Publications

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We demonstrate the effects of integrating a nanoantenna to a midwave infrared (MWIR) focal plane array (FPA). We model an antenna-coupled photodetector with a nanoantenna fabricated in close proximity to the active material of a photodetector. This proximity allows us to take advantage of the concentrated plasmonic fields of the nanoantenna. The role of the nanoantenna is to convert free-space plane waves into surface plasmons bound to a patterned metal surface. These plasmonic fields are concentrated in a small volume near the metal surface. Field concentration allows for a thinner layer of absorbing material to be used in the photodetector design and promises improvements in cutoff wavelength and dark current (higher operating temperature). While the nanoantenna concept may be applied to any active photodetector material, we chose to integrate the nanoantenna with an InAsSb photodiode. The geometry of the nanoantenna-coupled detector is optimized to give maximal carrier generation in the active region of the photodiode, and fabrication processes must be altered to accommodate the nanoantenna structure. The intensity profiles and the carrier generation rates in the photodetector active layers are determined by finite element method simulations, and iteration between optical nanoantenna simulation and detector modeling is used to optimize the device structure. © 2012 SPIE.

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This report describes a passive, optical component called resonant subwavelength gratings (RSGs), which can be employed as one element in an RSG array. An RSG functions as an extremely narrow wavelength and angular band reflector, or mode selector. Theoretical studies predict that the infinite, laterally-extended RSG can reflect 100% of the resonant light while transmitting the balance of the other wavelengths. Experimental realization of these remarkable predictions has been impacted primarily by fabrication challenges. Even so, we will present large area (1.0mm) RSG reflectivity as high as 100.2%, normalized to deposited gold. Broad use of the RSG will only truly occur in an accessible micro-optical system. This program at Sandia is a normal incidence array configuration of RSGs where each array element resonates with a distinct wavelength to act as a dense array of wavelength- and mode-selective reflectors. Because of the array configuration, RSGs can be matched to an array of pixels, detectors, or chemical/biological cells for integrated optical sensing. Micro-optical system considerations impact the ideal, large area RSG performance by requiring finite extent devices and robust materials for the appropriate wavelength. Theoretical predictions and experimental measurements are presented that demonstrate the component response as a function of decreasing RSG aperture dimension and off-normal input angular incidence.

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This project combined nanocomposite materials with microfabricated optical device structures for the development of microsensor arrays. For the nanocomposite materials we have designed, developed, and characterized self-assembling, organic/inorganic hybrid optical sensor materials that offer highly selective, sensitive, and reversible sensing capability with unique hierarchical nanoarchitecture. Lipid bilayers and micellar polydiacetylene provided selective optical response towards metal ions (Pb(II), Hg(II)), a lectin protein (Concanavalin A), temperature, and organic solvent vapor. These materials formed as composites in silica sol-gels to impart physical protection of the self-assembled structures, provide a means for thin film surface coatings, and allow facile transport of analytes. The microoptical devices were designed and prepared with two- and four-level diffraction gratings coupled with conformal gold coatings on fused silica. The structure created a number of light reflections that illuminated multiple spots along the silica surface. These points of illumination would act as the excitation light for the fluorescence response of the sensor materials. Finally, we demonstrate an integrated device using the two-level diffraction grating coupled with the polydiacetylene/silica material.

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The authors describe the design and microfabrication of an extremely compact optical system as a key element in an integrated capillary-channel electrochromatograph with laser induced fluorescence detection. The optical design uses substrate-mode propagation within the fused silica substrate. The optical system includes a vertical cavity surface-emitting laser (VCSEL) array, two high performance microlenses and a commercial photodetector. The microlenses are multilevel diffractive optics patterned by electron beam lithography and etched by reactive ion etching in fused silica. Two generations of optical subsystems are described. The first generation design is integrated directly onto the capillary channel-containing substrate with a 6 mm separation between the VCSEL and photodetector. The second generation design separates the optical system onto its own module and the source to detector length is further compressed to 3.5 mm. The systems are designed for indirect fluorescence detection using infrared dyes. The first generation design has been tested with a 750 nm VCSEL exciting a 10{sup -4} M solution of CY-7 dye. The observed signal-to-noise ratio of better than 100:1 demonstrates that the background signal from scattered pump light is low despite the compact size of the optical system and meets the system sensitivity requirements.