We consider a four-level atomic system with a degenerate or quasidegenerate ground-state doublet and two excited states. One of the upper levels is coupled to the ground doublet by a coherent field of variable strength, which we refer to as the Raman field, whose role is to create atomic coherence between the driven atomic states. The other higher lying state is coupled to the ground states when laser action develops. This model includes an incoherent pump mechanism to create some population in the top lasing state. In appropriate conditions, for the incoherent pump rate and the Raman field strength, the absorption profile of the laser transition develops gain features. We discuss the physical origin of this gain which we find to be the consequence of a population inversion between dressed atomic states. It is interesting to compare the results of this analysis with a similar study of a three-level lambda system which was previously recognized to yield lasing without inversion.1 In this case the appearance of gain can be attributed to the induced atomic coherence.
Theoretical and experimental studies of stimulated Raman spectrum with the D1 line of an 85Rb atomic vapor system are performed. In this Λ-type system, we discover that the influence of the pump on the probe field includes two parts: stimulated Raman excitation and optical pumping. Depending on the pump frequency detuning and field intensity conditions, the Raman spectrum can display either gain or loss. When the optically pumped absorption spectrum is taken as background, either a narrow transparent window or an absorption doublet with widely different linewidths appears. Our theoretical analysis agrees well with our experimental observations. Based on these studies, we present an interpretation from a new viewpoint of the physics of electromagnetically induced transparency (with on-resonance pumping) and Autler-Townes splitting (with far off-resonance pumping) in a Λ-type level atomic system, giving the relationship between these two important phenomena and the Raman spectrum.
Recently, there has been much interest in the Raman gain observed in a Λ system with closely spaced ground states such that both ground states are initially equally populated and are both coupled to the excited state by a single frequency pump. This system is of interest for applications such as frequency shifting and self phase conjugation and as an example of a system that exhibits large optical gain for very small excited state populations. We have studied the mechanism of this gain theoretically and have found qualitative agreement with previously observed experimental results. Briefly, a single frequency pump interacts with both legs of the system. The gain spectrum is interpreted in terms of the energy intervals of the dressed states of the pumped system. The gain mechanism is analogous to the process of sideband amplification in a strongly driven two level system. However, the width of the gain is not limited by the linewidth of the excited state and, for low pump power, is determined by the transit time only. This is consistent with the extremely narrow gains (less than 1 kHz) observed in sodium vapor.
We present the results of an experimental and theoretical study of the gain (or absorption) experienced by a weak probe beam propagating through a sodium vapor in the presence of an intense pump field that is nearly resonant with the 3s → 3p atomic transition. The probe-transmission spectrum includes three distinct features that result from the modification of the atomic-level structure by the ac-Stark effect. Two of these features can lead to amplification of the probe wave. We measured the dependence of the probe spectrum on the detuning of the pump beam from resonance and on the pressure of a helium buffer gas. The experimentally obtained spectra are in good agreement with the predictions of a theoretical model based on the solution of the density-matrix equations of motion for a two-level atom and including the effects of Doppler broadening. The maximum gain measured in these experiments occurs at one of the Rabi sidebands and leads to a 38-fold increase in the intensity of the probe wave. Under some experimental conditions, we also observed two additional resonances, which are due to stimulated Raman scattering involving the sodium ground-state hyperfine levels.
We describe theoretical and experimental investigations of steady-state gain without population inversion that can occur on the
D1 transition of potassium, upon strong cw-laser excitation of the
D2 transition. A Raman-driven four-level model, in which a noninverted population in the 4
2P1/2 level is established via collisional transfer from the 4
2P3/2 level, is in good qualitative agreement with a variety of experimental results. These results include, among other studies, an examination of the impact of coherence dephasing on the realizable gain, as well as measurements of atomic energy-level populations for a case of line-center pumping that clearly displays gain. The measured relative populations for this case are n(4
P1/2)/
ntotal=0.098\ifmmode\pm\else\textpm\fi{}0.034 and n(4
P3/2)/
ntotal=0.19\ifmmode\pm\else\textpm\fi{}0.08, where
ntotal is the total population. These measurements indicate that the probed level 4
P1/2 population is not inverted with respect to the ground state. \textcopyright{} 1996 The American Physical Society.
An electromagnetically induced transparency is observed at one arm of a three-level \ensuremath{\Lambda}-type system in a rubidium D1 line (794.8 nm) with an 85% reduction in absorption, when a pumping field is present at the other arm. This reduction in absorption for the weak probe field is due to the atomic coherence produced by the strong pumping field. This experiment is done in a Rb vapor cell at room temperature with cw diode lasers for both pumping and probe beams in a Doppler-free configuration. A simple theoretical treatment including Doppler broadening is in good qualitative agreement with the experimental measurement.