Surface height profile imaging with optically trapped spheres

表面在自然和技术中扮演着特殊的角色，因为它们不仅将外部和内部分开，而且控制化学反应，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在不破坏最小结构的情况下对其进行空间探测和解析。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)，说明了对具有最不同结构、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故事，但事实证明，AFM悬臂的尖端往往太硬，不适合许多应用，因此损坏了柔软的样品。与AFM类似，但更灵敏的是，光学捕获的探头可以扫描结构表面，根据探头的位移测量高度分布。这种技术被称为光子力显微镜(PFM)。光学陷阱由于能够在流体环境中对微小结构施加最小的作用力而在生物纳米科学中发挥着重要的作用。结合先进的3D粒子跟踪技术，如背焦平面干涉测量，它们可以传感施加在这些结构上的微小作用力。在最近的一份出版物(Friedrich 2015)中，我们展示了通过分时双光圈和纳米级精确的三维干涉粒子跟踪的组合，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。这项技术利用了被捕获的探测器的高能热位置波动，导致对表面的采样比AFM中的采样要软5000倍。在这项研究方案中，我们的目标是从三个不同的方向改进PFM技术：第一，通过使用更小的探测器来显著提高空间分辨率，这需要短(绿)激光波长来稳定地捕获它们。第二，实行所谓的攻丝模式，即探头垂直摆动，目的是减少探头卡住，提高扫描速度。第三个目标是通过对不透明表面成像来扩大这种扫描探头显微镜的应用范围，这需要在反射模式下对探头进行精确的光学捕获和跟踪。

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.