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Physical Review A

Coherent population trapping and four-wave mixing via dark states in a Doppler-broadened open Rb system

S. Wang, D. G. Ducreay, Rodolfo Pina, Min Yan, Yifu Zhu

Published Feb 11, 2000• 17 citations
Publisher

Abstract

We report an experimental study of coherent population trapping and four-wave mixing via multilevel dark states prepared in the ground hyperfine states of ${}^{87}\mathrm{Rb}$ atoms. The dark states are created by \ensuremath{\Lambda}-type coupling chains driven by a linearly polarized pump laser tuned to the ${D}_{1}$ $F=2\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=1$ transition. The Doppler shift in the coupling chains is compensated by the left- and right-circularly polarized components of the pump laser. The multilevel dark states are explored by a probe laser and their existence is manifested by a strong dependence of the probe excitation and four-wave-mixing emission on the probe polarization. Our experiment shows that coherent population trapping in multilevel dark states of a degenerate atomic system should be useful in the coherence control of optical nonlinearity.

DOI
10.1103/physreva.61.033805

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Physical Review A•Feb 1, 1996•95 citations
We report an experimental study of coherent population trapping and four-wave mixing via multilevel dark states prepared in the ground hyperfine states of 87Rb{}^{87}\mathrm{Rb}87Rb atoms. The dark states are created by \ensuremath{\Lambda}-type coupling chains driven by a linearly polarized pump laser tuned to the D1{D}_{1}D1​ F=2\ensuremath↔F\ensuremath′=1F=2\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=1F=2\ensuremath↔F\ensuremath′=1 transition. The Doppler shift in the coupling chains is compensated by the left- and right-circularly polarized components of the pump laser. The multilevel dark states are explored by a probe laser and their existence is manifested by a strong dependence of the probe excitation and four-wave-mixing emission on the probe polarization. Our experiment shows that coherent population trapping in multilevel dark states of a degenerate atomic system should be useful in the coherence control of optical nonlinearity.
We report an experimental observation of coherent population trapping in multilevel dark states prepared in the ground hyperfine states of 87Rb{}^{87}\mathrm{Rb}87Rb atoms confined in a room-temperature vapor cell. The dark states are created by a linearly polarized coupling laser that is tuned to the 87Rb{}^{87}\mathrm{Rb}87Rb D1{D}_{1}D1​ $F=2\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}=1$ or 2 transition and establishes \ensuremath{\Lambda}-type transition chains. The Doppler shift for the \ensuremath{\Lambda}-type transition chains is compensated by the left and right circularly polarized components of the coupling laser. The multilevel dark states that consist of three or two magnetic sublevels among the five degenerate magnetic sublevels in the F=2F=2F=2 ground hyperfine state are probed by a weak probe laser scanned across the same D1{D}_{1}D1​ transition and their existence is manifested by a strong dependence of the probe excitation on the probe polarization. Our experiment demonstrates the importance of coherent population trapping in a laser coupled degenerate multilevel open atomic system, which may play an important role in diverse physical phenomena such as atom cooling and trapping, optical pumping, and multiple laser excitation and ionization of atoms and molecules.
Summary form only given.We present here the first experimental proof for the existence of elliptical dark states (EDS), the multilevel analog of the well known three-level dark states. We demonstrate that an ensemble of multilevel atoms, when prepared by elliptically polarized light, becomes transparent to probe light of the same polarization. The effect stems from laser-induced coherences between many ground-state magnetic sublevels that reflect the polarization of the light that had created the dark state. The novelty and essential character of the dark states are elucidated by the experimental demonstration of their creation by incoherent light. The experiments were performed in a Na atomic beam.
We present the first experimental proof for the existence of elliptical dark states, the multilevel analog of the well known three-level dark states. An ensemble of multilevel atoms, when prepared by elliptically polarized light, becomes transparent to probe light of the same polarization. The effect stems from laser-induced coherences between many ground-state magnetic sublevels which reflect the polarization of the light that had created the dark state. The novelty and essential character of the dark states are elucidated by the experimental demonstration of their creation by incoherent light. It is anticipated that elliptical dark states will play an important role in the laser cooling and manipulation of molecules.
The atomic coherence effect can be used in enhancing the efficiencies of nonlinear optical processes. Particularly, nondegenerate four-wave mixing (NDFWM) in multilevel systems has recently gained attention in connection with the advantage of using coherent population trapping (CPT) or electromagnetically induced transparency (EIT) to enhance the efficiency of generated signals. Generally speaking, linear absorption competes with the nonlinear signal generation at the exact one-photon resonance condition. However, the linear absorption can, in principle, be reduced or even eliminated by preparing the system in the CPT or EIT state. We report experimental demonstrations of NDFWM in several multi-level configurations in rubidium atomic vapor. In one of the double-/spl Lambda/ systems an unusual behavior of signal output power as a function of pump power, i.e. a maximum in the generated signal power is reached as the pump power increases, is experimentally observed. By comparing with several related /spl Lambda/ and double-/spl Lambda/ systems, we have concluded that CPT is the mechanism for this interesting effect.
We present an experimental study of cw nearly degenerate four-wave-mixing emission in a Doppler broadened Rb atomic system. With the fixed detuning of an intense pump laser for the Rb D2{\mathit{D}}_{2}D2​ line, the nearly degenerate four-wave-mixing emission exhibits three peaks as a probe laser is scanned across the same Rb D2{\mathit{D}}_{2}D2​ line. The observed three-peaked spectrum can be interpreted as the resonances in the energy structure of the dressed atomic states.
We present a general formalism for studying the interaction between laser fields and degenerate-Zeeman-sublevel atoms correct up to the first order in the weak probe field. We derive from this theory the equations of motion for a \ensuremath{\Lambda}-type system involving S1/2{\mathit{S}}_{1/2}S1/2​, F=2\ensuremath{\leftrightarrows}P1/2{\mathit{P}}_{1/2}P1/2​, F\ensuremath{'}=1\ensuremath{\leftrightarrows}S1/2{\mathit{S}}_{1/2}S1/2​, F=1 transitions in 87Rb^{87}\mathrm{Rb}87Rb atoms. These equations are used to numerically investigate the coherent population trapping (CPT) schemes in the S1/2{\mathit{S}}_{1/2}S1/2​, F=2\ensuremath{\leftrightarrows}P1/2{\mathit{P}}_{1/2}P1/2​, F\ensuremath{'}=1 transition induced by a linearly polarized pumping field and the electromagnetically induced transparency exhibited in the weak probe spectrum in the S1/2{\mathit{S}}_{1/2}S1/2​, F=1\ensuremath{\leftrightarrows}P1/2{\mathit{P}}_{1/2}P1/2​, F\ensuremath{'}=1 transition. We discuss the effects of the CPT on the probe spectrum with and without the Doppler broadenings by comparing the probe spectrum derived from the real system with that derived from an ideal \ensuremath{\Lambda}-type system. We show that the atoms in the CPT states are shielded from interacting with the probe field while those in the absorbent states make contributions to the weak probe by the Raman anti-Stokes process, and the effect of the real system on the probe field becomes equivalent to the ideal \ensuremath{\Lambda} system only in the strong CPT parameter regime. \textcopyright{} 1996 The American Physical Society.