二元相位菲涅尔波带片近场俘获高低折射率粒子（英文）

Near-field Trapping High and Low Refractive Index Particles with a Binary Phase Fresnel Zone Plate

采用双环形涡旋偏振光照射二元相位亚波长菲涅耳波带片的方法,实现了对高低折射率瑞利(Rayleigh)粒子的近场俘获.利用角谱理论计算了菲涅耳波带片的衍射场分布.改变入射光的截断参数(β)和涡旋角(δ),可以在菲涅耳波带片的近场区域产生亮斑和暗斑.计算发现,当β=1.09和δ=0时,在近场区域产生亚波长三维亮斑,能够稳定俘获19nm的金粒子,金粒子的折射率大于周围介质,轴向和横向俘获距离分别为0.4921λ和0.2844λ.当β=1.45和δ=0.414π时,在近场区域产生光墙包围着的三维暗斑,可以将30nm的空气泡稳定地俘获在暗斑中心,空气泡的折射率小于周围介质的折射率.两种情况计算所得的俘获距离均小于传统远场俘获系统中的距离.该系统可以用来精确俘获两类折射率不同的Rayleigh粒子.

A method of near-field trapping high and low refractive index Rayleigh particles was proposed using a binary phase Subwavelength Fresnel Zone Plate（SFZP）illuminated by the double-ring-shaped vortex beam.Based on the angular spectrum representation,the distribution of SFZP′s diffraction field was calculated.Bright and dark spots can be generated in the near field of the SFZP by changing the truncation parameter（β）and the vortex angle（δ）of the incident beam.Numerical results show that the SFZP can generate a subwavelength three-dimensional（3D）bright spot in the near-field region whenβ=1.09andδ=0.The tiny bright spot can be used to stably trap agold particle with radius 19 nm,whose refractive index is higher than that of the ambient medium.The axial and transverse trap distances are respective 0.4921λand 0.2844λ.Whenβ=1.45andδ=0.414π,a 3Ddark spot surrounded by a light shell can be formed in the near-field region.An air bubble with radius 30 nm is stably trapped at the center of this subwavelength dark spot.The refractive index of the air bubble is lower than the ambient medium.The axial and transverse trap distances are 0.62λand 0.3081λ.The trap distance obtained under two conditions is less than that in the conventional far-field trap system.The single trap system can be used to accurately trap two types of Rayleigh particles with different refractive indices.