Surface height profile imaging with optically trapped spheres

表面在自然界和技术中起着特殊的作用，因为它们不仅将外部与内部分开，而且控制化学反应，并调节压力，光，热和水分的交换。为了在分子尺度上了解表面的性质，需要特殊的测量技术来空间探测和解析最小的结构，而不会破坏它们。几种不同的扫描探针显微镜（SPM）技术，其中最突出的是原子力显微镜（AFM）和扫描隧道显微镜（STM），说明了对具有最多各种结构、特征和功能的成像表面的巨大需求。然而，尽管AFM技术取得了令人印象深刻的成功，但事实证明，AFM杠杆的尖端对于许多应用来说往往过于僵硬，从而损坏了软样品。类似于AFM，但更灵敏，光学捕获探针可以扫描整个结构化表面，以测量探针位移的高度轮廓。这种技术被称为光子力显微镜（PFM）。光阱由于能够灵活地对流体环境中的微小结构施加最小的力，在生物纳米科学中发挥着重要作用。结合先进的3D粒子跟踪技术，如后焦平面干涉测量法，它们允许感测施加在这些结构上的微小力。在最近的出版物（Friedrich 2015）中，我们已经证明，通过分时双光阱和纳米精度三维干涉颗粒跟踪的组合，可靠的高度轮廓和表面成像是可能的，空间分辨率低于衍射极限。这种技术利用了被困探针的高能量热位置波动，导致表面的采样比AFM软5000倍。在这项研究中，我们的目标是改善PFM技术在三个不同的方向：第一，空间分辨率应显着提高，通过使用更小的探针，这需要一个短的（绿色）激光波长稳定地捕获它们。其次，应实施所谓的轻敲模式，其中探头垂直振荡，目的是减少探头粘附并增加扫描速度。第三个目标是通过成像也不透明的表面，这需要精确的光学捕获和跟踪的探针也在反射模式，以扩大这种扫描探针显微镜的应用范围。

Surfaces play a special role in nature and technology, since they do not only separate outside from inside, but control chemical reactions, and regulate the exchange of pressure, light, heat, and moisture. To understand the properties of surfaces on a molecular scale, special measurement technology is required to spatially probe and resolve smallest structures without destroying them. Several different scanning probe microscopy (SPM) techniques, among which the most prominent are the atomic force microscope (AFM) and the scanning tunneling microscope (STM), illustrate the enormous demand for imaging surfaces with the most various structures, features and functions. However, despite the impressive success-story of AFM technology, it turned out that the tips of AFM cantilevers are often too stiff for many applications, thus damaging the soft sample. Similar to AFM, but much more sensitive, an optically trapped probe can be scanned across a structured surface to measure the height profile from the displacements of the probe. This technique is called Photonic Force Microscopy (PFM). Optical traps have been playing important roles in the bio-nano-sciences due to their ability to flexibly apply smallest forces on tiny structures in fluid environments. Combined with advanced 3D particle tracking techniques such as back-focal-plane interferometry, they allow sensing miniscule forces exerted on these structures. In a recent publication (Friedrich 2015) we have demonstrated that by a combination of a time-shared twin-optical trap and nanometer-precise three-dimensional interferometric particle tracking reliable height-profiling and surface imaging is possible with a spatial resolution below the diffraction limit. This technique exploits the high energy thermal position fluctuations of the trapped probe, leading to a sampling of the surface 5000 times softer than in AFM. In this research proposal we aim to improve the PFM technology in three different directions: First, the spatial resolution shall be improved significantly, by using smaller probes, which requires a short (green) laser wavelength to stably trap them. Second, the so-called tapping mode shall be implemented, where the probe is oscillated vertically with the goal to reduce the probe sticking and to increase the scanning velocity. A third goal is to expand the range of applications for this scanning probe microscopy by imaging also opaque surfaces, which requires precise optical trapping and tracking of the probe also in reflection mode.