Generation Mechanism And Characteristics Of Dark Hollow Beam In Hot Atomic Rb Vapor

With the deeper application of dark hollow beam (DHB) in many areas such as, micro-particle guiding and alignment, optical trapping, super-resolution imaging technology and so on, the generation of DHB become increasingly important. To build a simple structure, and adjustable dark spot size experimental method has turned into a critical part in DHB applications. In this paper, we propose a new method to generate DHB by using the self-phase modulation (SPM). The laser frequency, power, atomic number density, and cell position for SPM effect were investigated both experimentally and theoretically. What's more, the generation of DHB and its polarization, transmission properties were also studied. Main efforts are listed as follows:We take Rubidium (Rb) atoms as the nonlinear medium. Firstly, the frequency dependence of the diffraction ring patterns caused by SPM effect was investigated, it is seen that the most obvious diffraction patterns will appear at the frequency detuning about 0.7 GHz. And then, the laser power, atomic number density, and cell position dependence of the diffraction ring patterns were studied at the frequency detuning about 0.7 GHz. The results show that the number of the rings increases with the increase of incident power and atomic number density, but the center will not change, and the center of the diffraction rings is bright for the cell vapor at positive axis and dark for negative. Finally, the nonlinear refractive index coefficient of Rb atoms was calculated to be 0.8 X 10-16W/W,1.5×10-16cm2/W and 2.0×10-16cm2/W at the temperature of 90 ?,100 ? and 105 ? respectively by SPM effect. This study can help people understand the system of the far-field diffraction patterns, which can also be a reference in generation of DHB.The studies for SPM effect indicate that for proper parameters, there will appear a single diffraction ring with dark center. The focal length of the lens and the position of sample were also studied and found with the decreasing of lens focal length the intensity of DHB center will decrease, but the size will increase. What's more, there will be a darker center of DHB for a bigger distance between cell position and lens focal point. Which indicates the feasibility of the method in generating a adjustable dark spot size DHB by SPM.Finally, the polarization and transmission properties of DHB generated by SPM were investigated in Rb atoms. The results indicate that the DHB is linear polarized, its polarization direction is as same as incident light. It is also found that the center of the DHB will not change for a long transmission distance. These properties demonstrate the potential of DHB in many areas.