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普通光学显微成像系统记录过程的简化模型如图 1所示。(x, y)为物体所在的平面,垂直于物平面的z轴为光的传播方向,物平面距显微物镜面的距离为d1, 显微物镜面的距离与像面的距离为d2, d1和d2满足物像关系。
当用一束平面光波垂直照射被测物体后,在物体表面,即z=0处,物体光波场的分布为[18-22]:
$ \begin{array}{c} u(\mathit{x}, \mathit{y}, 0) = \\ \int_{ - \infty }^{ + \infty } {\int_{ - \infty }^{ + \infty } U } \left( {{\mathit{\boldsymbol{k}}_x}, {\mathit{\boldsymbol{k}}_y};0} \right)\exp \left[ {{\rm{i}}\left( {{\mathit{\boldsymbol{k}}_x}x + {\mathit{\boldsymbol{k}}_y}y} \right)} \right]{\rm{d}}{\mathit{\boldsymbol{k}}_x}{\rm{d}}{\mathit{\boldsymbol{k}}_\mathit{y}} \end{array} $
(1) 式中, (kx, ky)为沿着x, y方向的空间波矢分量。
在空气中传播距离d1后的物体光波场为:
$ \begin{array}{c} u(\mathit{x}, \mathit{y}, {\mathit{d}_1}) = \int_{ - \infty }^{ + \infty } {\int_{ - \infty }^{ + \infty } U } \left( {{\mathit{\boldsymbol{k}}_x}, {\mathit{\boldsymbol{k}}_y}, 0} \right) \times \\ \exp \left[ {{\rm{i}}\left( {{\mathit{\boldsymbol{k}}_x}x + {\mathit{\boldsymbol{k}}_y}y + {\mathit{\boldsymbol{k}}_\mathit{z}}{\mathit{d}_1}} \right)} \right]{\rm{d}}{\mathit{\boldsymbol{k}}_x}{\rm{d}}{\mathit{\boldsymbol{k}}_\mathit{y}} \end{array} $
(2) 式中, kz2=k02-kx2-ky2,k0为不同方向叠加的空间波矢,k0=2π/λ为光波在传播方向的波数,kz为沿着z方向的空间波矢分量, λ为照明光波波长。
(2) 式表示物体光波场是由空间频率为(kx, ky)的无穷多组平面波沿不同传播方向的叠加,每组平面波可表示为[21-22]:
$ \mathit{U}\left( {{\mathit{\boldsymbol{k}}_x}, {\mathit{\boldsymbol{k}}_y}, 0} \right){\rm{exp}}\left[ {{\rm{i}}\left( {{\mathit{\boldsymbol{k}}_x}x + {\mathit{\boldsymbol{k}}_y}y + {\mathit{\boldsymbol{k}}_\mathit{z}}\mathit{z}} \right)} \right] $
(3) 当k02>kx2+ky2时,传播距离d1后其光场分布为:
$ \begin{array}{c} \mathit{U}\left( {{\mathit{\boldsymbol{k}}_x}, {\mathit{\boldsymbol{k}}_y}, 0} \right){\rm{exp}}\left[ {{\rm{i}}\left( {{\mathit{\boldsymbol{k}}_x}x + {\mathit{\boldsymbol{k}}_y}y} \right)} \right] \times \\ {\rm{exp}}({\rm{i}}\sqrt {\mathit{\boldsymbol{k}}_0^2 - \mathit{\boldsymbol{k}}_\mathit{x}^2 - \mathit{\boldsymbol{k}}_\mathit{y}^2} {\mathit{d}_1}) \end{array} $
(4) 引入一个相位延迟因子,表示传播距离d1后只引起了各个频谱分量的相对相位,属于低空间频率分量的传输波。
当k02 < kx2+ky2=k//2时,为纯虚数,其中,传播距离d1后其光场分布为:
$ \mathit{U}\left( {{\mathit{\boldsymbol{k}}_x}, {\mathit{\boldsymbol{k}}_y}, 0} \right){\rm{exp}}( - \mathit{\mu }{\mathit{d}_1}){\rm{exp}}\left[ {{\rm{i}}\left( {{\mathit{\boldsymbol{k}}_x}x + {\mathit{\boldsymbol{k}}_y}y} \right)} \right] $
(5) 此称为倏逝波,随着传播距离d1的增加,其振幅按指数规律衰减,当传播距离大于一个波长λ时很快衰减为0。由于d1≫λ,倏逝波没有达到显微物镜面,不能参与远场成像。成像系统频谱分布如图 2所示。
能参与远场成像的传输波被限制为k02>kx2+ky2,其最大的横向空间波矢为:
