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.
Unlike a semiconductor, where the absorption is limited by the band gap, a “microrectenna array” could theoretically very efficiently rectify any desired portion of the infrared frequency spectrum (25 - 400 THz). We investigated vertical metal-insulator-metal (MIM) diodes that rectify vertical high-frequency fields produced by a metamaterial planar stripe-teeth Al or Au array (above the diodes), similar to stripe arrays that have demonstrated near-perfect absorption in the infrared due to critical coupling . Using our design rules that maximize asymmetry (and therefore the component of the electric field pointed into the substrate, analogous to Second Harmonic Generation), we designed, fabricated, and analyzed these metamaterial-based microrectenna arrays. NbOx and Al2O3 were produced by anodization and ALD, respectively. Smaller visible-light Pt-NbOx-Nb rectennas have produced output power when illuminated by visible (514 nm) light .
The resonances of these new Au/NbOx/Nb and Al/Al2O3/Al microrectenna arrays, with larger dimensions and more complex nanostructures than in Ref. 1, were characterized by microscopic FTIR microscopy and agreed well with FDTD models, once the experimental refractive index values were entered into the model. Current-voltage measurements were carried out, showed that the Al/Al2O3/Al diodes have very large barrier heights and breakdown voltages, and were compared to our model of the MIM diode. We calculate expected THz-rectification using classical  and quantum  rectification models, and compare to measurements of direct current output, under infrared illumination.
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 R. M. Osgood III, et. al., Proc. SPIE 8096, 809610 (2011).
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 J. R. Tucker and M. J. Feldman, Rev. of Mod. Phys. 57, (1985)1055.
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.
Arrays of "nanorectennas" consist of diode-coupled nanoantennas with plasmonic resonances in the visible/near-infrared
(vis/nir) regime, and are expected to convert vis/nir radiative power into useful direct current. We study plasmonic
resonances in large format (~ 1 mm2 area) arrays, consisting of electron beam-patterned horizontal (e.g., parallel to the substrate) Ag lines patterned on ultrathin (< 20 nm) tunneling barriers (NiO, NbOx, and other oxides). Our e-beam fabrication technique is scalable to large dimensions, and allows us to easily probe different antenna dimensions. These
tunneling barriers, located on a metallic ground plane, rectify the alternating current generated in the nanoantenna at
resonance. We measure the plasmonic resonances in these nanoantennas, and find good agreement with modeling,
which also predicts that the electric field driving the electrons into the ground plane (and therefore the rectification
efficiency) is considerably enhanced at resonance. Various metal-insulator-metal tunneling diodes, incorporating the
afore-mentioned barrier layers and different metals for the ground plane, are experimentally characterized and compared
to our conduction model. We observe ~ 1 mV signals from NiO-based nanorectenna arrays illuminated by 532 nm and
1064 nm laser pulses, and discuss the origin of these signals.
Arrays of "nanorectennas", consisting of nanodiode-coupled nanoantennas, are of interest for converting
visible/near-infrared (vis/nir) light into useful direct current. For efficient energy conversion, the
nanoantenna array must have a high absorbance (for different polarizations and angles of incidence) and a
large fill factor; i.e., the nanoantennas must be tightly packed together. We fabricate hexagonal, close-packed
(~ 100 nm nearest neighbor separation), large area (~ 1 cm2) arrays of vertical (e.g., perpendicular
to the substrate) Au nanowires (length < 1 μm) on Si, by electrochemically depositing gold into a porous
aluminum oxide template (a potentially inexpensive process scalable to large dimensions). Coupling of
these nanowires causes a considerable blue-shift of the plasmonic resonance of a single Au nanowire when
illuminated by p-polarized light from the infrared to the blue-green portion of the visible spectrum (similar
to the s polarization resonance), enabling a nanorectenna with tuned response in the vis/nir regime, whose
absorption is roughly polarization-independent and relatively insensitive to angle of incidence. We measure
the off-normal reflectivity of these arrays, compare with simulations, and present experimental data on
rectification and power generation in the attached Au-Si Schottky nanodiodes.
The optical switching times of liquid-crystal cells using 5CB, 5OCB and PCH5 liquid crystal materials have been
characterized as a function of applied voltage, V, and temperature, T. The transition time from 90 to 10 % transmission
scales as V-2 and is limited to 50 to 30 ns by the breakdown electric field, ~ 106 V cm-1 of the liquid crystal. The time
from the initial voltage step to 90 % transmission, delay time, decreases with increasing voltage and approaches a
constant value at higher electric fields, >105 V cm-1. Both the transition and delay times decrease with increasing
temperature. The minimum transition times at temperatures a few degrees below the nematic-isotropic temperature are
32, 32, and 44 ns and delay times are 44, 25 and 8 ns for 5CB, 5OCB, and PCH5 respectively.
There is renewed interest in using rectennas (consisting of an antenna coupled to a rectifying diode) for energy conversion applications. Progress in nanofabrication has enabled nanoscale devices to be fabricated, such that "nanoantennas" exist that resonate at visible/near-infrared (vis/nir) wavelengths, and ultrafast "nanodiodes" exist that can rectify vis/nir frequencies (above 1014 Hz). Photon energies are so high at these frequencies that existing theories of diode responsivity may not apply, justifying new simulations and experiments. We present modeling and experiments of nanoantenna-coupled nanodiodes, such as metal-insulator-metal structures, and discuss how our findings influence models of energy conversion in these structures. We simulate and measure the properties of potential nanorectennas such as gold nanowires on ultrathin insulators.
Frequency Selective Surfaces (FSS) are comprised of periodic, geometric, metallic patterns that act like an array of horizontal antennas. They were originally designed as band-pass/band-block filters. Nanofabrication techniques allow for the realization of FSS structures that operate in the near infrared (NIR) and visible portions of the electromagnetic spectrum. Thus it is possible to create arrays of light antenna filters possessing optical properties that are unlike those of dye, dielectric, or holographic filters that are in common use today. Recent studies of arrays of gold, dipole
nanoantennas by our group and others offer an opportunity to compare modeled FSS response with experimental results elucidating the unique, off-normal reflectance stability of frequency selective surfaces operating in the NIR/visible portion of the spectrum.
Carbon nanotubes have been shown to exhibit light antenna behavior, such as polarization and length dependence,
and enhancement of incident electromagnetic radiation at resonance. We study and model resonance effects from
planar metallic nanoantennas, as a function of nanoantenna dimensions and material properties. We discuss the
challenges of designing a two-dimensional nanoantenna array with resonance in the short wavelength (blue-green)
region of the visible spectrum, constructed from different materials and in different environments.
Nanoantennas, coupled to rectifying nanodiodes ("rectennas"), could be used for converting broadband
visible/near-infrared energy to direct current, and could serve as fast, high-Q infrared detectors at designed
wavelengths. We study and model the efficiency of antennas coupled to metal-insulator-metal (MIM) and
thermionic emission diodes, over a wide range of incident wavelengths. We find that tuning the antenna's
reactance, so that the antenna acts as an inductor and resonantly cancels the diode capacitance, can enhance energy conversion efficiency by more than an order of magnitude above the broadband level, at the resonance frequency. We discuss maximizing the efficiency of a modern rectenna-based broadband energy conversion system, especially in the challenging visible regime, and recommend using nanodiodes with conduction via thermionic emission. We recommend further modeling of and experiments with nanoantennas, in order to calculate total efficiency of the nanorectenna's energy conversion.