Detectability of unresolved particles in off-axis digital holographic microscopy

Off-axis digital holographic microscopy (DHM) provides both amplitude and
phase images, and so may be used for label-free 3D tracking of micro- and
nano-sized particles of different compositions, including biological
cells, strongly absorbing particles, and strongly scattering particles.
Contrast is provided by differences in either the real or imaginary parts
of the refractive index (phase contrast and absorption) and/or by
scattering. While numerous studies have focused on phase contrast and
improving resolution in DHM, particularly axial resolution, absent have
been studies quantifying the limits of detection for unresolved particles.
This limit has important implications for microbial detection, including
in life-detection missions for space flight. Here we examine the limits of
detection of nanosized particles as a function of particle optical
properties, microscope optics (including camera well depth and substrate),
and data processing techniques and find that DHM provides contrast in both
amplitude and phase for unresolved spheres, in rough agreement with Mie
theory scattering cross-sections. Amplitude reconstructions are more
useful than phase for low-index spheres and should not be neglected in DHM
analysis.