Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, 9000 Rockville Pike, Bethesda, Maryland 20892, USA.
Anal Chem. 2009 Dec 1;81(23):9657-63. doi: 10.1021/ac901789w.
In order to facilitate nanoparticle-enhanced Raman imaging of complicated biological specimens, we have examined the use of higher order modes with radial and azimuthal polarizations focused onto a Au nanoparticle atomic force microscope (AFM) tip utilizing a backscattering reflection configuration. When comparing the Raman intensity profiles with the observed sample topography, the radial-polarized configuration demonstrates enhanced spatial resolution. This enhanced resolution results from the direction of the induced electron oscillation in the metal nanoparticle oriented by the electromagnetic field at the laser focus. The electric field component along the direction of laser propagation, attendant to the radial polarization, creates an enhanced field along the z-axis and normal to the sample. Substantial enhancement is observed utilizing an intermediate numerical aperture objective (NA = 0.7), necessary for backscattering measurements. The azimuthal polarization, similar to linear polarization, results in an enhanced field predominantly parallel to the sample, resulting in imaging artifacts. The Raman intensity profiles observed as the exciting laser polarization is switched between either a radially polarized or an azimuthally polarized state illustrate these imaging artifacts. Because azimuthal polarization arises readily from changes in the incident polarization onto the mode converter, the results presented here aid in identifying such artifacts when analyzing nanoparticle-enhanced Raman spectroscopic images. Due to the power law decay of the enhanced field, an enhancement orientation normal to the sample enables contrast between structures smaller than the tip dimensions as the apex of the nanoparticle tip, where the enhancement is strongest, passes over the sample. These effects are demonstrated using both carbon nanotube and fixed biological samples.
为了便于对复杂生物样本进行纳米颗粒增强 Raman 成像，我们研究了利用背反射配置将具有径向和角向偏振的高阶模式聚焦到金纳米颗粒原子力显微镜（AFM）尖端的方法。当将拉曼强度分布与观察到的样本形貌进行比较时，径向偏振模式显示出增强的空间分辨率。这种分辨率的提高归因于金属纳米颗粒中感应电子振荡的方向，而这种方向是由激光焦点处的电磁场决定的。沿激光传播方向的电场分量，伴随径向偏振，在 z 轴方向上产生增强的场，并垂直于样本。利用中间数值孔径物镜（NA=0.7）可以观察到显著的增强效果，这对于背散射测量是必需的。角向偏振与线性偏振相似，导致增强场主要平行于样本，从而产生成像伪影。当将激发激光偏振从径向偏振或角向偏振状态切换时观察到的拉曼强度分布说明了这些成像伪影。由于角向偏振很容易从模式转换器上的入射偏振的变化中产生，因此这里呈现的结果有助于在分析纳米颗粒增强 Raman 光谱图像时识别这些伪影。由于增强场的功率律衰减，垂直于样本的增强方向使得在纳米颗粒尖端的顶点（增强最强的地方）经过样本时，能够在小于尖端尺寸的结构之间产生对比度。这些效果在碳纳米管和固定生物样本中都得到了验证。
