Publications

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We present a broadband, all-dielectric, diffractive optical element (DOE) for spectral beam combining with optimized efficiency. We achieve maximal efficiency and polarization insensitivity for the sum of incident wavelengths by varying grating etch depth and duty cycle of a rectangular profile grating realized with the precision of ebeam mask definition. Design and fabrication considerations that maximize efficiency are quantified, including material options, e-beam defined lithographic parameters such as grating periods and aspect ratios, tailored wavelength dispersion, and polarization independence. These results are compared to published efficiency values of >95% diffraction efficiency for a single polarization and single wavelength and polarization-independent efficiency values of >98% also for a single wavelength. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

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We explore the spectral and angular selectivity of near surface normal transmission of grating modified metallic surfaces and their ultimate potential for application as narrow-band spectro-polarimetric planar filter components in the development of advanced infrared focal plane arrays. The developed photonic microstructures exhibit tailored spectral transmission characteristics in the long wavelength infrared, and can be fabricated to preferentially transmit a given linear polarization within the design band. Modification of the material and structural properties of the diffractive optical element enables sub-pixel tuning of the spectro-polarimetric response of the device allowing for intelligent engineering of planar filter components for development of advanced focal plane arrays in the long wavelength infrared. The planar nature of the developed components leaves them immune to fabrication issues that typically plague thin film interference filters used for similar applications in the infrared, namely, deposition of multiple low-stress quarter-wavelength films and modification of the film thicknesses for each pixel. The solution developed here presents the opportunity for subpixel modification of the spectral response leading to an efficient, versatile filter component suitable for direct integration with commercially available focal plane array technologies via standard fabrication techniques. We will discuss the theoretical development and analysis of the described components and compare the results to the current state-of-the-art. © 2012 SPIE.

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Radiation hard nonvolatile random access memory (NVRAM) is a crucial component for DOE and DOD surveillance and defense applications. NVRAMs based upon ferroelectric materials (also known as FERAMs) are proven to work in radiation-rich environments and inherently require less power than many other NVRAM technologies. However, fabrication and integration challenges have led to state-of-the-art FERAMs still being fabricated using a 130nm process while competing phase-change memory (PRAM) has been demonstrated with a 20nm process. Use of block copolymer lithography is a promising approach to patterning at the sub-32nm scale, but is currently limited to self-assembly directly on Si or SiO{sub 2} layers. Successful integration of ferroelectrics with discrete and addressable features of {approx}15-20nm would represent a 100-fold improvement in areal memory density and would enable more highly integrated electronic devices required for systems advances. Towards this end, we have developed a technique that allows us to carry out block copolymer self-assembly directly on a huge variety of different materials and have investigated the fabrication, integration, and characterization of electroceramic materials - primarily focused on solution-derived ferroelectrics - with discrete features of {approx}20nm and below. Significant challenges remain before such techniques will be capable of fabricating fully integrated NVRAM devices, but the tools developed for this effort are already finding broader use. This report introduces the nanopatterned NVRAM device concept as a mechanism for motivating the subsequent studies, but the bulk of the document will focus on the platform and technology development.

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LDRD Project 105876 was a research project whose primary goal was to discover the currently unknown science underlying the basic linear and nonlinear electrodynamic response of nanotubes and nanowires in a manner that will support future efforts aimed at converting forefront nanoscience into innovative new high-frequency nanodevices. The project involved experimental and theoretical efforts to discover and understand high frequency (MHz through tens of GHz) electrodynamic response properties of nanomaterials, emphasizing nanowires of silicon, zinc oxide, and carbon nanotubes. While there is much research on DC electrical properties of nanowires, electrodynamic characteristics still represent a major new frontier in nanotechnology. We generated world-leading insight into how the low dimensionality of these nanomaterials yields sometimes desirable and sometimes problematic high-frequency properties that are outside standard model electron dynamics. In the cases of silicon nanowires and carbon nanotubes, evidence of strong disorder or glass-like charge dynamics was measured, indicating that these materials still suffer from serious inhomogeneities that limit there high frequency performance. Zinc oxide nanowires were found to obey conventional Drude dynamics. In all cases, a significant practical problem involving large impedance mismatch between the high intrinsic impedance of all nanowires and nanotubes and high-frequency test equipment had to be overcome.

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Resistivity and piezoelectric response of individual ZnO nanorods were measured using scanning force microscopy. We found a variation in resistivity of 3 orders of magnitude, from 0.1 to 155 Ωcm and in piezoelectric coefficient ranging from 0.4 to 9.5 pm/V in ZnO nanorods grown from solution at the same time on the same substrate. However, there exists a clear correlation between these two properties: nanorods with low piezoelectric response display low resistivity. The relationship is explained by the reduction of the Madelung constant due to free electrons. The results highlight that slight differences in the local environment during synthesis can cause large variation in physical properties found among similar nanostructures. These variations cannot be revealed through ensemble measurements and may contribute to the confusion in the literature of individual nanostructure properties. We demonstrate that correlating multiple physical properties on individual nanostructures provides an insight into the origin of the varying physical properties. © 2008 American Chemical Society.

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We present a combined experimental and modeling study of the dependence of solution-based zinc oxide (ZnO) selective-area growth rates on pattern dimension. Selective growth is achieved by patterning a portion of the substrate with an organic template that inhibits growth. The density of ZnO nanorods and the mass grown per unit area of exposed surface increases as the distance between the exposed growth regions is increased and as the width of the exposed lines is decreased. A 2-D model was developed to calculate selective growth at the exposed surface regions, the loss of reactant material due to a competing reaction in solution, liquid-phase and surface diffusive mass transport to (or on) the growth surface, and the ZnO growth reaction at the surface. To explain the experimental results, we found it necessary to include a reaction by-product in the chemistry model, the desorption of which is the rate limiting step. A relatively simple, three-step reaction mechanism, combined with the species mass transport model, provides a good, semi-quantitative description of the experimental observations in the selective-area growth of ZnO from supersaturated solutions.

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