The photothermal effect is a way in which chemical contrast can be measured as an optical
pathlength or phase change. When a chemical species in a sample absorbs optical energy at
a particular wavelength, this absorption raises the temperature at these points in the sample
via the photothermal effect. This temperature change changes the local refractive index in
the sample. Quantitative phase imaging is an interferometric technique for measuring the
optical pathlength of sample features. Quantitative phase imaging is capable of detecting
the photothermally-induced refractive index change, and is thus a powerful method for performing photothermal imaging. In this work, a thermal wave model is derived from Fourier's
law of conduction in conjunction with a medium's heat capacity to derive the diffusion of
temperature in a medium. This diffusion theory is transformed to a thermal wave model by
applying a temporally modulated thermal source. Analytical expressions for the temperature
field surrounding such a modulated thermal source are derived in multiple dimensions. The
thermal wave equation is also simulated using a custom finite difference numerical method,
and the simulated results are compared to the theoretical expressions with good agreement.
The experimental apparatus for inducing such a thermal point source in a medium of water
is described using the quantitative phase imaging system of spectral modulation interferometry. The spectral modulation interferometry system is aligned with a visible light pumping laser in two configurations for point source measurement and cell imaging. Label-free
chemical imaging is then performed by pumping a field of cellular samples with wide-field
illumination, and the resulting photothermal signal is detected by temporal analysis of the
optical pathlength changes, generating the two-dimensional photothermal image. The measured photothermal cell image is qualitatively compared to predicted photothermal image
based on the application of the thermal wave model in the spatial frequency domain. The
chemical specificity of this technique is also verified by simultaneously pumping absorbing
and non-absorbing biological cells in the same field-of-view. / Generating image contrast is a fundamental challenge in optical microscopy. Samples of interest in optical microscopy typically do not have visible absorption contrast without modification. A method of contrast that could provide information about a sample's absorption at
different optical wavelengths would be useful for characterizing a sample's chemical content.
The photothermal effect is an effect in which the small absorption of light by microscopic
samples can be detected as a temperature change. With quantitative phase imaging, this
temperature change can be measured by detecting the change in optical density of a sample due to its increase in temperature. Thus, quantitative phase imaging can be used to
detect the small absorption of light by microscopic samples and generate two-dimensional
images with chemical contrast. This work describes the theory of how thermal energy produced by optical absorption diffuses through a sample immersed in water. A thermal wave
model is derived theoretically and compared to a custom simulation of the thermal wave
physics with strong agreement. This thermal theory is verified with the quantitative phase
imaging system used in this work to characterize the photothermal imaging technique. The
photothermal imaging method is then applied to cellular samples, which are pumped with
green light. The photothermal image is then generated and compared qualitatively to the
image predicted by the thermal theory. The chemical imaging ability of the technique is
then demonstrated by simultaneous imaging of absorbing and non-absorbing cells.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/111226 |
| Date | 18 January 2021 |
| Creators | Thomas, Joseph Gabriel |
| Contributors | Electrical Engineering |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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