Fundamental studies of degenerate four -wave mixing in a two-level atomic system

Binh Trong Do, Purdue University

Abstract

We perform two series of degenerate four-wave mixing (DFWM) experiments in an optically pumped, two-level atomic system (the 3s 2S1/2, F = 2, mF = 2 → 3p2P 3/2, F = 3, mF = 3 transition in a diffuse, collision free, thermal beam of sodium) interacting with two equal-counter-propagating pump and a probe laser fields. In the first series, we perform these DFWM experiments whose pump and probe beams are derived from a narrow-band laser field under conditions in which the Doppler shift, the natural line width and the Rabi frequency are comparable with one another. We study the detailed lineshape and strength of the DFWM signal as a function of the atomic velocity and pump beam intensity. For comparison with our experimental results, we analyze the DFWM interaction under the above conditions through direct integration of the optical Bloch equations. From our experimental measurements and computational results, we show that the lineshapes of the DFWM spectra of non-zero velocity atoms are very complex when the pump intensity exceeds the saturation intensity. We find the relative contributions of atoms of different velocities to the DFWM signal and these contributions are dependent on the pump intensity. We can also predict the limit of the atomic velocity beyond which the atoms give negligible contribution to the DFWM signal and the optimal atomic velocity at which the atoms whose Doppler shift is close to the pump Rabi frequency contribute strongest to the DFWM signal. In the second series of DFWM experiments, the pump and probe beams are also derived from a single narrow-band cw laser source on which we have imposed phase and frequency fluctuations conforming to the phase diffusion model. We vary the pump intensity, laser bandwidth and the temporal delay between pump and probe fields, and we measure the peak signal strengths of the DFWM spectra of the phase diffusion field and normalize them with respect to those of the narrow band field. The numerical results are calculated by direct integration of the optical Bloch equations which model the same interaction, and they agree with the experimental measurements qualitatively, sometimes quantitatively. From this series of experiments, we study the effect of coherence of the laser field on DFWM interactions in atomic media.

Degree

Ph.D.

Advisors

Fischbach, Purdue University.

Subject Area

Atoms & subatomic particles|Optics

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