New synthesis techniques for highly concentrated colloidal C60 suspensions were developed. The nonlinear absorption and nonlinear scattering behavior of colloidal C60 suspensions and benchmark materials (carbon black suspension and C60 solution) were studied with an apparatus that simultaneously measured the total scattered and transmitted energy, inferring absorbance. These experimental results were compared to simple thermodynamic and reverse saturable absorption models, as well as a hybridized model proposed for the nonlinear optical behavior of C60 colloids. All samples followed an attenuation pattern in the nonlinear scattering regime that was fit by a single extinction coefficient, indicating that the energy in excess of that required to reach the sublimation threshold does not significantly affect the size of the induced scattering centers. C60 colloids evidenced strong quenching of the first excited singlet band, leading to weak intersystem-crossing to the triplet manifold. The degree of quenching was morphology dependent. Tighter crystalline packing led to stronger quenching. Samples with higher triplet quantum yield evidenced less efficient heating of the particles. Consequently, for otherwise similar C60 colloids, stronger nonlinear absorption response was found to diminish the nonlinear scattering response. Large, crystalline C60 colloids had a stronger nonlinear optical response than benchmarks.

This paper describes optically tandem solar cells with improved spectral efficiency for the blue, UV, and near infrared portions of the solar spectrum. Optically tandem cells are constructed by stacking frequency converting materials and photovoltaic devices so that they are optically coupled.

We examine the photophysics of a colloidal suspension of C(60) particles in a micellar solution of Triton X-100 and water, prepared via a new synthesis which allows high-concentration suspensions. The particle sizes are characterized by transmission electron microscopy and dynamic light scattering and found to be somewhat polydisperse in the range of 10-100 nm. The suspension is characterized optically by UV-vis spectroscopy, femtosecond transient absorption spectroscopy, laser flash photolysis, and z-scan. The ground-state absorbance spectrum shows a broad absorbance feature centered near 450 nm which is indicative of colloidal C(60). The transient absorption dynamics, presented for the first time with femtosecond resolution, are very similar to that of thin films of C(60) and indicate a strong quenching of the singlet excited state on short time scales and evidence of little intersystem crossing to a triplet excited state. Laser flash photolysis reveals that a triplet excited-state absorption spectrum, which is essentially identical in shape to that of molecular C(60) solutions, does indeed arise, but with much lower magnitude and somewhat shorter lifetime. Z-scan analysis confirms that the optical response of this material is dominated by nonlinear scattering.

One approach to CBRNE detection is analytical monitoring with portable spectroscopy systems. Such a technique needs to work in adverse environments, be amenable to use by field operators, and, given the sensitive nature of the target materials, should have an extremely rapid response time with no false negatives. This research demonstrates that surface-enhanced Raman scattering (SERS) is capable of detecting ppb levels of CBRNE materials with high sensitivity and no false positives. We present reproducible and selective detection using novel SERS structures that exhibit an inherently uniform surface morphology, leading to rapid, reproducible manufacturing. Our work includes receiver-operator characteristic (ROC) curves for the detection of both conventional and improvised nitro explosives at low signal-to-noise ratios. We also present the detection of added CBRNE materials including chemical and biological agents as well as nuclear enriching materials. Our expertise extends to instrumentation of portable, robust Raman spectrographs that can be packaged with our sensors for a versatile security tool with applications extending from points of entry to points of production, from people to objects and freight.

Surface-enhanced Raman scattering (SERS) is emerging as a versatile and powerful technique for the detection of various defense related hazardous materials. This work illustrates the level of sensitivity and reproducibility achieved using SERS substrates with structural features engineered at the nanometer scale. Nanostructured substrates show significant sensitivity toward a number of different analytes. Pinacolyl methyl phosphonic acid (PMPA), a nerve-agent degradation product, was detected in less than 30 seconds at 1ppb. Para-nitroaniline, an explosives simulant, was detected in the same amount of time at 10 ppm. Multiple tests showed signal reproduction of PMPA at 100 ppb below a 7% standard deviation. The substrates are small and lightweight. In addition, a portable SERS spectrometer, equipped with a fiber coupling for excitation and detection, can act as the sensor body. On a previous occasion, electrochemically roughened SERS substrates were loaded into this portable spectrometer and deployed in the field for the successful blind detection of buried, defused, landmines. Such a system accommodates multiple substrate technologies, allowing sensing in the vapor and liquid phase as well as via solids extraction, and is compatible with nanoscale substrates.