用波晶片相位板产生角动量可调的无衍射涡旋空心光束

本文提出了一种用波晶片产生无衍射涡旋空心光束的新方案. 根据晶体双折射的性质, 设计波晶片的厚度, 在一块晶体薄片上对o光和e光分别形成各自的四台阶相位板, 线偏振光入射到该相位板后, o光和e光衍射按强度叠加, 利用准伽利略望远镜系统聚焦, 得到近似无衍射涡旋空心光束. 光路简单, 调节方便. 在近轴条件下, 运用菲涅耳衍射理论和经典电磁场角动量理论, 数值模拟计算了周期数不同的两块波晶片相位板衍射光强和角动量的分布, 结果表明: 两块相位板都能在较长距离内产生近似无衍射涡旋空心光束, 光强和轨道角动量的分布与螺旋相位板产生的涡旋光束基本相同. 在衍射光路中加入相位补偿器, 调节o光和e光的相位差可以调节自旋角动量的大小, 从而可以调节总角动量密度和平均光子角动量的大小. 用这种空心光束导引冷原子或冷分子, 原子在与光子相互作用过程中可获得可调的转动力矩.

Generation of no-diffraction hollow vertex beams with adjustable angular momentum by wave plate phase plates

In this article, a new scheme is proposed to generate approximately no-diffraction hollow vertex beams by wave plates. By selecting the appropriate thickness values of wave plates based on the properties of the double refraction, four-step-phase plates for o-light or e-light are formed. With linearly polarized light irradiated at the phase plate, the diffractions of o-light and e-light would overlap according to their intensities. By focusing effect of quasi-Galileo telescope system, a no-diffraction hollow vertex beam can be generated. In this scheme, the optical path is simple and convenient to adjust. Under the adaxial condition, the distributions of diffraction intensity and angular momentum of two wave plates at the numbers of cycles, s=1 and s=4, are numerically simulated according to Fresnel diffraction theory and classical electromagnetic field angular momentum theory. Simulation results indicate that the approximately no-diffraction hollow vertex beams can be generated by each of two phase plates within a long distance. The distributions of intensity and the angular momentum are essentially the same as those generated by spiral phase plates at the same number of cycles. The distributions of intensity and the angular momentum are different at different numbers of cycles s. If s increases, the diffraction bright ring radius increases, the intensity decreases and the average orbital angular momentum increases. At s=4, the length of no-diffraction region is significantly greater than at s=1 and the average orbital angular momentum is four times that at s=1. Within the no-diffraction region, the distribution of orbital angular momentum intensity varies with distance but the total angular momentum is constant. A phase compensator is inserted in the diffraction path to adjust the phase difference between o-light and e-light. Whereas the spin angular momentum of the diffraction light can be adjusted by them, and thus the total angular momentum intensity and average photon angular momentum can be adjusted. This scheme can be utilized to guide the cold atoms or molecules to obtain the adjustable torque throughout the interacting process of atoms and photons.