| Surface Enhanced Raman Scattering is a very important effect in Chemical Physics due to its ability of bringing small signals to light, even allowing Raman Spectra of single molecules. The effect is limited, however. The maximum enhancement seen from SERS is approximately 7-8 orders of magnitude. In order to observe even more enhancement, one would think that using another method of Raman enhancement in conjunction with SERS would be great. Here we introduce SE CARS (Coherent Anti-Stokes Raman Scattering). CARS in a nonlinear optical process, in which two electric fields are incident upon a molecule in order to prepare a particular vibrational state (a stimulated Raman type process). A third field is then introduced in order to probe the vibration, which emits a field blue-shifted from the original fields. Because it is a four-wave mixing process, CARS is thought of as a Raman Squared process, thereby increasing the amount of signal observed. By combining Surface Enhancement with CARS, the hope is to surpass the enhancement of both methods. The goal that we wish to achieve is to see a CARS signal from a single molecule.;The molecule chosen for these studies is Di-Styryl Benzene (DSB), due to its strong conjugation and therefore high scattering cross section. In order to achieve that Surface enhancement, we have used molecules that are terminated by thiols on each end and attached to silver nanospheres. The samples were all made for us by the Bazan group at UC Santa Barbara. They have developed a method to tether single molecules to nanoparticles in a dimer-like geometry. They used this method to create the DSB dimers in the following experiments. The samples were used over several years, in order to observe the stability of the chosen geometry over time.;In order to move to CARS, a two-color laser system was needed. I have designed and constructed two synchronously pumped non-linear Optical Parametric Amplifiers for use in these experiments. The NOPAs were used to measure time-resolved CARS signals in a non-collinear BOXCARS geometry. Because the desired molecule is DSB, molecules with similar Raman spectra were needed. Calibration of the NOPAs was done using Styrene to target the 1631 cm-1 shift, and ThioPhenol to target the 997 cm-1 vibration. Time-Resolved CARS data was taken out to 10ps for both liquid samples. Forward Raman Simulations were carried out in order to predict what the CARS spectra should look like. |
