Scanning sub-wavelength structured surfaces with optically trapped probes

Characterization of nanometer scaled surface structures is an important task in many scientific applications. Various surface scanning methods have already been established, e. g. the atomic force microscope (AFM). The photonic force microscope (PFM) utilizes an optically trapped bead as a probe to scan a surface. The displacements of the probe are detected in three dimensions and with nanometer accuracy by back focal plane interferometry. The interference pattern of unscattered light and the light scattered by the probe is recorded with a quadrant photo diode with sample rates up to one MHz. Phase disturbances caused by the structured surface are corrected for. The stiffness of the optical trap can be tuned to values 1000 times softer than a typical AFM cantilever. Soft surfaces can be examined without destroying them. Because the probe is suspended without mechanical contact, surfaces inside cavities can be measured. The aim of this scanning probe technique is to image both the relief of a surface and its interaction potentials as well as local viscosities. Measurement results reveal a spatial resolution of sample structures clearly beyond Abbe’s optical resolution limit.