Coherent Raman Scattering: Methods Towards Imaging with High Sensitivity.

Abstract

Coherent Raman spectroscopy is a powerful tool for molecular identification. For imaging applications, Raman spectroscopy techniques have offered a way of achieving endogenous chemical contrast without the need for fluorescent labeling. Increasing the sensitivity of Raman scattering microscopy is vital to performing high speed, chemically selective imaging. This thesis presents three experiments with the ultimate goal of increasing the sensitivity and quantifying limitations of different Raman techniques. The first experiment is a comparison of spontaneous and coherent Raman signal strengths under biological imaging conditions. While it is commonly stated in the literature that coherent Raman techniques provide orders of magnitude higher signal than spontaneous Raman, such a comparison has not been done under the low concentration, low excitation power conditions relevant for biological imaging. We determine a critical power above which coherent Raman methods provide higher signal and below which spontaneous Raman methods provide higher sensitivity. Contrary to what is commonly stated in the literature, spontaneous Raman can provide higher signal levels under common biological imaging conditions.

The second experiment is a demonstration of the first multiplexed Raman-induced Kerr effect (RIKES) microscopy setup to date. We compare the signal-to-noise ratios between femtosecond stimulated Raman (FSRS) and RIKES for spectroscopy applications in solution and for microscopy applications with polystyrene beads. We find that for our samples, RIKES consistently provides a higher signal-to-noise ratio than FSRS for both applications. We also use RIKES microscopy to map out the distribution of cytoplasm in onion cells.

Finally, a surface-enhanced Raman experiment is performed to observe and compare highly enhanced signals for spontaneous and coherent Raman spectroscopy. We use a commercial SERS substrate and observe significantly enhanced spontaneous Raman signals from benzenethiol adsorbed onto the substrate. For coherent Raman techniques, we observe a large background signal that contains no chemical information. We investigate the nature of the background, finding that it is a four-wave mixing signal, likely arising from radiation coupling with surface plasmons on the nanostructured substrate. This background compromises the ability to use surface enhancement for increasing the sensitivity of coherent Raman imaging and sensing applications.