Measuring translational, rotational, and vibrational dynamics with digital holographic microscopy
This paper discusses the application of digital holographic microscopy (DHM) to study the dynamics of single and multiple colloidal spheres. Translational dynamics of colloidal particles were useful in examining Brownian motion both historically and more recently as probes for viscoelastic media, while vibrational dynamics have elucidated interesting effects due to depletion attraction or at liquid-liquid interfaces. DHM offers improvements over other optical tools such as video microscopy, which is only two-dimensional; confocal microscopy, which is comparatively slower due to long acquisition times; or light-scattering methods, which often have limited resolution for clusters of multiple particles. DHM provides improvements in these areas by measuring ~10 nm distances between particles at time steps ~1 ms and effectively images multiple particles.
"Theory" The basic principle behind DHM is that laser light (in a regime well-approximated by plane waves) is shone on the object under study. The scattered light from the object interferes with the unscattered light from the beam, forming a hologram. This interference pattern is then magnified, captured by a microscope and recorded digitally by a camera. This hologram is then fitted to extract parameters of interest from the scattering object. Rather than using Rayleigh-Kirchoff reconstruction, which can be thought of as back-projecting light through the hologram so that the the object is reconstructed from where the light converges, the authors chose to use the Lorenz-Mie scattering solution to generate predicted holograms, then use them to fit for optical spatial and optical properties of the scatterer. This method gives results that are more precise than Rayleigh-Kirchoff reconstruction.