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在横截面z定义椭圆坐标:x=f(z)coshξcosη,y=f(z)sinhξsinη,其中f(z)是与z有关的半焦距,ξ∈[0, ∞), η∈[0, 2π)分别是径向、角向变量,并且ξ的等值线为共焦的椭圆,η的等值线为共焦的双曲线。近轴波动方程在椭圆坐标系下可以得到两组独立波动方程解,分别为[14, 16]:
$ $\begin{array}{*{20}{c}} {G_{p, m}^{\rm{e}}(r, \varepsilon ) = \frac{{C{w_0}}}{{w(z)}}C_p^m({\rm{i}}\xi , \varepsilon )C_p^m(\eta , \varepsilon )\exp \left[ {\frac{{ - {r^2}}}{{{w^2}(z)}}} \right] \times }\\ {\exp \left\{ {{\rm{i}}\left[ {kz + \frac{{k{r^2}}}{{2R(z)}} - (p + 1){\Psi _{{\rm{GS}}}}(z)} \right]} \right\}} \end{array}$ $
(1) $ $\begin{array}{l} G_{p, m}^{\rm{o}}(r, \varepsilon ) = \frac{{S{w_0}}}{{w(z)}}S_p^m({\rm{i}}\xi , \varepsilon )S_p^m(\eta , \varepsilon )\exp \left[ {\frac{{ - {r^2}}}{{{w^2}(z)}}} \right] \times \\ \;\;\;\;\;\;\exp \left\{ {{\rm{i}}\left[ {kz + \frac{{k{r^2}}}{{2R(z)}} - (p + 1){\mathit{\Psi} _{{\rm{GS}}}}(z)} \right]} \right\} \end{array}$ $
(2) 式中,上标e和o分别表示偶模和奇模,r为半径,C和S为归一化常数,w0为束腰半径,w(z)=w0×$\sqrt {\left({1 + {z^2}/z_{\rm{R}}^2} \right)} $是激光在z处的横截宽度; Cpm(η, ε)和Spm(η, ε)分别表示带有阶数p和级数m的偶次和奇次因斯多项式,p和m满足(-1)p-m=1,即始终具有相同的奇偶性,其中对于偶模0≤m≤p,对于奇模1≤m≤p; ε=2f02/w02是由束腰半径w0和束腰面半焦距f0决定的椭圆参量,其表示椭圆率的变化程度; k是波数; R(z)=z+zR2/z为光波前的曲率半径,-(p+1)×ΨGS(z)=-(p+1)arctan(z/zR)为阶数为p的IG模的Gouy相移,其中ZR=kw02/2为瑞利长度。由于C00(η, ε)=1,因此p=0, m=0时IG模式其实就是最简单的高斯模式[13]。
一般地,柱对称矢量光场可以看成是两个正交偏振分量的叠加[24]。同样,两个具有偶模和奇模IG模式的正交偏振分量的叠加,可以构建具有IG模式的矢量光场,即IGV(Ince-Gaussian vectorial)光场。广义的IGV光场可以用琼斯矢量表示为:
$ {\mathit{\boldsymbol{E}}_{{\rm{IGV}}}}(x, y) = \left[ {\begin{array}{*{20}{c}} {G_{{p_x}, {m_x}, {\varepsilon _x}}^{\rm{e}}}\\ {G_{{p_y}, {m_y}, {\varepsilon _y}}^0\exp ({\rm{i}}\delta )} \end{array}} \right] $
(3) 式中,Gpex, mx, εx, Gopy, my, εy分别是IGV光场的x, y偏振分量,下标px, mx, εx和py, my, εy分别表示IGV光场x分量和y分量IG模式的阶数、级数、椭圆参量,δ是x, y分量之间的初始相位延迟。利用具有不同参量p, m, ε的正交偏振分量,可以构建具有不同强度与偏振分布的复杂矢量结构的IGV光场。为了方便,用EIGV(epxmxεx, opymyεy)来表示由两个正交偏振分量生成的IGV矢量光场。
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IGV光束的紧聚焦特性可以通过Richards-Wolf矢量衍射方法来进行数值分析[25-26]。图 1为IGV光束的紧聚焦示意图。一个高数值孔径(numerical aperture,NA)的透镜聚焦入射的IGV光束,该透镜满足正弦条件, 半径r=f sinθ(f为焦距)。透镜光瞳面Pi(xi, yi)处的IGV光束经过聚焦之后,在焦点(xf, yf, zf)附近的光场为:
$ \begin{array}{l} \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\mathit{\boldsymbol{E}}\left( {{x_{\rm{f}}}, {y_{\rm{f}}}, {z_{\rm{f}}}} \right) = A\iint_{ - 1}^1 {\frac{1}{{\sqrt {\cos \theta } }} \times } \\ \;\;\;\;\;\;\left\{ {G_{{p_x}, {m_x}, {\varepsilon _x}}^{\rm{e}}\left[ {\begin{array}{*{20}{c}} {\left. {\left( {\cos \theta {{\cos }^2}\varphi + {{\sin }^2}\varphi } \right){\mathit{\boldsymbol{e}}_x}} \right\}}\\ {(\cos \theta \sin \varphi \cos \varphi - \sin \varphi \cos \varphi ){\mathit{\boldsymbol{e}}_y}}\\ {\sin \theta \cos \varphi {\mathit{\boldsymbol{e}}_z}} \end{array}} \right] + } \right.\\ \left. {G_{{p_y}, {m_y}, {\varepsilon _y}}^{\rm{o}}{\rm{exp}}({\rm{i}}\delta )\left[ {\begin{array}{*{20}{c}} {(\cos \theta \sin \varphi \cos \varphi - \sin \varphi \cos \varphi ){\mathit{\boldsymbol{e}}_x}}\\ {\left( {\cos \theta {{\sin }^2}\varphi + {{\cos }^2}\varphi } \right){\mathit{\boldsymbol{e}}_y}}\\ {\sin \theta \sin \varphi {\mathit{\boldsymbol{e}}_z}} \end{array}} \right]} \right\} \times \end{array} \\ \exp \left\{ { - {\rm{i}}k\left[ {r\sin \theta \cos (\varphi - \phi ) + {z_{\rm{f}}}\cos \theta } \right]} \right\}{\rm{d}}{x_{\rm{i}}}{\rm{d}}{y_{\rm{i}}} $
