Study On The Interaction Between Quantum Interference And Nonlinear Effects

Since the invention of laser, the interaction of light and medium has attracted extensive attention. Quantum interference has received considerable theoretical and experimental investigation recently, such as electromagnetically induced transparency, lasing without inversion, coherent adiabatic population transfer, interference enhancement or cancellation of two-photon absorption and resonant enhancement of refractive index. Nonlinear optical processes play a very important role on micro-structural characters of atom or molecular. Meanwhile, they have potential applications such as making all-optical switches. Four-wave mixing and two-photon absorption related to the third nonlinear effect has been studied for a long time while the interaction between the electromagnetically induced transparency and four-wave mixing is the subject of the present review. Of course, two-photon absorption of atom or molecular is also the focus of recent study.In this letter, an atomic system consists of ground state |c>, lower excitation state|b > and upper excitation state a > which based on the 5S1/2 -5P3/2 -5D5/2 scheme within rubidium is considered. A strong coupling field ha>j with Rabi frequency d interacts with state |b> and |a> (5P3/25D5/2), two-photon probe field Pr with Rabi frequency P couples transition |c > -|a > (5S1/2 -5D5/2 ). Obviously, coherent radiation field f with Rabi frequency f is certain to generate and couples transition |b >-| c >(5S1/2 - 5P3/2). The energy of each level is a , h, and c respectively.This letter emphasizes to the interaction of electromagnetically inducedtransparency and four-wave mixing, which causes considerable new phenomena unlike the ordinary A-type:1) In our model, HIT affects four-wave mixing directly. Adjust the intensity of probe field properly, and gain without inversion will occur between level |b> and |c>. Select other parameters, and once absorption is negative, gain between |b> and |c> must be inversionless.2) Because of the existence of four-wave mixing, the upper level obtains more population on resonance when the probe field is comparatively weak. At the same time, the population porfile becomes asymmetric as is the result of reference paper[39]. Therefore, the occurrence of asymmetry of the HIT porfile is not an accidental phenomena but an inevitable outcome of the existence of four-wave mixing.3) Whether the coupling and the probe field is weak or not, level |b> is always populated on resonance. The population of level |b> increases where the FWM is strong. The profile of the lower excitation state associates with the profile of FWM.4) If the probe field intensity is increased, e.g. P=1, several ultra-narrow HITdots occur in level |a> and populations are trapped in level |b> and |c> at different offset positions. When parameters are varied, the population of the upper excitation level is much more complicated.5) Different propagation means between coupling field and probe field will affect the realization of electromegnetically induced transparency. Their co-propagation can realize HIT more easily.Experiment part of this letter achieves interference enhancement of two-photon absorption firstly in sodium dimmer rhomb-type four-level system. A pair of superposition levels | 2 > and | 3 > because of the interaction of orbit and spin act asfortified middle levels.1) The two-photon absorption sign becomes the strongest when the probe fieldscans at the middle of the superposition levels. On the contrary, two-photon absorption exhibits weakest on resonant without the pair of middle superposition levels.2) When the detuning exceeds the width of Doppler broaden, two-photon absorption sign presents still.