Linear and nonlinear photothermal spectroscopy and hyperspectral imaging with a fiber laser probe
Embargo Date
2018-11-01
OA Version
Citation
Abstract
Recent years have seen a push to provide a fast, sensitive, and quantitative diagnostic tool
for biomedical applications. A search for new methods that can perform label-free and
bond-specific determination of tissue and disease types with high spatial resolution is
much desired. To address these needs, we have developed a mid-infrared photothermal
system for sensitive and non-destructive characterization of samples. Our system utilizes
a mid-infrared pump with a near-infrared probe for label-free spectroscopy and high
spatial resolution imaging. In particular, this research focuses on optimization of the
photothermal system, exploration of novel nonlinear photothermal phenomena, and
development of a sub-diffraction limited mid-infrared imaging system.
Photothermal spectroscopy is a pump-probe technique that utilizes a thermal lens
effect in the sample for contrast. With the use of a high brightness mid-infrared pump
laser, we extend photothermal spectroscopy into the mid-infrared regime for sensitive
detection with high signal contrast. Targeting vibrational modes intrinsic to the sample
allows for label-free characterization. Use of a fiber laser probe provides improved
spatial resolution and takes advantage of the well-developed detector technology at near-infrared
wavelengths.
The research presented will be divided into three parts: optimization of the
photothermal system, investigation of novel nonlinear photothermal phenomena, and
photothermal spectroscopy and imaging for biomedical applications. Optimization of
fiber laser design and experimental setup results in >100x increase in signal strength and
over an order of magnitude improvement in signal contrast. With an optimized system,
linear and nonlinear mid-infrared photothermal spectroscopy of a liquid crystal sample is
demonstrated. For the first time, multiple bifurcations are reported in the nonlinear
regime, shedding insight on the photothermal laser-matter interaction across phase
transitions of a liquid crystal sample. Using a raster-scanning approach, sub-diffraction
limited mid-infrared imaging is demonstrated. With this technique, various tissue types
within the brain can be distinguished from one another, including differentiation between
healthy and tumor tissue. Hyperspectral imaging of biological tissues demonstrates the
potential of this technique to combine both spectral and spatial information for sample
characterization. We present a photothermal system with the potential to meet the
demands in drug and food safety, environmental monitoring, biomedicine, and security.