Coherent Raman generation is widely utilized both in fundamental
research and in a variety of applications. Once coherent molecular motion of
an ensemble is established, it results in efficient generation of
femtosecond Raman sidebands, allowing synthesis of single-optical-cycle
pulses [1, 2], 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 by 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 .
1. Zhi, M., et al., Pulse-shaper-assisted phase control of a coherent
broadband spectrum of Raman sidebands. Optics Letters, 2011. 36(20): p.
2. Zhi, M.C., K. Wang, and A.V. Sokolov, Toward single-cycle pulse
generation in single-crystal diamond, in 17th International conference on
ultrafast phenomena. 2010: Snowmass, Colorado.
3. Pestov, D., et al., Single-shot detection of bacterial endospores
via coherent Raman spectroscopy. Proceedings of the National Academy of
Sciences of the United States of America, 2008. 105(2): p. 422-427.
4. Zhi, M.C., et al., Generation of femtosecond optical vortices by
molecular modulation in a Raman-active crystal. Optics Express, 2013.
21(23): p. 27750-27758.
5. Voronine, D.V., et al., Time-Resolved Surface-Enhanced Coherent
Sensing of Nanoscale Molecular Complexes. Scientific Reports, 2012. 2.
6. Hua, X., et al., Nature of surface-enhanced coherent Raman
scattering. Physical Review A, 2014. 89(4): p. 043841.