Gold nanostructures focus light to a molecular length scale at their surface, creating the possibility to visualize molecular structure. The high optical intensity leads to surface enhanced Raman scattering (SERS) from nearby molecules. SERS spectra contain information on molecular position and orientation relative to the surface but are difficult to interpret quantitatively. Here we describe a ratiometric analysis method that combines SERS and unenhanced Raman spectra with theoretical calculations of the optical field and molecular polarizability. When applied to the surfactant layer on gold nanorods, the alkane chain is found to be tilted 25° to the surface normal, which matches previous reports of the layer thickness. The analysis was also applied to fluid phase phospholipid bilayers that contain tryptophan on the surface of gold nanorods. The lipid double bond was found to be oriented normal to the bilayer and 13 Å from the nitrogen atom. Tryptophan was found to sit near the glycerol headgroup region with its indole ring 43° from the bilayer normal. This new method can determine specific interfacial structure under ambient conditions, with microscopic quantities of material, and without molecular labels.

Clutter from air bubbles can significantly impact the performance of object detection sonar in shallow water environments where bubbles are present due to breaking waves, wakes, or organic matter. Nonlinear acoustical effects can be excited in bubbly liquids at sound pressures below that required in bubble-free media and hence can be used to discriminate between bubbles and solid objects or to suppress bubble-related clutter altogether. Whereas such effects are widely exploited for ultrasonic biomedical imaging enhancement and it is hypothesized that dolphins exploit them in their biosonar, relatively less attention has been given to their use in naval, commercial, or oceanographic manmade sonar. Here, we describe laboratory tank experiments and modeling efforts aimed at exploiting these effects to improve object detection sonar. A bubble dynamics model was employed to investigate parameter space for nonlinear effects such as subharmonic and higher-order harmonic generation from regions of bubbly liquid and sonar-induced bubble translation. Laboratory experiments were conducted to verify the presence of these effects. Finally, a laboratory demonstration of a nonlinear bubble/object discriminating sonar, in which nonlinear effects are used to place markers on returns from regions of bubbly liquid, will be presented. [Work supported by ONR.]

No abstract

No abstract