$ {\mathit{\boldsymbol{k}}_{//, {\rm{max}}}} = {\mathit{\boldsymbol{k}}_0} $
(6) 因此, 远场成像的横向最高空间频率被限制在[7, 20-21]:
$ {f_{//, {\rm{max}}}} = \frac{{{\mathit{\boldsymbol{k}}_{//, {\rm{max}}}}}}{{2{\rm{ \mathsf{ π} }}}} = \frac{1}{\mathit{\lambda }} $
(7) 如图 2所示,横向频率小于1/λ时,为传输波; 横向频率大于1/λ时,为倏逝波,沿着传播方向衰减,在探测器处无法探测到。因此远场成像系统的空间分辨率为[7, 20]:
$ \mathit{\delta = }\frac{1}{{2{\mathit{f}_{//, {\rm{max}}}}}} = \frac{\mathit{\lambda }}{2} $
(8) 因此普通光学显微镜的极限分辨率为λ/2。由于倏逝波携带了纳米结构样品的更多亚波长细节信息,为了在远场实现超越衍射极限的分辨率,需要收集到倏逝波。
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当横向波矢量满足k02 < kx2+ky2=k//2时,由(5)式可知,在传播距离为λ处倏逝波会衰减为0,在远场不能参与成像。当将微球放置于物体的表面, 如图 3所示。为了简单起见,分析y=0的情况。半径为R、折射率为n的微球放置于物体的表面,其与物体的接触点为O。取微球边缘的P点作为入射点,由于倏逝波在空气中传播波长λ的距离处时会衰减为0,因此需满足h < λ,其中h为P点到物体表面的垂直距离。u和w分别表示过P点的切向和法向方向,w与z之间的夹角为θ。在空气中,k0分解到x和y方向的波矢量分别为kx和ky。
微球中,分解到u和w方向的波矢量分别为ku和kw,且满足:
$ \mathit{\boldsymbol{k}}_u^2 + \mathit{\boldsymbol{k}}_w^2 = {\mathit{n}^2}\mathit{\boldsymbol{k}}_0^2 $
(9) 当倏逝波入射到微球时,通过Snell定量可知,微球内沿着u方向的波矢量ku可以表示为[18-19]:
$ \begin{array}{c} \mathit{\boldsymbol{k}}_u^2 = \mathit{\boldsymbol{k}}_x^2{\rm{co}}{{\rm{s}}^2}\mathit{\theta + }\mathit{\boldsymbol{k}}_\mathit{z}^2{\rm{si}}{{\rm{n}}^2}\mathit{\theta } = \\ \mathit{\boldsymbol{k}}_x^2{\rm{co}}{{\rm{s}}^2}\mathit{\theta } - {\left| {{\mathit{\boldsymbol{k}}_z}} \right|^2}{\rm{si}}{{\rm{n}}^2}\mathit{\theta } \end{array} $
(10) 当kx2cos2θ-|kz|2sin2θ>n2k02时,传播到微球中仍然为倏逝波,在远场衰减为0。
当kx2cos2θ-|kz|2sin2θ < n2k02时,在微球中将倏逝波转换成了传输波。因此,倏逝波通过微球后,微球将部分倏逝波转换成传输波,通过显微物镜成像在探测器上。f为显微物镜的焦距,d1和d2同样满足物像关系。由此可得kx满足如下的关系[18-20]:
$ \mathit{\boldsymbol{k}}_0^2 \le \mathit{\boldsymbol{k}}_x^2 \le \frac{{\mathit{\boldsymbol{k}}_0^2({\mathit{n}^2} - {\rm{si}}{{\rm{n}}^2}\mathit{\theta })}}{{1 - 2{\rm{si}}{{\rm{n}}^2}\mathit{\theta }}} $
(11) 式中, ,代入(11)式可得:
$ \mathit{\boldsymbol{k}}_x^2 \le \frac{{\mathit{\boldsymbol{k}}_0^2({\mathit{n}^2}\mathit{R} - 2\mathit{h})}}{{\mathit{R} - 4\mathit{h}}} $
(12) 物体散射后产生的倏逝波在微球里传播与微球的半径和折射率有关,且需满足(12)式。
由(12)式可知,通过微球后可获得最大的空间频率为:
$ {\mathit{f}_{{\rm{max}}}} = \frac{{{\mathit{\boldsymbol{k}}_{//, {\rm{max}}}}}}{{2{\rm{ \mathsf{ π} }}}} = \frac{1}{\mathit{\lambda }}\sqrt {\frac{{{\mathit{n}^2}\mathit{R} - 2\mathit{h}}}{{\mathit{R} - 4\mathit{h}}}} $
(13) 微球光学显微成像系统可分辨的最小距离为:
$ \mathit{d} = \frac{1}{{2{\mathit{f}_{{\rm{max}}}}}} = \frac{\mathit{\lambda }}{2}\sqrt {\frac{{\mathit{R} - 4\mathit{\lambda }}}{{{\mathit{n}^2}\mathit{R} - 2\mathit{\lambda }}}} $
