Conducting nanoparticles with plasmon resonances create local, nanoscopic field enhancements that boost an analyte
molecule’s surface-averaged Raman scattering cross-section orders of magnitude above the bulk Raman cross-section by an amount known as the enhancement factor (EF). Demonstrations of single-molecule sensitivity with EF ~ 1013 have been reported from small “hot spots” (e.g., regions of enhanced electromagnetic near fields) on specialized substrates, but realistic chemical sensing requires high average EF over large substrates for practical sampling.1 By using simple wet chemical methods, NSRDEC scientists have fabricated large-area arrays of novel, highly conducting, anisotropic Ag and Al nanoparticles. The nanoparticles adhere to an ultrathin layer of poly-4(vinyl pyridine), and are anchored by submicron coating of poly-methyl methacrylate on glass and SiO2-coated Si substrates. The average interparticle spacing is determined by the dilution of the nanoparticle-water suspension. We present surface-enhanced Raman spectroscopy (SERS), spectrophotometry, and microscopy data from these nanoparticle arrays, model this data and the nanoscopic field enhancement, and determine the SERS EF. We compare the observed absorption resonances and SERS EF with those predicted by finite difference time domain modeling of the nanoscale fields and optical properties, and find good agreement between measured and calculated reflectivity, achieving EF ~ 106 for benzenethiol adsorbed onto a monolayer array of 120 nm Ag nanoparticles over an area of ~ 0.5 cm2. We discuss a way forward to increase SERS EF to 107 with large-area samples assembled using chemical methods, by using spiky Ag “nano-urchins” with very large predicted field enhancements.
We have synthesized nanostructured rare-earth doped silicates by two different methods: combustion flame - chemical vapor condensation (CF-CVC) and sol-gel processing. Substantial rare-earth concentrations (~ 8 wt. %) were achieved with no signs of concentration quenching. We have observed unprecedented spectrally broad/flat fluorescence emissions at 1.55 μm from the Er3+-doped materials, which we attribute to their unique nanostructures developed during heat treatments. In depth results of a combined XRD/TEM study monitoring the evolution of the nanostructure will be presented. The role of processing conditions, chemistry, and particle size will also be discussed.
The strength of optical fiber at low temperature is an important parameter since it approximates the inert strength, i.e. the starting strength of the material before degradation by fatigue. Published data suggest that the fatigue may abruptly slow below some temperature. However, published data are limited to strength vs temperature or fatigue in liquid nitrogen. We report strength and fatigue data for both bare (stripped) and metal coated fused silica optical fiber at temperatures down to 77 K. While fatigue slows as the temperature is reduced (i.e. the stress corrosion parameter increases with falling temperature) fatigue is still measurable at 77 K. This is the case even for hermetic metal coated fiber with extremely low water activity at the glass surface. The results confirm that fused silica exhibits "intrinsic" fatigue, i.e. fatigue in the absence of moisture.