Surface height profile imaging with optically trapped spheres

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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.