Electrodynamic properties of fluorine-doped tin oxide films grown by aqueous-spray-based heterogeneous reaction on heated hydrophilic substrates were investigated with emphasis on applications to infrared plasmonics. These properties were correlated with physical ones such as crystallinity, dopant and electron concentrations, conductivity, and mobility. The degree of crystallinity for the nanocrystalline films increases with F concentration and growth temperature. The F concentration in the films is proportional to that in the starting solution. Electron concentration and Hall mobility rise more slowly with F concentration. At their highest, both F and electron concentrations are ∼2% of the Sn concentration. In more lightly doped films, the electron concentration significantly exceeds the F concentration. The achieved resistivity of the doped films is lower than for undoped SnO2 film by 20 to 750 times. The infrared complex permittivity spectrum shows a shift in plasma wavelength from 15 to 2 μm with more than two orders increase in F concentration.
In recent years, infrared plasmonics has turned towards materials that are wavelength and application tailorable, and which are geared towards CMOS processing. The transparent conductive oxides are very favorable towards infrared plasmonic applications for a number of reasons, one of which being the natural visible transparency due to their relatively large bandgap. Fluorine-doped tin oxide (FTO) is one such transparent and doping-tunable material that in addition is low cost due to spray deposition techniques that result in perfectly conformal coatings. In this work, a deposition recipe that gives high free carrier concentration was used to fabricate structures for demonstration of surface plasmon excitation. 1D gratings with a range of structural parameters were etched in silicon. Then the gratings were conformally coated with FTO by aqueous spray deposition. Excitation of surface plasmon polaritons (SPP) at mid- and long- wave infrared wavelengths on these gratings was demonstrated. The observed (SPP) excitation resonances agree will with analytical excitation calculations and numerical simulations. We show that grating heights of ~10-15% of the wavelength are optimum for achieving the strongest sharpest coupling to plasmonic resonances in the mid- and longwave infrared. The presented results are compared with similar etched silicon gratings coated with Ga-doped ZnO (GZO). The dominant difference between our FTO and GZO measurements is the free carrier concentration. The useful wavelength range is predicted for FTO based plasmonics and compared with other plasmonic host materials. The work presented here could play a key role in novel decreased-cost detectors, filters, and on-chip optoelectronics.
An electronic detector of surface plasmon polaritons (SPP) is reported. SPPs optically excited on a metal surface using a prism coupler are detected by using a close-coupled metal-oxide-semiconductor capacitor. Semitransparent metal and graphene gates function similarly. We report the dependence of the photoresponse on substrate carrier type, carrier concentration, and back-contact biasing.
Resonantly absorbing thin films comprising periodically sub-wavelength structured metal surface, dielectric spacer, and metal ground plane are a topic of current interest with important applications. These structures are frequently described as “metamaterials”, where effective permittivity and permeability with dispersion near electric and magnetic resonances allow impedance matching to free space for maximum absorption. In this paper, we compare synchrotron-based infrared spectral microscopy of a single isolated unit cell and a periodic array, and we show that the resonances have little to do with periodicity. Instead, the observed absorption spectra of usual periodically structured thin films are best described as due to standing-wave resonances within each independent unit cell, rather than as due to effective optical constants of a metamaterial. The effect of having arrays of unit cells is mainly to strengthen the absorption by increasing the fill factor, and such arrays need not be periodic. Initial work toward applying the subject absorbers to room-temperature bolometer arrays is presented.
Optical constants for evaporated bismuth (Bi) films were measured by ellipsometry and compared with those published for single crystal and melt-cast polycrystalline Bi in the wavelength range of 1 to 40 μm. The bulk plasma frequency ωp and high-frequency limit to the permittivity ε∞ were determined from the long-wave portion of the permittivity spectrum, taking previously published values for the relaxation time τ and effective mass m . This part of the complex permittivity spectrum was confirmed by comparing calculated and measured reflectivity spectra in the far-infrared. Properties of surface polaritons (SPs) in the long-wave infrared were calculated to evaluate the potential of Bi for applications in infrared plasmonics. Measured excitation resonances for SPs on Bi lamellar gratings agree well with calculated resonance spectra based on grating geometry and complex permittivity.
