Study On Atomic Coherence Properties Of N-type Four-level Atomic System

In this thesis, we mainly study the coherent hole-burning (CHB), slow light,coherent absorption and gain, and four-level fluorescence effect in a N-type atomicsystem. There are three parts in the thesis.1．The coherent hole-burnings and the slow light in a four-level N-type atomicsystemIn this part, we study the steady optical response of a four-level N-type atomicsystem, which is driven by three laser fields (i.e. one coupling, one perturbing, andone saturating), containing CHB and slow light. Two system configurations areconsidered:(a) The saturating laser count-propagates with the probe laser, while thecoupling and the perturbing lasers co-propagate with the probe laser;(b) The saturating and the perturbing laser count-propagate with the probe laser,while the coupling laser co-propagates with the probe laser.Firstly, the dependence of the number, the location as well as the depth ofburning holes on system parameters are analyzed for the two situations, separately.Our numerical results show that, for situation (a), six CHBs and one EIT windowcan be found in the probe absorption spectrum. The width and the depth of the holeburnings can be changed by adjusting the Rabi frequencies of the coupling andperturbing laser fields. Only when Ω_c=Ωd=60MHz, the hole burning is themost best. If the perturbing laser is very weak, the probe absorption spectrum willrecover the CHB features (i.e. four CHBs and one EIT window) in a three-levelLambda system. Adjusting the saturating field detuning, more CHBs may beobserved but the EIT window will become shallower and a little deformed. Instead, for situation (b), we find that only two CHBs can be observed while the others areconcealed in two EIT windows separated by an electromagnetically inducedabsorption (EIA) peak. Gradually weakening the perturbing laser, an third centralEIT window appears as a result of the EIA peal splitting. Moreover, when theperturbing laser is not very strong the two side EIT windows are in fact dependedby the CHBs in it, and when the perturbing laser finally becomes zero Ω_d=0thetwo side EIT windows turns out to be two CHBs. We also analyze the abovenumerical results through the theory of dressed state.Secondly, the probe dispersion of the system is analyzed numerically, and thegroup index of refraction and the group velocity are computed. Our results showthat, slow light can be obtained in the burning holes and EIT windows. The probedispersion and the group index of refraction depends on the Rabi frequencies of thecoupling, the perturbing, and the saturating lasers. The change of probe dispersionis large and the group index of refraction is more bigger, when Ω_s=6MHz,Ω_c=Ω_d=60MHz.2. The absorption and gain of the probe laser of four-level N-type atomicsystemIn this section we calculate separately the probe absorption (gain) anddispersion coefficients of a four-level system driven by three laser fields. Thedependence of the probe absorption and dispersion coefficients on the couplingfield intensity and radiation coefficients is discussed. Our results show that, threegain peaks for Ω_p1 can be observed when Ω_p2=Ω_c=5.0MHz, and the gain ishighest at the resonant point. With the decreasing of Ω_c, the peak of gain atresonance is divided, the gain coefficient decreases gradually, and the dispersioncurve shifts from the normal dispersion to abnormal dispersion at resonance. Thenan enhanced superluminal or lossless subluminal light can be observed finally.Gain for Ω_p2(ω_p2)cannot be observed no matter what Rabi frequency of Ω_cis choosed, and the probing laser on resonance is absorbed increasingly with theincrease of Ω_c.3. The fluorescence effect of coherent radiation field in N-type four-levelatomic systemIn this section we discuss the resonant fluorescence effect in a four-levelatomic system driven by one or two coupling lasers. The fluorescence coefficientsare calculated for different spontaneous radiations, laser strengths and frequencies.When the system is driven by one coupling laser, the resonant fluorescence splitsfrom a single peak to two peaks with the increase of the coupling laser strength. Inboth cases, the height and location of the peak of resonant fluorescence spectrarelate with the coupling and probing detunings, from which we can suppress orenhance a signal light by adjusting the frequency of coherent laser. Correspondingto the same atomic structure, different intensities, numbers and locations of thefluorescence spectrum can be obtained in different laser paths with differentnumbers of strong coherent lasers, so we can change some parameters of theoptical path to achieve the control of the emission level resonant fluorescence.