| dc.description.abstract | Since the discovery of Raman scattering at the beginning of the 20th century, spontaneous Raman spectroscopy has developed into a powerful tool for the study of matter. By probing vibrational and rotational molecular oscillations, Raman spectroscopy facilitates remote detection and identification of chemicals, allows to study a chemical structure of materials, permits close monitoring of chemical reactions, and more. The main disadvantage of the spontaneous Raman effect is its low efficiency, as typically only a small fraction of the scattered photons carries information about vibrational modes of the analyte. Hence, one has to increase the light intensity and use long acquisition times to achieve a reasonable signal-to-noise ratio. These factors limit the application of spontaneous Raman spectroscopy to probe low concentrations of analyte, to analyze chemicals with low damage threshold, and to monitor rapidly changing systems. Fortunately, this drawback can be overcome by exploiting the phenomena of resonant, coherent and surface enhancements of Raman scattering. Actually, these effects not only increase the Raman scattering efficiency but represent distinct spectroscopic techniques: deep ultraviolet (DUV) Raman spectroscopy, coherent anti-Stokes Raman scattering (CARS) spectroscopy, surface-enhanced Raman scattering, and surface-enhanced CARS spectroscopies (SERS and SECARS).
Besides yielding higher signals than spontaneous Raman spectroscopy, these techniques provide additional spectroscopic data which is complementary to the vibrational-rotational spectrum. They offer additional means of examining electronic molecular structure, evolution of chemical reactions, and imaging of dynamic systems such as gas flows, among other phenomena.
In this dissertation we will demonstrate the advantages of the DUV Raman spectroscopy, CARS and, especially, SECARS over spontaneous Raman scattering spectroscopy. We will describe the mechanism responsible for the superiority of the abovementioned techniques over spontaneous Raman spectroscopy, however we will focus on the experimental implementation of each of these methods. We will briefly outline the basic physical principles of non-resonance and resonance Raman scattering and demonstrate how the resonant enhancements can be exploited in DUV Raman spectroscopy. As an experimental demonstration, we will show the tunable laser system specifically designed for this application. Then, we will provide an example of a CARS spectroscopic system designed to measure gas concentrations and to image gas flows. Finally, we will show the surface enhancement of the semiconductor nanoparticles used in SECARS spectroscopy of pyridine-ethanol complexes. The application of the semiconductor nanoparticles results in over 9 orders of magnitude stronger signals than regular CARS spectroscopy. This application promises to yield highly customized and affordable semiconductor nanoparticles (instead of metallic) in SECARS spectroscopy. Ultimately, the demonstrated combination of methods and enhancements opens a path towards the spectroscopy of nano-analytes and, ultimately, of single molecules. | en |