Streaming Process for Electrode-less Electrochemical Deposition (SPEED) method is used to create complex thin-film structures, such as KBNNO, in a single step, in contrast to hydrothermal approaches with separate nanoparticle growth and deposition processes. This new ferroelectric oxide [KNbO<sub>3</sub>]<sub>1-x</sub>[BaNi<sub>1/2</sub>Nb<sub>1/2</sub>O <sub>3-δ</sub>]<sub>x</sub> or “KBNNO” has an alloy-tunable band gap as low as 1.1 eV, so that its absorption can be tailored to match the solar spectrum. At the same time, it has a reasonably large polarization allowing for charge separation across the bulk, sizeable photocurrents, and open-circuit voltages V<sub>oc</sub> that exceed the band gap, potentially leading to efficiencies that exceed those possible for standard pnjunction cells. Physical characterization of KBNNO films demonstrate the microstructure and stoichiometry of SPEEDproduced thin-films, ratio of elements needed to achieve an ideal band gap of ~1.39 eV, the effect on film chemistry, microstructure, and band gap of annealing, the practical separation of excited carriers at room temperature, the maximum achievable polarization and its temperature dependence, and the conditions for ideal poling. Photovoltaic characterization of KBNNO cells will determine the efficiency, the relative strengths of dark and photo currents, the open circuit voltage, the short circuit current, and cell fill factor (FF).
The convergence of silicon photonics and infrared plasmonics allows compact, chip-scale spectral sensors. We report on
the development of a compact mid-IR spectrometer based on a broad-band IR source, dielectric waveguides, a
transformer to convert between waveguide modes and surface plasmon polaritons (SPP), an interaction region where
analyte molecules are interrogated by SPPs, an array of ring resonators to disperse the light into spectral components,
and photodetectors. The mid-IR light source emits into a dielectric waveguide, leading to a region that allows coupling
of the incident photons into SPPs. The SPPs propagate along a functionalized metal surface within an interaction region.
Interactions between the propagating SPP and any analytes bound to the surface increase loss at those wavelengths that
correspond to the analyte vibrational modes. After a suitable propagation length the SPP will be coupled back into a
dielectric waveguide, where specific wavelength components will be out-coupled to detectors by an array of ring
resonators. We have selected a 3.4 micron LED as the IR source, based on both cost and performance. Initial
experiments with circular waveguides formed from GLSO glass include measurement of the loss per mm.
Electrodynamic simulations have been performed to inform the eventual Si taper design of the proposed
photonic/plasmonic transformer. The SPP propagation length necessary for a discernible change in the signal due to
absorption in the interaction region has been estimated to be on the order of 1 mm, well within the bounds of calculated
propagation lengths for SPPs on Au.
Coatings of conducting gold-black nano-structures on commercial thin-film amorphous-silicon solar cells enhance the
short-circuit current by 20% over a broad spectrum from 400 to 800 nm wavelength. The efficiency, i.e. the ratio of the
maximum electrical output power to the incident solar power, is found to increase 7% for initial un-optimized coatings.
Metal blacks are produced cheaply and quickly in a low-vacuum process requiring no lithographic patterning. The
inherently broad particle-size distribution is responsible for the broad spectrum enhancement in comparison to what has
been reported for mono-disperse lithographically deposited or self-assembled metal nano-particles. Photoemission
electron microscopy reveals the spatial-spectral distribution of hot-spots for plasmon resonances, where scattering of
normally-incident solar flux into the plane increases the effective optical path in the thin film to enhance light harvesting.
Efficiency enhancement is correlated with percent coverage and particle size distribution, which are determined from
histogram and wavelet analysis of scanning electron microscopy images. Electrodynamic simulations reveal how the
gold-black particles scatter the radiation and locally enhance the field strength.
Small metal particles are investigated as scattering centers to increase the effective optical thickness of
thin-film solar cells. The particular type of particles used is known as "metal-black", well known as an IR
absorber for bolometric infrared detectors. Gold-black was deposited on commercial thin-film solar cells
using a thermal evaporator in nitrogen ambient at pressures of ~1 Torr. A broad range of length scales, as
revealed by scanning electron microscope images gives rise to effective scattering over a range of
wavelengths across the solar spectrum. The solar cell efficiency was determined both as a function of
wavelength and for a solar spectrum produced by a Xe lamp and appropriate filters. Up to 20% increase in
short-circuit photo-current, and a 5% increase in efficiency at the maximum power point, were observed.