Surface height profile imaging with optically trapped spheres
Final Report Abstract
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. 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 such miniscule forces exerted on these structures. Similar to Atomic Force Microscopy AFM, but much more sensitive, an optically trapped probe can be scanned across a structured surface to obtain the height profile from the displacements of the probe. This technique is called Photonic Force Microscopy (PFM). We demonstrated reliable height-profiling and surface imaging by a combination of a timeshared twin-optical trap and nanometer-precise three-dimensional interferometric particle tracking at the beginning of this project. However, repetitive sticking of the probe to the sample and the complexity of a twin optical trap limited the applicability. Within the scope of this research proposal, we went a similar path as the AFM technology in its beginnings: We developed an optical tapping mode, where the trapped probe is oscillated vertically by two switching laser foci at up to 100 Hz. Because of the resulting short contact times between probe and sample, probe sticking could be significantly reduced without loss in image profiling quality. With the help of Brownian dynamics computer simulations, we developed a new analysis algorithm leading to better background stability and height profile extraction in the images. Furthermore, we found a way to avoid the second laser focus, which has had the task to measure the scattered light from the sample only, necessary for precise 3D probe tracking and height reconstruction. In the new approach the sample scattering signals can be recorded when the trapped probe is still in the upward position, i.e. not been in contact with the sample. PFM scans in tapping mode could be performed across the typically sticky cell periphery in high quality and without losing the probe.
