Hole-burning And Slow Light In Atomic Vapors Coherently Driven Into The N Configuration

Atomic coherent effect mainly refers to the quantum correlation among various atomic levels. Coherent laser fields interacting with an atomic system will cause Autler-Townes splitting which leads to atomic coherent effect. The dressed states are different from atomic eigenstates on light absorption, dispersion, and spontaneous radiation, which benefit the coherent control of the interaction between light and matter on demand. Since 1980s, people have proposed several ideas based on atomic coherent effect, for example, coherent population trapping (CPT), electromagnetically induced transparency (EIT), amplification without inversion (AWI), and reduction of group velocity. Studies of atomic coherent effect and its applications have been further developed in the last decade, for example, a new concept, coherent hole-burning phenomenon, which is the union of electromagnetically induced transparency and hole-burning, is proposed. Coherent hole-burning phenomenon has attracted a great deal of interests. It has been investigated for many potential applications, not only in classically optical information storage, but also in control of group velocities of light pulses.In this thesis, we mainly study how to obtain coherent hole-burning phenomenon by utilizing a saturating and two coupling laser fields in a Doppler-broadened four-level N-type atomic system (as shown in Fig.1), and analyze the probability of obtaining slow light by utilizing this technique. Detailed theoretical process is as follows: firstly, we try to resolve the steady solution of the density-matrix equations in atomic-field interaction semiclassical theory, and then carry out the probe absorption spectrum by utilizing the linear response theory, the quantum regression theory, and the Laplace transform.First part: studies of coherent hole-burning phenomenon in a four-level N-type atomic system. In this part, we mainly analyze the coherent burning holes'characteristics in four different propagation schemes.First propagation scheme:(1) The saturating laser field s and the probe field p counter-propagate, while both the coupling laser fields (c andd ) and the probe field p co-propagate. In this situation, we could observe an EIT window and six burning holes in the probe absorption spectrum. Keeping the Labi frequency of the saturating laser field unchanged while increasing the Labi frequencies of both the coupling laser fields simultaneously, the EIT window will broaden, and all the burning holes will move to the outside, while their depths have no obvious changes. Conversely, keeping the Labi frequencies of the coupling laser fields unchanged while increasing the Labi frequency of the saturating laser field gradually will deepen the coherent burning holes, while keep their positions unchanged. Thus, the positions of the coherent burning holes depend on the coupling laser fields'Labi frequencies, while the depths of the coherent burning holes depend on the saturating laser field's Labi frequency. When the Labi frequency of the coupling laser field c decreases to zero gradually, the N-type atomic system will degenerate into a three-levelΛ-type atomic system, so that only four coherent burning holes remain in the probe absorption spectrum. When the Labi frequency of the coupling laser field d decreases to zero gradually, the N-type atomic system will degenerate into a simple two-level Ladder-type atomic system, so that only one burning hole remains in the probe absorption spectrum. Thus, the increasing of the coupling fields'number leads to the increasing of the burning holes'number. (2) The saturating laser field s and the probe field p counter-propagate, while the coupling laser field d ( c ) and the probe field p co-propagate (counter-propagate). In this situation, we could observe three high-amplitude absorption peeks in the probe absorption spectrum, which are typical characteristics of electromagnetically induced absorption (EIA). Between the three absorption peeks there are absorption decrescences, which correspond to two EIT windows. If we decrease the Labi frequency of the coupling laser field c gradually, the mid absorption peek will split, which leads to a third EIT window's appearance. When the Labi frequency of the coupling laser field c decreases to zero, the four-level N-type atomic system again degenerates to a three-levelΛ-type atomic system, so that the outside EIT windows will evolve into mixtures of EIT windows and coherent burning holes gradually, and into pure coherent burning holes at last.Second propagation scheme:Both the coupling laser fields c and d counter-propagate with the probe field p . In this situation, the coupling laser fields and the probe field can't eliminate the Doppler broadening, so no EIT window occurs in the probe absorption spectrum. However, we could observe five coherent burning holes (as shown in Fig.2) in the probe absorption spectrum simultaneously, whose appearances needn't elimination of the Doppler broadening. We note that the mid coherent burning hole is twofold degenerate, namely it is superposed by two independent coherent burning holes, and thus it is deeper than the other four coherent burning holes.Third propagation scheme:All the saturating ( s ) and the coupling (c andd ) laser fields counter-propagate with the probe field p . In this situation, the saturating laser field will propagate through the medium without absorption, thus no coherent burning hole occurs. Namely, without the coupling laser fields, there will be a burning hole in the probe absorption spectrum, while with the coupling laser fields, the number of the burning holes changes to zero. Thus, the coupling laser fields increase the probe absorption, meanwhile they decrease the saturating absorption, which means we can control two beams by one beam.Fourth propagation scheme:All the saturating ( s ), coupling (c andd ), and probe ( p ) fields propagate in the same direction. In this situation, the coupling and probe fields eliminate the Doppler broadening, therefore EIT windows will occur in the probe absorption spectrum. Fig.2. Probe absorption spectrum in the four-level N-type atomic system driven by a saturating and two coupling laser fields. Meanwhile, two hollows occur in the probe absorption spectrum, i.e. the coherent burning holes. Namely, the saturating absorption isn't suppressed completely by the coupling laser fields, although the fields propagate in the same direction.Second part: studies of slow light in the four-level N-type atomic system with coherent hole-burning phenomenon.In this part, to analyze the probability of obtaining slow light utilizing EIT windows and coherent burning holes in the four-level N-type atomic system, we resort to D1 line of rubidium (Rb) atoms in the room temperature as an example. In the first propagation scheme, keeping the coupling laser fields'Labi frequencies unchanged, we can obtain the slow group velocities in various degrees by controlling the saturating laser field's Labi frequency. As the increase of the saturating laser field's Labi frequency, the EIT windows become deeper and narrower, which correspond to steeper normal dispersions. Meanwhile, the coherent burning holes become deeper, which correspond to greater group refractivities. In the second propagation scheme, the mid coherent-hole burning is twofold degenerate, so it's obviously deeper than the other coherent burning holes, which is benefit for obtaining greater group refractivity (as shown in Fig.3, which correspond to the absorption spectrum shown in Fig.2). Keeping the coupling laser fields'Labi frequencies unchanged and increasing the saturating laser field's Labi frequency, we can obtain deeper and narrower coherent burning holes, which lead to greater Fig.3. (a) Refractivity as a function of probe detuning. (b) Group refractivity as a function of probe detuning. dispersions. In addition, we can control the coherent burning holes'positions by tuning corresponding parameters, and further control the spectral positions of slow light.In summary, in this thesis we study the coherent hole-burning phenomenon and slow light in atomic vapors coherently driven into the Doppler-broadened four-level N configuration by coupling and saturating laser fields. We analyze coherent hole-burning phenomenon in four different propagation schemes, and focus on the study of obtaining slow light in the two former schemes. Our results enrich the theory of interaction between light and matter, and supply substantial base for applications of the coherent hole-burning technique to optical information storage.