  We report a novel lensless on-chip microscopy platform based on near-field
blind ptychographic modulation. In this platform, we place a thin diffuser in
between the object and the image sensor for light wave modulation. By blindly
scanning the unknown diffuser to different x-y positions, we acquire a sequence
of modulated intensity images for quantitative object recovery. Different from
previous ptychographic implementations, we employ a unit magnification
configuration with a Fresnel number of ~50,000, which is orders of magnitude
higher than previous ptychographic setups. The unit magnification configuration
allows us to have the entire sensor area, 6.4 mm by 4.6 mm, as the imaging
field of view. The ultra-high Fresnel number enables us to directly recover the
positional shift of the diffuser in the phase retrieval process, addressing the
positioning accuracy issue plagued in regular ptychographic experiments. In our
implementation, we use a low-cost, DIY scanning stage to perform blind diffuser
modulation. Precise mechanical scanning that is critical in conventional
ptychography experiments is no longer needed in our setup. We further employ an
up-sampling phase retrieval scheme to bypass the resolution limit set by the
imager pixel size and demonstrate a half-pitch resolution of 0.78 micron. We
validate the imaging performance via in vitro cell cultures, transparent and
stained tissue sections, and a thick biological sample. We show that the
recovered quantitative phase map can be used to perform effective cell
segmentation of the dense yeast culture. We also demonstrate 3D digital
refocusing of the thick biological sample based on the recovered wavefront. The
reported platform provides a cost-effective and turnkey solution for large
field-of-view, high-resolution, and quantitative on-chip microscopy.
