Determine The Energy Level Diagram Of Atoms By F-STIRAP Technique

This thesis is composed of two parts, one is theoretical and experimental discussion of some details ralated to the storage and read out of the optical pulses in a three-levelΛ-type of 87Rb atomic system, the other is the detection of energy level of atoms.Theory part: in this thesis, we first studied the population transfer and the coherence in a three-levelΛ-type 87Rb atomic system in theory. And we also give the numerical simulations. The diagram of three-levelΛ-type system in 87Rb atoms is shown in Fig.1. In Fig.1,Ω1andΩ2are the Rabi frequency of the coupling pulses(λ1 =794.9842nm,λ2 =794.9698nm).Ω3andΩ4are the Rabi frequency corresponding to the probe pulse and the signal pulse, respectively. It has been demonstrated in the third section that maximal coherence between level 1 and 3 can enhance the intensity of the signal pulse (CARS signal). Here,we make the two write control optical pulses with the same back edge to prepare the Rb atoms with maximal coherence between hyperfine levels 5 2 S 1/2,F= 1,MF=?1,0and 5 2 S 1/2,F= 2,MF=+1,+2(which frequency shift is 6.8Ghz). After a time interval, we turn on the retrieve control pulse at 794.9842nm(or 794.9698nm)the recovered probe pulse at 794.9698nm(or 794.9842nm). That means we experimentally demonstrate that optical pulses stored in an atomic system can be controllably released into two different channels. And the retrieved pulse is also the enhanced CARS signal.The interaction Hamiltonians in the rotating wave and dipole moment approximations is:The elements of the density matrix are given by the Liouville equation:We use coordinatesξandτ,which are related to the laboratory coordinates byξ=z and cτ=t -z, The propagation of optical pulse fields in a medium is described by the equation:The equations (1), (2), (3)form a self-consistent system of equations. We performed numerical simulations of the above self-consistent equations for the case of maximal atomic coherence between ground levels in 87Rb vapor cell. The result of the numerical simulations is shown in Fig.2. Fig2 is the numerical simulations of the three level type of Rb atoms.As illustrated in Fig2,Ω3is the writing pulse(probe pulse),Ω4is the released pulse(CARS signal). Fig.2(a) shows the input pulse envelopes with the write control pulse preceding the probe pulse and the retrieve control pulse coming behind of them at the entrance of the medium.The variation of the write control pulse,the probe pulse,the retrieve control pulse and the restored pulse at the exit of the cell is shown in Fig.2(b). Fig.2(c)shows the population transfer as a function of time. The initial population is equally distributed between state 1 and 3 , as the incidencing ofΩ1andΩ2, The coherence termρρ13reaches its maximum value.The result is similar when the position ofΩ3andΩ4is changed.Experiment part: Fig.3 is the experimental arrangement we use.In Fig.3, ECDL1 and ECDL2 are the DL100 external-cavity diode lasers;Ω1 ,Ω3, are generated by ECDL1(λ1 =794.9842nm),Ω2andΩ4are generated by ECDL2(λ2 =794.9698nm), AOM1 and AOM2 are the acousto-optic modulators which are used to turn on an off the laser to generate the probe pulse. L1~L6 are the lens. PD1 and PD2 are the photodiodes.(1) The storage and read out of the optical pulse.Fig.4, Fig.5 is result of the optical storage.Fig.4 shows the result of the storage in the three-levelΛsystem.Ω3 andΩ4 are Rabi frequency of the reading pulse and the retrieved pulse, respectively. The delay between the pulseΩ1andΩ3is 80ns. Fig.4 (a) the pulses at the entrance of the Rb cell, Fig.4 (b) The pulses after Rb cell,Ω4 is the released pulse. The result is similar whenΩ4 is the reading pulse. We notice that, as the absorption of second coupling field, the peak at the back of the first coupling pulse arises. We believe that it is caused by the stimulated Raman resonance scattering(SRS).Fig.5 shows the intensity of restored pulse versus the storage time. We can see that the intensity of restored pulse become weaker as the storage time increases. We believe that this phenomenon is caused by the effect of decoherence.(2) Determine of the energy level.Fig.6 is the CARS signal on the oscillograph:The wavelength ofΩc1andΩp isλ1=794.9842nm, the wavelength ofΩc2andΩCARSisλ2=794.9698nm, the duration ofΩc1,Ωc2,Ωp are. 170ns,30ns,30ns,respectively. The delay between the pulseΩc1andΩpis 80ns.After the observation of the enhanced CARS signal, we use the technique of F-STIRAP to determine the energy level diagram of atoms. In our experiment, we set ECDL1 scanning from 794.9839nm～794.9844nm, as shown in Fig.7, we observe a maximal CARS signal corresponding to the two-photon resonance. Then we are able to calculate the position of the energy level.In summary, we experimentally demonstrated that the optical signal can be stored and controllably released in the system of a three-levelΛ-type of 87Rb. And based on it, we design a experiment to determine the position of the energy level diagram by F-STIRAP technique. Comparing the result of theory with the experiment, we can see that they match well with each other.