Quantitative phase imaging: from image to knowledge

by Gabriel Popescu (Beckman Institute for Advanced Science & Technology)

Most living cells do not absorb or scatter light significantly, i.e. they
are essentially transparent, or phase objects. Phase contrast microscopy proposed by
Zernike in the 1930’s represents a major advance in intrinsic contrast imaging, as it reveals
inner details of transparent structures without staining or tagging. While phase contrast
is sensitive to optical path-length changes in the cell, the information retrieved is only
qualitative. Quantifying cell-induced optical pathlength shifts permits nanometer scale
measurements of structures and motions in a non-contact, non-invasive manner. Furthermore, the phase
information allows for retrieving tomography of transparent structures such as cells. This is an
inverse problem analog to that in X-ray diffraction applications (e.g., crystallography), where the
phase signal is not readily available. Thus, quantitative phase imaging (QPI) has recently
become an active field of study, with applications in both materials and life sciences.

We have developed Diffraction phase microscopy (DPM) and Spatial Light
Interference Microscopy (SLIM) as two complementary methods for QPI. They share the idea of
common-path interferometry, which adds stability to the phase measurements. I will review several
applications of this method in basic science and clinical diagnosis.  I will end with a discussion of
recent results using QPI for pump-probe applications of nonlinear laser-matter interactions.