AMO/QO Physics Seminar

"Highly Efficient Coherent Raman Generation"

Xia Hua
Department of Physics and Astronomy, Texas A&M University


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 [3].

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 [4]. 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 [5]. 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 [6].

1. Zhi, M., et al., Pulse-shaper-assisted phase control of a coherent broadband spectrum of Raman sidebands. Optics Letters, 2011. 36(20): p. 4032-4034.
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.

Wednesday, June 11, 2014
MPHY 578, 9:00 a.m.
Mitchell Physics Building

Institute for Quantum Science and Engineering
Texas A&M University