Shinning light that has a wavelength that matches the surface plasmon resonances of nanostructured metals, such as gold and silver, results in a large enhancement of this incident optical radiation field.  Technologies can be developed for imaging and sensing for example that provide greater sensitivities by exploiting this combination of metal nanoparticles and selective excitation colors.  This is called plasmonic enhancement (PE).  When laser light is used to excite and detect PE vibrational resonant molecular signatures, oddly shaped spectral features were obtained in such PE stimulated experiments. However, sensitivities at the single molecule level are achieved in the stimulated Raman experiments that have been carried out as evidence of the sensitivity achievable with this PE approach.  Two explanations for these unusual lineshapes have been discoverd and both may be operative.  One is based on interference effects from the emission of the nanoparticles themselves and the other is due to the complex character (i.e. real and imaginary part)  of the enhancement factor that only comes into play for stimulated, i.e. two laser, Raman experiments. Predicted lineshapes due to plasmon emission interference are shown in an accompanying figure for different enhancement strengths (g) and matches what has been observed.  Understanding the lineshape of these strongly enhanced vibrational features is important for the use of this approach for imaging capabilities so the identity of molecules  of interest can be established.  In another component of this project it was discovered that controlled deposition of silver on gold nanostructures (nanobipyramids) allowed tuning of the localized plasmon resonance to both higher or lower energies permitting better resonance with a selected molecular state of interest.

Sum frequency generation (SFG), another vibrationally resonant nonlinear laser technique, was used to probe how self assembled monolayers align on metal surfaces.  Plasmonic resonance effects are surface roughness dependent and contribute to the dispersive lineshapes found in these SFG spectra on Au and Ag as well.  Self-assembling monolayers of n-alkylthiols show remarkable odd-even (n) chain length effects that are dependent on the orientational structural details of the molecules with these metal surface as evident in the accompanying figure.  Such molecular level details are  important to understanding the macroscopic properties of these materials which have applications in sensing and chromatography.

Dense gases and supercritical fluids are high temperature and pressure environments for important chemical activity spanning applications from internal combustion engine reactions to coffee decaffeination.  An ultrafast, laser multidimensional IR technique (2DIR) was used to study dense gases and supercritical fluids (SCF) to understand how chemistry might be different in these solvents than in “normal” liquid solvents, and to develop a better detailed description of these dense phase regions that are common to all materials. Although we are taught in general chemistry that SCFs are a homogenous region with properties in between those of the gas and liquid, our ultrafast laser experiments find that high dense and supercritical solutions are better described as a mix of co-existing freely rotating molecules, that is a gas population, and hindered molecules trapped in liquid population regions below finite sized barriers (see picture). The supercritical region corresponds to the panels in between the gas and liquid depictions in this figure.

Furthermore, excited rotational and vibrational energy relaxation appears to be governed by independent binary collisions in the supercritical regions studies thus far, a gas-like response in a dense fluid.  Near the critical point dynamics at the molecular scale is found to slow as long length scale spatial correlations increase at the state point.  Thus the molecular environment “knows” that longer scale, macroscopic critical effects are occurring and has an influence on the local molecualr environment. This technique will be used to divide the supercritical region into liquid and gas like regions and confirm theoretical predictions about the existence of such boundaries in the supercritical regime.

Students working on these projects are trained in understanding matter-radiation interaction, nonlinear laser physics, the design and characterization of plasmonic nanostructures, their fabrication, near- and far-field  analysis through electromagnetic simulations, data collection and processing protocols and lab safety best practices, all skills transferable to both commercial as well as subsequent academic laboratories. 

Last Modified: 10/03/2019
Modified by: Lawrence D Ziegler