Observation Of Spontaneously Generated Coherence And Gain Spectrum Induced By Spontaneously Generated Coherence

Spontaneously generated coherence (SGC) refers to the interference of two ormore decay channels with nonorthogonal electric-dipole transition matrix elements.It gives rise to a variety of novel quantum effects, such as spontaneous emission,resonance fluorescence, absorption, lasing without inversion, dark state, Kerrnonlinear, photon correlations, coherent population trapping transfer, entanglement,quantum photocell, photonic bandgaps and electromagnetically induced grating.However, it is very difficult to find a real atomic system with SGC toexperimentally demonstrate these phenomena, for the existence of SGC requirestwo ormore rigorous close-lying levels subject to the conditions that these levels arenear degenerate and the corresponding dipole matrix elements are not orthogonal.In order to observe the phenomena based on SGC in atomic systems without thenear-degenerate levels, a few methods have been proposed to simulate thisintriguing effect. But most works of this type are theoretic and have not beenverified experimentally.1. Theory:a) We examine the absorption of a weak probe beam in a laser-driventripod-type atom with three closely lying ground levels, where both thedriving and probe lasers interact simultaneously with the three transitions.The effects of spontaneously generated coherence (SGC) are taken intoaccount. We introduce dipole moments in the dressed-state picture and theHamiltonian in terms of the dressed states describing the interactionbetween the probe and the atom. Gain spectrum under various conditionsare presented and analyzed. We show that the spectral structure and the gainamplitude of the probe are strongly influenced by the effect of SGC and the frequency separation of the three closely lying ground levels. While in thenondegenerate case, the gain spectra not only are strongly dependent on theeffect of SGC but also are dependent on the energy separation of the threeclosely lying ground levels. When the separation is not large, theamplitudes of the gain peaks are influenced by the SGC factor because thevariation of the SGC factor can modify the population of the dressed states.In the absence of quantum interference, the spectrum may exhibit two pairsof gain peaks in the near sidebands, which is caused by populationinversion in the dressed states. When quantum interference is maximal, thetwo pairs of gain peaks evolve into absorption peaks. This is because all thematrix elements of the atomic dipole moment in the dressed-state pictureare nonzero and the population inversion in the dressed states is notachieved. In the case of larger energy separation of the ground levels, thespectrum exhibits two pairs of absorption peaks in the near sidebands, forunder this condition population inversion between dressed states cannot beachieved. When the SGC factor increases, the absorption peaks will beenhanced because of the larger population difference in the dressed states.Finally, we discuss a possible experiment to attain these results. Theexperimental suggestion is applying two coherent fields on three of the fourlevels in an N-type four-level system without SGC.b) We investigate the effects of spontaneously generated coherence on boththe steady and transient gain (absorption) properties in a four-levelinverted-Y-type atomic system in the presence of a weak probe, two strongcoherent fields, and an incoherent pump. For the steady process, we findthat the inversionless gain origins from both spontaneously generatedcoherence and dynamically induced coherence, but the former contributesmore to the inversionless gain. In particular, we can modulate theinversionless gain just by changing the relative phase between the twofields. And when detuning of the coupling field is not zero, we can obtaintwo gain peaks. As for transient process, the transient gain (absorption)properties can also be dramatically affected by the spontaneously generatedcoherence. The transient gain can be greatly enhanced with completelyeliminated transient absorption just by choosing the proper relative phase between the two fields. The experiment can be designed to attain theseresults avoiding the conditions of spontaneously generated coherence.2. Experiment:a) We report the experimental observation of the effect of spontaneouslygenerated coherence on absorption without the rigorous requirement ofclose-lying levels. The experiments are studied in both a four-level N-typeand a four-level inverted-Y-type atomic system in a rubidium atomic beam.With the coupling and controlling field, the N-type system is equivalent to asystem with three closely upper levels coupled to one lower level by thesame vacuum modes. The quantum interference can induce two prominentand nearly transparent holes where the slope of the refractive index is verysteep. This special situation could allow the simultaneous propagation oftwo weak pulses with different frequencies.When we tune the wavelengthof the controlling field, the N-type systemturns to be the inverted-Y atomicsystem. Under the two-photon resonance condition, the system isequivalent to a V-type systemwith two closely upper levels, and theinterference can reduce one broad transparency windowin the middle of theabsorption spectrum. Besides we can control the number of thespontaneously decay channels by the detuning of the controlling field, thusthe effect of spontaneously generated coherence can exist in three or twoclosely space levels.b) We investigate the resonance fluorescence spectrum of an atomicthree-level ladder system driven by two laser fields. We show that such asystem emulates to a large degree a V-type atom with parallel dipolemoments—the latter being a system that exhibits spontaneously generatedcoherence and can display ultrasharp spectral lines. We found a suitableenergy scheme in85Rb atoms and experimentally observed the narrowingof the central peak in a rubidium atomic beam. The corresponding spectrumcan convincingly demonstrate the existing of spontaneously generatedcoherence. And we also find that the fluorescence spectrum can bemodified by the detuning of the controlling field. When the detuning issmall, the fluorescence spectrum may exhibit seven peaks and thelinewidths of the central peak is narrower than the standard resonance fluorescence of a two-level atom; whereas under the large value of thedetuning, the fluorescence spectrum exhibits no more than three peaks andno narrowing of the central peak can be observed. And the detuning of thecontrolling field can also modify the intensity of the peaks. From theexperimental results we can see that the height of the fluorescence spectrumis increased while the outer sidebands and one pair of inner sidebands aredecreased as the detuning of the controlling field is increased. We haveexplained these spectral features in terms of the dressed-atom model of thesystem. The experimental measurements agree quantitatively withtheoretical calculations.