We report experimental and theoretical/simulation results from 1-D and 2-D photonic crystals of novel biomaterials that are synthesized in-house at DEVCOM SC. Inspired by these biomaterials and related phenomena, we experiment with sensing analyte molecules for chem/bio detection. Because these biomaterials have high indices of refraction, they confine photons tightly. Optical properties are predicted computationally based on experimental measurements of indices of refraction using variable-angle continuous spectra ellipsometers, with both unfocused and focused probing spots. This biomaterial, when combined with polymers that enable smooth films (polyvinyl acetate, ethyl cellulose, etc.), would be a natural, environmentally friendly, non-toxic, and toxicologically safe material appropriate for scaling up for large-area optical sensing of molecules, especially toxic industrial molecules. We carry out initial research on the detection of analyte molecules in solution via optical methods and compare to simulations. We contrast with inorganic materials for remote sensing, reconnaissance, UAVs, etc. and compare challenges in scalable fabrication including synthetic biology.
We report results from thin films of novel biomaterials based on natural minerals, never before synthesized in the laboratory using primarily non-toxic and environmentally friendly materials and characterized optically. These biomaterial films have high indices of refraction and would be a natural and toxicologically safe material to use for large-area optical sensing of molecules, including toxic industrial molecules. Adhering modest concentrations of molecules in solution (water/humidity, ethanol, glucose, ammonia, etc.) to the surface of a Fabry-Perot cavity is shown experimentally to sufficiently alter the index of refraction and thickness of the Fabry-Perot films to enable detection of the molecules via optical methods (reflectance, ellipsometry, transmission, etc.). We report laboratory sensing of 3 types of molecules in solution with controlled high-quality Fabry-Perot cavities. We discuss different and better biominerals to use and discern potential applications.
Laser-induced shock waves have been gaining attention for biological and medical applications in which shock waves influence cell permeation. However, the mechanisms of permeation remain mostly unclear because of the difficulty of observing the transient and dynamic behaviors of the shock waves and the cells. Here we present an all-optical measurement method that can quantitatively capture the pressure distribution of the propagating shock wave and simultaneously monitor the dynamic behavior of cell membranes. Using this method, we find that a sharp pressure gradient causes cell membrane permeation. Our measurement will further advance biological and medical applications of shock waves.
Lightweight, portable solar blankets, constructed from thin film photovoltaics, are of great interest to
hikers, the military, first responders, and third-world countries lacking infrastructure for transporting
heavy, brittle solar cells. These solar blankets, as large as two square meters in area, come close to
satisfying specifications for commercial and military use, but they still have limited absorption due to
insufficient material efficiency, and therefore are large and too heavy in many cases.
Metasurfaces, consisting of monolayers of periodic and semi-random plasmonic particles patterned in
a scalable manner, are explored to enhance scattering into thin photovoltaic films (currently of
significant commercial and military value), in order to enhance absorption and efficiency of solar
blankets. Without nano-enhancement, absorption is limited by the thickness of the thin photovoltaic
active layer in the long-wavelength region. In this study, lithographically patterned, periodic Al
nanostructure arrays demonstrate experimentally a large absorption enhancement, resulting in a
predicted increase in short-circuit current density of at least 35% and as much as 70% for optimized
arrays atop 200-nm amorphous silicon thin films. Optimized arrays extend thin-film absorption to the
near infrared region. This impressive absorption enhancement and predicted increase in short-circuit
current density may significantly increase the efficiency and reduce the weight of solar blankets,
enabling their use for commercial and military applications.
Nanoparticles and nanostructures with plasmonic resonances are currently being employed to enhance the efficiency of solar cells. 1-3 Ag stripe arrays have been shown theoretically to enhance the short-circuit current of thin silicon layers. 4 Monolayers of Ag nanoparticles with diameter d < 300 nm have shown strong plasmonic resonances when coated in thin polymer layers with thicknesses < d.5 We study experimentally the diffuse vs. specular scattering from monolayer arrays of Ag nanoparticles (spheres and prisms with diameters in the range 50 – 300 nm) coated onto the front side of thin (100 nm < t < 500 nm) silicon films deposited on glass and flexible polymer substrates, the latter originating in a roll-to-roll manufacturing process. Ag nanoparticles are held in place and aggregation is prevented with a polymer overcoat. We observe interesting wavelength shifts between maxima in specular and diffuse scattering that depend on particle size and shape, indicating that the nanoparticles substantially modify the scattering into the thin silicon film.
Nanoparticles and nanostructures with plasmonic resonances are currently being employed to
enhance the efficiency of solar cells. Ag stripe arrays have been shown theoretically to enhance the
short-circuit current of thin silicon layers. Such Ag stripes are combined with 200 nm long and 60
nm wide “teeth”, which act as nanoantennas, and form vertical rectifying metal-insulator-metal
(MIM) nanostructures on metallic substrates coated with thin oxides, such as Nb/NbOx films. We
characterize experimentally and theoretically the visible and near-infrared spectra of these “stripeteeth”
arrays, which act as microantenna arrays for energy harvesting and detection, on silicon
substrates. Modeling the stripe-teeth arrays predicts a substantial net a.c. voltage across the MIM
diode, even when the stripe-teeth microrectenna arrays are illuminated at normal incidence.
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.
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