带有相位拓朴数的环形相位板调控高斯光束的梯度力

 用基于德拜近似条件的矢量衍射理论研究了带有相位拓朴数的环形相位板调控高斯光束的光梯度力分布,改变相位板环形区的相对半径或当相位板的相位以一定拓扑数呈现拓扑变化时,可以调节光学系统焦点区域的光梯度力分布,形成各种非常有规则的几何形状的光陷阱,如当拓朴数分别取3,4,5,6时,可形成三角形、四边形、五边形和六边形光陷阱。此种相位板可用来构建可调的光镊系统,且通过调节相位板的各区域半径和拓扑数可以得到所希望的光陷阱。     

相位板偏振偏转角对径向偏振光的梯度力分布的影响

 研究了同心分区偏振偏转相位板对径向偏振光梯度力的调制效应。给出了相位板各部分偏振偏转角不同时,光学梯度力的分布情况。模拟结果表明:随着同心分区相位板各部分的半径和偏振旋转角的改变,光学梯度力方向及大小明显变化,且会产生许多可控的梯度力分布模式,可应用于微粒的收集、分离和合并。结果显示同心分区相位板对径向偏振光的调制可以用来生成可调光镊。     

Gradient force pattern, focal shift, and focal switch in an apodized optical system

In this paper, the evolution of the gradient force pattern, focal shift, and focal switch induced by a three-portion pure phase-shifting apodizer is numerically investigated in detail. The results show that the proposed apodizer may induce tunable gradient force on the particles in the focal region, focal shift, and focal switch. By adjusting the geometrical parameters of the phase-shifting apodizer, multiple traps may occur with changeable distance between them, and the shape of the optical trap also evolves evidently. More interestingly, for certain geometrical parameters of the proposed apodizer, by changing the phase shift of inner annular portion, the considerable focal shift may occur with focal switch accompanying, which is discussed to show that this kind of apodizer may be a very promising method of transporting trapped particles.     

Tunable gradient force of hyperbolic-cosine–Gaussian beam with vortices

Gradient force plays an important role in optical tweezers technique. In this paper, the tunable gradient force in focal plane of the hyperbolic-cosine–Gaussian (ChG) beam is investigated numerically. The ChG beam contains one spiral vortex and one non-spiral vortex. Simulation results show that the gradient force distribution can be altered considerably by decentered parameters of ChG beam, topological number of the spiral vortex, and vortex parameter of the non-spiral vortex. Many novel gradient force patterns can occur, which means corresponding optical traps may come into being, including ring optical trap, multiple-point trap pattern, line optical trap, rectangle trap pattern, and rhombus trap pattern. In addition, force pattern evolution principle may also differ significantly.     

Focusing properties of Gaussian beam with mixed screw and conical phase fronts

Vector diffraction theory is employed to investigate the focusing properties of the Gaussian beams with mixed screw and conical phase fronts. Numerical simulations show that the Gaussian beams with screw-conical phase fronts are different from both the ordinary Laguerre-Gaussian beams and the higher-order Bessel beams. Rather than forming the ring-shaped intensity distributions characteristic of optical vortices, focusing the Gaussian beams with screw-conical phase fronts produce non-symmetric spiral intensity distributions at the focal plane. The intensity distribution forms a counter-clockwise non-symmetric screw path around the focus. The rotation of intensity distributions was observed in the focal plane. The gradient force patterns of these beams focused with high NA are also investigated. The results show that the gradient force pattern shape depends principally on parameter topological charge n of the phase distribution. The gradient force pattern expands with increase in the parameter m of the phase distribution. Therefore, one can change the topological charge n or the parameter m of the phase mask to construct the tunable optical trap to meet different requirements. Its potential application might include rotational positioning of particles and accumulation of smaller non-symmetric particles towards the focus.     

