Highly Efficient Coherent Raman Generation

Coherent Raman generation is widely utilized both in fundamental research and in a variety of applications. Once the coherent molecular motion is established, it results in efficient generation of femtosecond Raman sidebands, allowing synthesis of single-optical-cycle pulses, or, for example, enabling detection of bacterial endospores via coherent Raman spectroscopy.

We explore the utility of Raman coherence in two sets of experiments. In the first part of this work, we generate multi-color optical vortices in a Raman-active crystal PbWO_(4) using two-color femtosecond laser pulses. We verify that the topological charge transfer among the Raman sidebands obeys the expected orbital angular momentum algebra. In the second part of this work, we explore detection and sensing applications, and achieve further improvement of efficiency by using field enhancement due to surface plasmon resonances in aggregates of gold nanoparticles. By scanning the time delay of the probe pulse, we demonstrate a new vibrational spectroscopic technique called time-resolved surface-enhanced coherent anti-Stokes Raman scattering (tr-SECARS). We demonstrate the application of tr-SECARS by detecting hydrogen-bonded molecular complexes of pyridine with water in the near field of gold nanoparticles. We discuss the discrepancy in SECARS enhancement factors, observed in the experiment and calculated theoretically. To understand this discrepancy, we develop a model and simulate the dependence of SECARS spectra on the position and linewidth of the surface plasmon resonance. Finally, we propose strategies for increasing experimental enhancement factors towards theoretical predictions.