(4) 式中,入射光瞳面处的圆柱坐标系(xi, yi)是由半径$\rho = \sqrt {x_{\rm{i}}^2 + y_{\rm{i}}^2} $和方位角$\varphi = {\tan ^{ - 1}}\left({\frac{{{y_{\rm{i}}}}}{{{x_{\rm{i}}}}}} \right)$表示的,A是积分常数,收敛角θ=sin-1(dNAρ),其中dNA表示数值孔径,ex,ey和ez分别为x,y和z方向上的单位矢量。聚焦场E(xf, yf, zf)位于由半径$r = \sqrt {x_{\rm{f}}^2 + y_{\rm{f}}^2} $和方位角$\phi = {\tan ^{ - 1}}\left({\frac{{{y_{\rm{f}}}}}{{{x_{\rm{f}}}}}} \right)$表示的笛卡尔坐标系中。这就是一般IGV光束通过高NA透镜聚焦后的聚焦场表达式。
Ince-Gaussian矢量光场束腰位置对紧聚焦特性影响的研究
Research on the effect of waist position changing of Ince-Gaussian vectorial beam on tightly focusing characteristics
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摘要: 为了获得Ince-Gasussian矢量光场束腰位置对紧聚焦特性的影响规律,采用正交偏振的奇偶模式叠加理论和Richards-Wolf矢量衍射积分理论,对不同束腰位置的Ince-Gaussian矢量光场紧聚焦特性进行了研究。结果表明,在高数值孔径聚焦条件下,入射Ince-Gaussian矢量场的束腰距离透镜位置zi在一定瑞利长度zR范围内(zi < 0.5zR)改变时,其聚焦场的横向场结构即光强与偏振态分布,依然可以保持稳定;通过聚焦场相位结构分析,给出了在束腰距离透镜位置zi超过一定瑞利长度zR范围(zi>0.5zR)时,影响横向场结构不稳定的原因;聚焦场纵向偏振分量作为聚焦场的一个自由度,被证明可以用来构建更加丰富的矢量结构光场。此研究结果对复杂结构矢量光场在光学微操控与光信息存储方面的研究有重要参考价值。
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关键词:
- 物理光学 /
- 偏振 /
- 矢量光场 /
- 紧聚焦 /
- Ince-Gaussian模式
Abstract: In order to research the influence of the beam waist position on the tightly focusing of the Ince-Gaussian vectorial beam, the superposition theory of orthogonally polarized even and odd modes and the Richards-Wolf vector diffraction integral theory were utilized in this study and the tightly focusing feature of the Ince-Gaussian vectorial beam at the different waist position was analyzed. The results show that under the condition of high numerical aperture focusing, the transverse field structure of the focusing field possessing distribution of light intensity and polarization state can still remain stable when the distance zi between the beam waist position of the Ince-Gaussian vectorial field and the lens changes within a certain Rayleigh length zR (zi < 0.5zR). By analyzing the phase structure of the focusing field, the reason of the instability of the transverse field structure is given when the distance zi between the beam waist position and the lens exceeds a certain Rayleigh length zR (zi>0.5zR). At the same time, the longitudinal polarization component of the focusing field, as a degree of freedom of the focusing field, can be used to construct a more abundant vector structured light field. The results can provide great value for the research of complex structure vector optical field in optical micromanipulation and optical information storage.-
Key words:
- physical optics /
- polarization /
- tightly focusing /
- vector optical field /
- Ince-Gaussian mode
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