Focus evolution of Gaussian beam induced by phase plate

In this article, vector diffraction theory is employed to investigate the focusing properties of the Gaussian beam through a phase plate. The phase plate may alter the wavefront phase of an incoming beam by topological charge. Both the circular phase distribution and the annular phase distribution plates are investigated. Numerical simulations show that the focal intensity distribution depends on topological charge. With changing topological charge, focal intensity distribution may evolve into ring shape, and some novel focal spots may occur. Focal intensity distribution evolving process with integer topological charge differs considerably from that with fraction topological charge. When the concentric annular phase plate is placed in the laser path, the focal intensity distribution depends on both the inner radius and topological charge. For small inner radius of the phase plate, doughnut-shape focal pattern occurs. With increasing inner radius, the diameter of the doughnut focal pattern decreases, and doughnut shape disappears slowly in some cases.     

Concentric piecewise azimuthally polarized beam by a pure phase plate

Focusing properties of the azimuthally polarized beam induced by a pure phase plate are investigated theoretically. The pure phase plate consists of two concentric portions, one center circle portion and one outer annular portion, through which the azimuthally polarized beam passed evolves into concentric piecewise azimuthally polarized beam. When the phase shift of the center portion is π, one ring focus may evolve into novel focal patterns with increasing radius of the center circle portion, such as cylindrical crust focus, two-ring focus, and three-ring focus. And if the geometrical parameters are unchanged, focal patterns also changes considerably with tunable phase of the center portion. Ring focus shifts along the optical axis on the increasing phase. Some optical gradient force distributions and dependence of focal shift on phase shift are also illustrated. This kind of concentric piecewise azimuthally polarized beam can be used in optical manipulation technology.     

Focusing properties of hyperbolic-cosine-Gaussian beams combining a helical axicon

Focus shape plays an important role in many optical systems, and has attracted much attention. In this article, the focusing properties of hyperbolic-cosine-Gaussian (cosh-Gaussian) beams through a helical axicon were investigated theoretically. Results show that intensity distribution in focal region can be altered considerably by the decentered parameter of cosh-Gaussian beam, axicon parameter and topological charge of the helical axicon. Many novel focal pattern may occur, including annular focus, rectangular focal shape, multiple-peak focal pattern. And in order to show the possible applications of these alter focal pattern, some optical gradient force distributions were also calculated and illuminated.     

Focusing properties of concentric piecewise cylindrical vector beam

The focusing properties of a concentric piecewise cylindrical vector beam is investigated theoretically in this paper. The beam consists of three portions with different and changeable phase retardation and polarization. Numerical simulations show that the evolution of the focal shape is very considerable by changing the radius and polarization rotation angle of each portion of the vector beam. And some interesting focal spots may occur, such as two- or three-peak focus, dark hollow focus, ring focus, and two-ring-peak focus. Corresponding gradient force patterns are also computed, and novel trap patterns, including cup shell shape trap with one trap at its each side along axis, rectangle shell shape trap with one trap at its each side, dumbbell optical trap, spherical shell optical trap, may occur, which shows that the concentric piecewise cylindrical vector beam can be used to construct controllable optical tweezers.     

Focus evolution of linearly polarized Lorentz beam with sine-azimuthal variation wavefront induced by one on-axis optical vortex

Focus evolution of linearly polarized Lorentz beam with sine-azimuthal variation wavefront induced by one on-axis optical vortex was investigated theoretically in this article. Calculation results show that the focal pattern can be altered considerably by the charge number of on-axis optical vortex under condition of certain beam parameters and phase parameter that indicates the sine phase change frequency on increasing azimuthal angle. And the focal evolution principle differs remarkably for different beam parameters and the phase parameter. In focus evolution process, some novel focal patterns appear, including annular focal pattern, two-peak focal pattern, intensity lines, hexagon containing two peaks, swallowtail shape, multipede shape, and complex focal pattern. Introduction of optical vortex adds one controllable parameter to alter focal pattern, which may extend application of Lorentz beam in some focusing systems.     