(14) 由(14)式可知,微球光学显微成像系统可分辨的最小距离与微球的折射率和半径有关。由于n2R-2λ-(R-4λ)>0,由(14)式可得:
$ d < \frac{\mathit{\lambda }}{2} $
(15)
基于微球透镜远场超分辨率成像方法研究
Far-field super-resolution imaging based on microsphere lens
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摘要: 在可见光波段,传统光学显微镜的成像分辨率被限制到200nm。为了突破衍射极限,采用了将微球与传统光学显微镜相结合的方法来获得远场超分辨率成像。首先通过理论分析平行光通过微纳结构物体后物光波在空气中的传输,进而分析微球将倏逝波转换成传输波实现远场超分辨的成像机理;其次通过仿真研究了微球的光纳米喷射特性,可知微球光纳米喷射的半径尺寸小于入射光波长的一半;最后搭建了基于微球与传统光学显微镜相结合的超分辨率成像实验系统。结果表明,将蓝光光盘作为被测物体,通过该成像系统可获得100nm的远场超分辨率成像; 该成像系统可以对微纳元件结构进行检测。这一结果对光刻技术、生物医学等领域是有帮助的。Abstract: The imaging resolution of a conventional optical microscope is limited to 200nm by the diffraction in the visible spectrum. In order to overcome the resolution limit of the imaging, the microsphere combing with the traditional optical microscope was used to obtain the super-resolution imaging in far field. Firstly, the transport of the object light waves in the air was analyzed theoretically after the parallel light interacted with the micro-nano structure object, and the mechanism of the far-field super-resolution imaging was analyzed that the evanescent wave was converted into the transmission wave by the microsphere. Secondly, the photonic nanojet characteristics of the microspheres were researched. The results show that the radius of the photonic nanojet by the microsphere is less than half of the incident wavelength. Lastly, the blue-ray disc was used as the object, the experimental system of the super-resolution imaging based on the microsphere combining with traditional optical microscope was set up. The resolution of the imaging system is 100nm in the far-field. The results show that the imaging system can be used in the detection of the micro-nano structure. The results are helpful to the lithography, bio-medicine, etc.
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Key words:
- imaging systems /
- super-resolution /
- microsphere lens /
- blue-ray disc
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Figure 7. The imaging results of the blu-ray disc
a—the image of the blu-ray disc by SEM b—the image of the blu-ray disc by optical microscopy c—the image of microsphere by optical microscopy& d—the image of the blu-ray disc by microsphere-based microscopy e—the magnified rectangle area marked with solid blue line in Fig.7d
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