Directional Dependence and Polarization Anisotropy in Pump-probe Microscopy
Nonlinear optical microscopies provide a flexible, inexpensive, and useful way to do medical imaging. Among the imaging modalities, pump-probe microscopy is a versatile technique that shows great potential to image melanomas (a skin cancer) for which there is still a great need for improved early clinical diagnosis. But image analysis often suffers from an incomplete understanding of the signal properties. In this dissertation, I discuss several unusual properties observed in pump-probe signals using model samples (e.g., gold nanoparticles, natural and synthesized melanin particles, historic artwork pigments) and an upgraded pump-probe microscope. The experiments improve understanding in pump-probe signals and help for better data analysis.Chapter 2 demonstrates my experimental results regarding the directional dependence of pump-probe signals. I upgrade imaging system for angle-resolved measurements, and use melanin particles and gold nanoparticles to study the role of scattering in the generation of pump-probe signals. The results show the reason for signal discrepancy between transmission (for thin sliced samples) and backward (for in-vivo imaging) detections observed in melanoma imaging. Chapter 3 discusses an intrinsic polarization anisotropy in the pump-probe signal of melanin. I develop a formalism based on transition dipole moments to model this polarization anisotropy, and use it to decompose pump-probe signals in several pigments (melanin, historic artwork pigments). I improve the imaging system to accurately control and calibrate the polarization angle of the lasers for polarization measurements. The polarization angle serves as a useful imaging parameter. Combined with the directional dependence discussed in chapter 2, a global fitting is performed on the melanin signals to extract strengths and decays of the various components in the signal. Chapter 4 discusses studies on remaining puzzles observed in melanoma imaging, including depth dependence, signal dynamics change due to particle aggregation, and a long-lived signal component of unclear origin. While these anomalies are not fully understood, their properties are presented for future studies. Chapter 5 summarizes the key achievements presented in this dissertation, and discusses new questions that should be explored in the future.
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