Focal shift of three-portion concentric piecewise cylindrical vector beam

Focal shift of a concentric piecewise cylindrical vector beam is investigated by means of vector diffractive theory. A section of the beam consists of three concentric portions. The center circle portion and outer annular portion are radial polarized, and the inner annular portion is generalized polarized. The phase of the inner annular portion is tunable. For certain geometric parameters of the beam, the focal shift occurs and can be adjusted by phase shift and polarized direction of the inner annular portion. For some cases, a three-peak focal pattern may occur, and peaks shift in the same direction, and one local minimum also shifts. For some other cases, a two-peak focal pattern may occur with shifting peaks. The tunable quantitative focal shift is investigated in detail.     

Radially polarized hollow Gaussian beam with on-axis spiral optical vortex

The focusing properties of radially polarized hollow Gaussian beam (HGB) with on-axis spiral optical vortex are investigated theoretically by vector diffraction theory. The phase wavefront of HGB is the function of radial coordinate. Calculation results show that the focusing properties can be altered considerably by beam order of HGB, topological charge of the on-axis optical vortex, and phase parameter that characterizes the radial phase wavefront distribution. Higher topological charge induces focal evolution from one focal spot to annular focal pattern in transverse direction, while phase parameter can lead to focal shift along optical axis remarkably. In addition, focal shift direction can also be adjusted by changing varying direction of phase parameter.     

Focusing properties of Gaussian beam with superimposed left-handed and right-handed helical phase fronts

Vector diffraction theory is employed to investigate the focusing properties of the Gaussian beams with superimposed left-handed and right-handed helical phase fronts theoretically. Numerical simulations show that the intensity distribution in focal region can be altered considerably by adjusting topological charge m corresponding to right-handed helix and topological charge n corresponding to left-handed helix. Many novel focal pattern may occur. It was shown that the focal pattern evolves from one intensity peak to multiple intensity peaks with changing the topological charge m and n, and all the intensity peaks form in a circle. As the number of intensity peaks is the sum of m and n, the focal pattern can be controlled through adjusting the topological charge m and n. And in order to show the possible applications of these alterable foci pattern, some optical gradient force distributions were also calculated and illuminated.     

Tight Focusing Properties of Radially Polarized Gaussian Beams with Pair of Vortices

The tight focusing properties of a radially polarized Gaussian beam with a nested pair of vortices having a radial wave front distribution are investigated theoretically by the vector diffraction theory.The results show that the optical intensity in the focal region can be altered considerably by changing the location of the vortices nested in a radially polarized Gaussian beam.It is noted that focal evolution from one annular focal pattern to a highly confined focal spot in the transverse direction is observed corresponding to the change in the location of the optical vortices in the input plane.It is also observed that the generated focal hole or spot lead to a focal shift along the optical axis remarkably under proper radial phase modulation.Hence the proposed system may be applied to construct tunable optical traps for both high and low refractive index particles.     

Focal patterns of higher order hyperbolic-cosine-Gaussian beam with one optical vortex

Intensity distribution in focal region plays an important role in many optical systems. In this paper, the focal patterns of higher order hyperbolic-cosine-Gaussian (HOCG) beam in focal plane were investigated. The HOCG beam contains one spiral optical vortex on its optical axis. Results show that the focal pattern can be altered considerably by beam order of the incident HOCG beam, and some novel focal patterns may occur, including foursquare focal pattern, cross-shaped dark focal focus, foursquare intensity peaks chain, and multiple intensity peaks array. Focal pattern evolution principle on increasing beam order also differs remarkably under condition of different topological charge of the optical vortex and displacement parameter associated with the cosh parts. And like topological charge and displacement parameter, the beam order and numerical aperture may affect focal pattern.     

