  We propose and experimentally demonstrate a differential microscopy method to
obtain simultaneous amplitude, phase, and quantitative polarization gradient
imaging in a single experimental embodiment. A full-field optical spatial
differentiator is achieved in a relatively simple setup by placing a glass
cover slip as a Signum phase mask in the Fourier plane of a standard 4-f
imaging system and accordingly named Signum phase mask differential microscopy.
The longstanding requisite of polarized light to obtain the spatial
differentiation of the field at the object plane is eliminated in our scheme
and, hence, leads to the emergence of quantitative differential polarization
contrast imaging by integrating polarization degree of freedom as an additional
contrast agent in the framework of differential microscopy. Implementation of
the proposed differential imaging scheme in high-resolution microscopy is
experimentally demonstrated alongside its functionality for a broad wavelength
range. Simultaneous acquisition of differential phase, amplitude, and
polarization (anisotropy) gradient imaging in a rather elementary optical setup
enables a low-cost multi-functional differential microscopy system that is
anticipated to emerge as a revolutionary tool in label-free imaging and optical
image processing.
