mixing. We find for energy levels having magnetic degeneracies, the third‐order nonlinear response resulting from two counterpropagating pump waves orthogonally polarized to an incident probe wave generates a signal whose polarization is phase conjugate to that of the incident probe wave. This effect is due to the coupling of different magnetic states resulting from coherent excitation giving rise to a net electric quadrupole. Experimental studies in Na on the D2 line at 5890 A confirm the analysis.

Continuous-wave dye laser with a phase-conjugate mirror

We have experimentally demonstrated a cw dye laser incorporating a phase-conjugate mirror (PCM). The mirror was generated by using four-wave mixing in sodium. The unique bandwidth and pump-probe detuning properties of such a PCM permitted the first reported demonstration of the c/4L spaced paired half-axial modes. In addition, the aberration correction ability is demonstrated.