  Imaging of biological cells and tissues often relies on fluorescent labels,
which offer high contrast with molecular specificity. The use of exogenous
labeling agents, however, may alter the normal physiology of the bio-specimens.
Complementary to the established fluorescence microscopy, label-free
quantitative phase imaging provides an objective morphological measurement tool
for bio-specimens and is free of variability introduced by contrast agents.
Here we report a simple and low-cost microscope add-on, termed Ptychographic
Modulation Engine (PME), for super-resolution quantitative phase imaging. In
this microscope add-on module, we attach a diffuser to a 3D-printed holder that
can be mechanically moved to different x-y positions. We then use two
vibrational motors to introduce random positional shifts to the diffuser. The
add-on module can be placed between the objective lens and the specimen in most
existing microscope platforms. Thanks to the diffuser modulation process, the
otherwise inaccessible high-resolution object information can now be encoded
into the captured images. In the ptychographic phase retrieval process, we
jointly recover the complex object wavefront, the complex diffuser profile, and
the unknown positional shifts of the diffuser. We demonstrate a 4-fold
resolution gain over the diffraction limit of the employed 2X objective lens.
We also test our approach for in-vivo cell imaging, where we are able to adjust
the focus after the data has been captured. The reported microscope add-on
provides a turnkey solution for super-resolution quantitative phase imaging. It
may find applications in label-free bio-imaging where both large field-of-view
and high resolution are needed.
