  The refractive index (RI) of cells and tissues is crucial in pathophysiology
as a noninvasive and quantitative imaging contrast. Although its measurements
have been demonstrated using three-dimensional quantitative phase imaging
methods, these methods often require bulky interferometric setups or multiple
measurements, which limits the measurement sensitivity and speed. Here, we
present a single-shot RI imaging method that visualizes the RI of the in-focus
region of a sample. By exploiting spectral multiplexing and optical transfer
function engineering, three color-coded intensity images of a sample with three
optimized illuminations were simultaneously obtained in a single-shot
measurement. The measured intensity images were then deconvoluted to obtain the
RI image of the in-focus slice of the sample. As a proof of concept, a setup
was built using Fresnel lenses and a liquid-crystal display. For validation
purposes, we measured microspheres of known RI and cross-validated the results
with simulated results. Various static and highly dynamic biological cells were
imaged to demonstrate that the proposed method can conduct single-shot RI slice
imaging of biological samples with subcellular resolution.
