Polarization and Spectral Interferometric Techniques for Quantitative Phase Microscopy - Biomedical Optical Phase Microscopy and Nanoscopy - page 276

We quantified the temporal phase stability by recording the millisecond-scale temporal path-
length fluctuations originating within a diffraction-limited spot, as defined by the imaging
optics. The stability was first measured without the presence of a sample and then with the
sample at three representative locations: on the cell center, on the cell edge, and at the
background (through the medium only), as shown in Figure 14.2C . For these four situations,
the standard deviations of the measured phase fluctuations were determined to be 0.67, 1.31,
1.83, and 0.87 nm, respectively, as shown in Figure 14.2E . The first result represents the
fundamental temporal stability of the entire optical system, demonstrating subnanometer
path-length sensitivity on the millisecond time scale. The slightly degraded stability in the
three latter cases may be attributed to small vibrations occurring inside the entire sample.
To illustrate the utility of DQPM for imaging dynamic cellular phenomena, we recorded a
2-s movie at 120 fps that shows the phase profiles of a beating rat neonatal cardiomyocyte
(myocardial cell isolated from the heart of a neonatal rat) in growth medium.
Here, we have demonstrated a new polarization-based method for imaging fast biological
phenomena with high accuracy. Experimental results show good spatial resolution for
dynamic cell phase imaging and excellent temporal phase stability. Due to its simultaneous
acquisition, the system's ability to observe fast cellular phenomena is only limited by the
full frame rate of the camera, and thus this method provides a powerful tool for dynamic
studies of biological cells.
14.3 Spectral-Domain Differential Interference Contrast Microscopy
In Section 14.2 , an improvement in system stability was realized by utilizing light polarized
in two orthogonal directions. Now, we discuss another polarization-based system for
quantitative measurement of phase gradient. More importantly, this technique, termed
spectral-domain differential interference contrast microscopy (SD-DIC), integrates spectral
techniques to greatly improve the sensitivity of these measurements.
First invented in the 1950s, DIC microscopy has since become a standard imaging modality
in modern optical microscopes, and today it sees widespread use for enhanced contrast in
imaging unstained and transparent biological specimens [14] . Similar to phase-contrast
microscopy, DIC takes advantage of optical phase variations across an object to achieve
better visualization, with the help of interferometry. In a typical DIC microscope, the
interference occurs between the two orthogonally polarized components of the input light,
which are split in space to pass the sample with a slight lateral shear and hence experience
a differential optical phase/path-length change between them. When subsequently
recombined, they interfere to produce drastically improved contrast compared with
brightfield images. DIC microscopy hence measures the phase/path-length gradient of a
sample and generates images with a shadow-casting effect.