Focal depth and focal splitting of truncated hyperbolic-cosine-Gaussian beam induced by phase plate

Focal depth and focal splitting of hyperbolic-cosine-Gaussian beams induced by a phase plate were investigated. The pure phase plate consists of three concentric zones: a center circle zone, an inner annular zone and an outer annular zone. The phase variance of the inner annular zone is adjustable. Simulation results show that the focal depth can be adjusted by changing the radii of zones. With the increase of the inner radius of the outer annular zone, the focal spot broadens along the optical axis and splits into two peaks. Then the two peaks combine back into one peak. There are two critical values for the inner radius of the outer annular zone, at which focal spot changes sharply. The tunable range of the focal depth varies considerably. The phase variance of the inner annular zone affects focal depth also; when the phase variance is π, the effect attains maximum. The parameters of cosh parts of the beam affect both focal splitting and focal depth evidently; focal splitting disappears with increasing parameters of cosh parts, and focal depth increases with increasing the parameters of cosh parts in both the low and the high numerical-aperture optical systems.     

Focal patterns of sine-azimuthal wavefront-modulated cosh-Gaussian beams by half space phase plate

The pure phase plate was introduced to alter the half-space phase value of the incident linearly polarized hyperolic-cosine-Gaussian (cosh-Gaussian) beams with sine-azimuthal variation wavefront, and focusing properties of the cosh-Gaussian beams by half space phase plate were investigated in detail. Simulation results show that focal pattern can be altered very considerably by the phase plate under condition of different beam parameters, numerical aperture, and phase parameter that indicates the phase change frequency on increasing azimuthal angle. Symmetry of the whole focal pattern can be altered remarkably, polysymmetrical focal patterns evolve into nonsymmetrical focal patterns. And some novel focal shape may appear, including cross-shape, multiple-peak focal pattern, multiple intensity lines, wheel focal pattern, swallowtail focal pattern, and dark hollow focal spots.     

余弦平方高斯光束通过无光栏透镜的梯度力

基于Collins公式推导了余弦平方高斯光束通过无光栏透镜光学系统聚焦后的光强分布及梯度力,采用数值模拟分析梯度力分布及光阱的数量、位置及尺度随光束参数、光束入射距离及系统菲涅耳数的变化。结果表明:光束相关参数b越大光阱的数量越多,光阱的位置随光束入射距离而改变,大小随系统菲涅耳数的增加而减小,但不影响其位置。     

Focusing properties of the hyperbolic-cosine-Gaussian beam induced by phase plate

Focusing properties of hyperbolic-cosine-Gaussian beam induced by phase plate were investigated theoretically in this Letter. The phase plate consists of three concentric zones: center circle zone, inner and outer annular zones with 0, π, 0 phase variations, respectively. Numerical simulations show that the evolution of the focal shape is altered considerably by changing the radii of zones and the decentered parameters in cosh parts of the light beam. Increase inner radius of the outer annular zone, focal spot broadens along optical axis, splits into two peaks, and then the two peaks combine back into one peak. In this process, the focal depth increases and then decreases three times, so three focal depth peaks occur. And the distance between the two peaks for bigger inner radius of outer annular zone is very small. With decreasing decentered parameters in cosh part of the beam, position of the three focal depth peaks shift toward to small radius, and the peak values also fall simultaneously.     

Sine-azimuthal wavefront-modulated cosh-Gaussian beams with half-space amplitude modulation

Half-space amplitude modulation was introduced into the linearly polarized cosh-Gaussian beams with sine-azimuthal variation wavefront, and corresponding tunable focal patterns were investigated in detail. Simulation results show that focal pattern can be changed remarkably by the amplitude modulation. And under condition of different beam parameters and phase parameter that indicates the sine-azimuthal variation wavefront, the focal pattern evolution also differs considerably. Some novel focal shape may appear, including cross-shape, multiple-peak focal pattern, multiple intensity lines, wheel focal pattern, bell shape, and swallowtail focal spots. Symmetry of the whole focal pattern can be altered remarkably, the whole focal pattern evolves into uniaxial symmetry about perpendicular axes for odd and even phase parameter, respectively.