Intensity-dependent Effects On Four-wave Mixing Based On Electromagnetically Induced Transparency

Fig.1Theoretical model of the four-level double-Λ system of87Rb atomic. This thesis introduced how the signal intensity is influenced by intensities of thecoupling, pump, and probe fields in the FWM process which based on EIT.The energy model for FWM is shown in Fig.1. There are two FWM channels inthis model. The first FWM channel (population flow:1â†'3â†'2â†'4â†'1) isthe generation of internal FWM signal coming from the input probe field in EITscheme; the second FWM channel (population flow:1â†'4â†'2â†'3â†'1) iscaused by the reabsorption of the internally generated FWM signal. The FWMefficiency is greatly enhanced because of the constructive quantum interferencebetween two concurrent and competitive FWM processes. Fig.2Schematic of experimental setup BS:beam splitter;λ/2:half-wave plate; PBS:polarizing beam splitter;EOM:electrooptical modulator.The experimental setup is shown in Fig.2. A Ti:sapphire ring laser is tuned to transition|5S1/2,F=2>â†'|5P1/2,F=2> and acts as the coupling field of Rabi frequency Ω1; an external cavity diode laser-ECDL1(DL100) is tuned to transition|5S1/2,F=1>â†'|5P1/2,F=2> and acts as the probe field of Rabi frequency Ωp another an external cavity diode laser-ECDL2(DL100) is tuned to transition|5S1/2,F=2>â†'|5P3/2) and acts as the pump field of Rabi frequency Ω2. In this case, a signal field of Rabi frequency Ωf may be generated on transition|5S1/2,F=1>â†'|5P3/2>as a result of FWM. The coupling, probe, and pump beams of linear polarizations (either horizontal or vertical) propagate collinearly into temperature-stabilized vapor cell with the help of a beam splitter (BS), a λ/2wave plate, and a polarization beam splitter (PBS1). The vapor cell of87Rb atoms has a sample length of L=3.0cm and an volume density of2.2×10cm at the temperature of T=62℃. After the vapor cell, the probe field Ωp and the pump field Q2of horizontal polarizations pass through another polarization beam splitter (PBS2) to arrive in a grating (G1) while the coupling field Ω1and the signal field Ωf of vertical polarizations are reflected by PBS2to another grating (G2). The grating G1(G2) with a groove density of1200lines/mm can spatially separate theprobe (coupling) fieldΩ p(Ω1) of795nm and the pump (signal) fieldΩ2(Ω f) of780nm.Photodiodes D1and D2are used to monitor intensities of the probe fieldΩ pand the signal fieldΩ f, respectively.First of all, we complete a classic EIT experiment. We show in Fig.3the probeabsorption and the signal intensity as a function of the probe detuningΔ pwhen thecoupling field has a vanishing detuning. As we can see from the black curves, thesignal field is not generated in the absence of the pump field and a typical EIT dip isobserved on the Doppler broadened spectrum of probe absorption. On the other hand,the red curves clearly show that the signal field is generated inside the EIT dip whenthe pump field is turned on. And we also find that EIT dip becomes much shallowerdue to the nonlinear energy transfer from the probe field to the signal field along theclosed pathway.Next we show in Fig.4the signal intensity as a function of the coupling intensityfor four different values of the pump intensity. It is clear that, for a given pumpintensity, the signal intensity first quickly increases to a maximum and then slowlydecreases after this maximum when the coupling field becomes stronger and stronger.Moreover, for a larger (smaller) pump intensity, the maximal signal intensity always corresponds to a larger (smaller) coupling intensity. This means that the maximalsignal field can only be attained when the coupling field and the pump field are wellmatched in intensity. We can conclude that there exists a linear relationship betweenthe pump intensity and the coupling intensity for attaining a efficient FWM process.We have studied how the signal intensity is influenced by intensities of thecoupling, pump, and probe fields in the FWM process. To attain an efficient FWMprocess, we adopted a special scheme, in which the probe flied and the coupling fieldwork in the EIT regime with vanishing detunings while the pump field and the signalfield work in the coherent Raman regime with large detunings. We also find that themaximal signal intensity approximately corresponds to a constant ratio between thecoupling intensity and the pump intensity when the probe intensity is fixed. That is,the pump and coupling fields should be well matched in intensity to attain the optimalFWM efficiency. In our experiment, the probe intensity should not be too large to gobeyond the EIT condition. Otherwise, the FWM efficiency will be greatly reduced.