Phase-Sensitive Optical Coherence Microscopy (OCM) - Biomedical Optical Phase Microscopy and Nanoscopy - page 263

13.4.5 Refractive Index Measurements

In Ref. [19] , the use of common-path phase OCM was demonstrated for the measurement
of the refractive index (RI) of a biomimetic material (glucose solution in water having
intralipid as the scattering medium) and a single biological cell (keratinocyte). The RI of
glucose solutions was measured with a precision of B 0.00015, which corresponds to a
precision of B 2 nm in the OPD measurement using this setup. The precision obtained in
the measurement of the RI of a single keratinocyte was B 0.0004. By taking advantage of
the phase stability and robustness of the common-path geometry, the enhanced repeatability
of the results was demonstrated.

To measure the RI ( n ) of glucose solution, the OPDs were measured first with an empty
chamber with a geometrical thickness t , followed by OPD measurements in a liquid-filled
chamber. This was used to estimate the optical thickness, that is, n 3 t . The observed changes
in RI for different glucose concentrations in water and in two different concentrations of
intralipid (0.5% and 1%) were measured. A linear increase in RI was observed with an
increase in concentrations of glucose, in agreement with the literature.

To measure the RI of a single cell, the setup was coupled with a microscope. A CCD
camera was placed in the viewing port of the microscope, and it displayed the bright field
image from the same objective lens with which the OCM signal was collected. The cell
diameter ( D ) of keratinocyte cells was measured using the bright field image. This cell
diameter was then used to estimate the cell thickness by assuming that the cells are
spherical in shape. In order to measure the RI, two OPD measurements are taken, that of
the sample chamber when the light passes through the cell L c and that of the outside of the
cell L s . From these measurements the RI of the cell can be derived as:

L c 2 L s
D
where n c is the RI of the cell, and n s is the RI of the cell media.

n c 5 n s 1

13.4.6 Nerve Displacement during Action Potential

Nerve fibers undergo rapid outward lateral surface displacements during propagation of action
potentials. This swelling phenomenon, which is generally attributed to water influx into axons,
has been observed in a number of invertebrate and vertebrate preparations. Measurement of
nerve displacements requires a system that is capable of measuring nanometer-scale
displacements in surfaces with low reflectivity and at a rate of B 2 kHz bandwidth. Techniques
such as laser Doppler vibrometry are sensitive to small displacements but are susceptible to
external perturbations and may be difficult to apply to weakly reflecting objects.