Surface Plasmon Resonance Enhanced Scanning Near-Field Pockels Microscope Observation of Polar Structure

我们开发了一种扫描近场光学显微镜，可以在超薄薄膜（如单分子层）中成像波克尔斯效应。波克尔斯效应，有时称为线性电光效应，是在电偶极子近似下发生在无反转中心系统中的一种二阶非线性光学过程。因此，用这种技术可以探测极性结构。此外，这种显微镜允许我们观察极性结构，甚至在非薄膜中，因为表面等离子体共振效应提供了在介质中折射率的机会的高灵敏度。这有望在超薄薄膜中观测到折射率i的微小变化（<10^<-6>）。这种显微镜的特点是：(1)一种能证明非薄膜中极性结构的光学显微镜。(2)无需高功率激光器即可进行观测；(3)便于进行光谱测量。(4)我们可以用尖头来证明纳米尺寸的区域。我们观察到图案染料-单分子层吸附在45nm厚的薄镀金膜上。图案尺寸为40x40μm。将表面等离子体共振条件下的Pockels图像与同样可以探测极性结构的二次谐波生成显微镜所获得的图像进行了比较，证实了两者之间的高度对应性。虽然目前的分辨率限制在10μm左右，但使用高分辨率的尖头和压电级可能会提供更高的分辨率。

We have developed a scanning near-field optical microscope that can image Pockels effects in ultrathin films such as a monomolecular layers. Pockels effect, sometimes called "linear electrooptic effect" is one of the second-order nonlinear optical processes that occur in a system without any inversion center under electric dipole approximation. Hence a polar structure can be probed with this technique. In addition, this microscope allows us to observe polar structure even in untrathin films because the surface plasmon resonance effect offers high sensitivity for the chance in the refractive index in a media. This promises possible observation of a small change in the refractive index i(<10^<-6>) in ultrathin films.The features of this microscope are :(1)an optical microscope that can prove the polar structure in a untrathin films.(2)the observation can be preformed without any high power lasers(3)we can make spectroscopic measurement easily.(4)we can prove nanometer sized region using a sharp tip.We have observed pattered dye-monomolecular layers adsorbed on a 45nm-thick thin gild film. The pattered size was 40x40μm. The Pockels images taken in the surface plasmon resonance condition were compared with the images obtained by second-harmonic generation microscopy that can also probe a polar structure, and confirmed a high correspondence between them. Although the present resolution is limited within about 10μm, it is likely that use of a sharp tip and a piezo stage with high resolution will provide much higher resolution.