Three-dimensional particle tracking and localization has various applications in biology and medicine, where it may be used to analyze contrast agents, or in flow analysis, e.g. for localizing dust particles in a gas stream or to analyze turbulence in a flow. Moreover, particle localization finds applications in IT-security, where a random arrangement of particles in a transparent environment may represent a Physically Unclonable Function (PUF), which is interesting for individual labeling of high value goods.
In conventional systems, such as bright field microscopy, a three-dimensional representation of particles is rather difficult, as it is challenging to acquire depth information about the sample. Quantitative phase imaging techniques provide phase and amplitude and thus in-depth information. Furthermore, they offer single shot measurements while providing images from multiple focal planes. Concerning the stability, which is an important aspect in localizing particles of diffraction limited size, common-path digital holographic microscopy is a reliable tool in particular in combination with a self-referencing system.
In this article, we show a common-path digital holographic microscope for particle localization. Firstly, the setup is characterized with a test chart in order to evaluate lateral and axial resolution properties. Afterwards a sample with particles distributed in a three-dimensional medium is analyzed. For reconstruction of the holograms, we use the angular spectrum method, numerical phase unwrapping as well as Zernike polynomials for aberration correction. All in all, the system is able to achieve stable particle localization in 3D with lateral resolution in the sub-micrometer range and an axial sensitivity of at least 100 nm.