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46067-G6
Single Molecule Vibrational Coherence on a Carbon Chain

Eric O. Potma, University of California, Irvine

1. Research goal
We aim to measure the vibrational motion of two CH₂ oscillators on a hydrocarbon chain and determine their mutual correlation. Such measurements will reveal important information on the ultrafast structural motions in the chain, essential molecular dynamics that has remained inaccessible thus far. The mutual correlation of two vibrating molecular groups can only be probed on a single molecule level. We, therefore, propose a series of coherent vibrational spectroscopy experiments on a single molecule, the very first of their kind.
2. Research steps taken
The most suitable optical technique for probing coherent vibrational motions is coherent anti-Stokes Raman scattering (CARS) spectroscopy. We have constructed a special CARS microscope with high sensitivity to measure the vibrational response form molecules in small sample volumes. The following steps have been taken:

• Sensitivity. We have found that the most sensitive detection level of the conventional CARS probing technique corresponds to ~10⁵ CH₂ oscillators in a volume of 0.1 fL Since we cannot reduce the probing volume any further, a mechanism for signal amplification had to be found. Inspired by the success of surface enhanced Raman scattering (SERS), we have chosen to explore field enhancement by surface plasmon resonances at metallic surfaces. Most of our efforts during the review period have been devoted to optimize the surface-enhanced (SE-)CARS approach.
• Silver and gold nanoclusters as SE-CARS substrates. We examined the suitability of colloidal clusters of gold and silver nanospheres for SE-CARS experiments. We have found that, unlike in SERS, the ultrafast excitation conditions in the CARS experiment give rise to the generation of luminescence and coherent emission from the nano-metallic substrates. This form of emission was much stronger than the tentative vibrational response of surface tethered molecules. To exercise more controllability over the substrate emission, we have chosen to work with nanowires instead.
• Luminescence from gold nanowires. Nanowires, fabricated by our collaborator Reginald Penner, are excellent substrates candidates for SE-CARS experiments. Unlike colloidal samples, the wires have a precisely defined plasmon mode that can be selectively addressed through polarization control of the excitation light. We have examined the incoherent emission of these wires under ultrafast excitation conditions. We have fully characterized the two-photon induced luminescence of the wires. Importantly, by controlling the size and aspect ratio of the wires, we were able to tune the strength of the luminescence response. Such controllability will be crucial in selecting the optimum excitation conditions in a SE-CARS experiment. Our findings have been published in the Journal of Physical Chemistry C 112 (33), pp 12721-12727 (2008), and support from ACS-PRF is acknowledged.
• Coherent emission from gold nanowires. In addition to incoherent emission, we also found that the nanometallic substrate produces coherent emission under ultrafast excitation. Besides the nanometal’s well-known second harmonic generation (SHG) response, we have identified and characterized the coherent anti-Stokes emission of the nanowires. We have shown that the coherent anti-Stokes emission can be controlled and boosted relative to the luminescence background. These findings show that the third order optical response (coherent anti-Stokes emission) of plasmon resonances can be favored over the second order optical response (SHG, two-photon induced luminscence) through precise dimension control of the wires. This notion is of critical importance to SE-CARS studies. Our results have been published in Nano Letters 8(8), 2373-2377 (2008), and support from ACS-PRF is acknowledged

3. Proposed plan of action
After fully characterizing the nonlinear response from metallic nanosubstrates, we are now in a position to isolate the vibrational response from tethered molecules from the intrinsic emission from the metallic substrate. The following steps are proposed:

• Nano-gap SE-CARS assay. We have found that when a small 2nm gap is made in the nanowire, enormous field enhancements can be obtained. The nano-gap approach combined all the advantages of controlled engineering of the plasmon mode in nanowires with the enormous enhancement offered by the proximate surfaces in the gap. We have tethered alkane thiols in the gap and have recorded signals that indicate that a molecular response can be detected. We will further explore this strategy and optimize the nano-gap assay for the purpose of SE-CARS single molecule detection.
• Time-resolved SE-CARS. Once the nano-gap assay has been developed, we will perform time-resolved SE-CARS measurements to unravel the coherent vibrational response from targeted CH₂ oscillators.

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