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arXiv:quant-ph/0105084 · Physical Review A

Observation of transient gain without population inversion in a laser-cooled rubidium Λ system

S.R. de Echaniz, Andrew D. Greentree, Alan V. Durrant, D. M. Segal, J. P. Marangos, Joan A. Vaccaro

Published Oct 10, 2001• 24 citations
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Abstract

We have observed clear Rabi oscillations of a weak probe in a strongly driven three-level $\ensuremath{\Lambda}$ system in laser-cooled rubidium. When the coupling field is nonadiabatically switched on using a Pockels cell, transient probe gain without population inversion is obtained in the presence of uncoupled absorptions. Our results are supported by three-state computations.

DOI
10.1103/physreva.64.055801

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We have observed clear Rabi oscillations of a weak probe in a strongly driven three-level \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ system in laser-cooled rubidium. When the coupling field is nonadiabatically switched on using a Pockels cell, transient probe gain without population inversion is obtained in the presence of uncoupled absorptions. Our results are supported by three-state computations.
Summary form only given. Many studies have been made of electromagnetically induced transparency (EIT) and probe gain under steady-state conditions. We have carried out experimental studies of transient effects in rubidium lambda systems in the non-adiabatic regime where the coupling field is rapidly switched on using a Pockels cell in a time that is short compared with transient time scales. Our preliminary work has confirmed predictions that the steady-state EIT is approached through damped oscillations characterised by the coupling field Rabi frequency /spl Omega//sub c/ and the spontaneous decay rate /spl Gamma/. We report results of our recent investigations into the effects of strong coupling fields conducted in several different lambda schemes in the rubidium 5S/sub 1/2/-5P/sub 3/2/ hyperfine levels in a magnetooptical trap. We also studied the effects of the initial conditions of population and optical coherence, probe absorption on uncoupled Zeeman transitions, and incoherent pumping.
Observation of transient effects in electromagnetically induced transparency (EIT) is reported in Rb85{\mathrm{Rb}}^{85}Rb85 cooled in a magneto-optical trap. The transmission of a weak probe beam in resonance with the 5S1/2(F=3){5S}_{1/2}(F=3)5S1/2​(F=3) to 5P3/2(F=3){5P}_{3/2}(F=3)5P3/2​(F=3) hyperfine transition increased transiently when a relatively strong coupling field in resonance with the 5S1/2(F=2){5S}_{1/2}(F=2)5S1/2​(F=2) to 5P3/2(F=3){5P}_{3/2}(F=3)5P3/2​(F=3) hyperfine transition was switched on rapidly using a Pockels cell. The probe transient showed an initial Rabi half-cycle overshoot before settling down to steady-state EIT. The results agreed well with computations using a three-state model of the \ensuremath{\Lambda} system. The computations also suggest that transient gain should be observed with coupling field power only four times larger than that presently available to us.
Measurements of stimulated Raman gain in rubidium vapor are made and compared with a detailed model. Amplification of a weak probe laser is observed in a \ensuremath{\Lambda}-type three-level system that is driven by a strong cw coupling laser. The intensity of the coupling laser is sufficiently strong that it both optically pumps the atomic levels, causing a population inversion in the ground-state hyperfine levels, and drives the stimulated Raman process producing gain. A theory that models all aspects of the gain mechanism is presented. The model includes all interactions between the external fields and the atomic transitions, including those processes that involve multiple fields interacting with the same transition. Numerical calculations are compared with gain measurements made at several coupling laser intensities and detunings from the D1D1D1 line in 85Rb.{}^{85}\mathrm{Rb}.85Rb.
We report the observation of steady-state amplification of a weak probe laser without population inversion in the bare states in a Doppler-broadened \ensuremath{\Lambda}-type Rb atomic system. The probe amplification was observed with the Rabi frequency of a coupling laser much smaller than the Doppler width and can be viewed as due to the dressed-state inversion. A 10% single-pass amplification of a weak probe laser was obtained, and the measured linewidth of the gain spectral feature approaches the Rb natural linewidth. A theoretical analysis based on a three-level Doppler-broadened \ensuremath{\Lambda} system is presented, and qualitative agreement with the experiment results is achieved. \textcopyright{} 1996 The American Physical Society.
We explore a three-level Lambda scheme to demonstrate that the phenomenon of lasing without inversion (LWI) can be observed in the transient regime. We demonstrate that the pure LWI contribution to the gain of a probe field is distinct from both the resonant absorption (or gain) and the coherent Raman gain (or absorption) by choosing specific initial populations in the dressed-state basis. The suppression of the non-LWI (resonant and Raman) processes is followed by the "rich get richer" (capitalistic) effect for the ground-state population dynamics: Initially, the more populated ground state becomes even more populated. The conditions for the observation of the effect are specified.