This talk will introduce some of our recent work on using nanostructures to tailor thermal radiation with applications from solar and thermal energy for electrical generation and storage, to desalination. We fabricated solar photovolatic cells with efficiency >15% using 10 micron-thick crystalline silicon films. We demonstrated that aerogels can be used for concentrated solar thermal power, eliminating the need for vacuum and wavelength selective coatings. Photovoltaic cells can couple to terrestrial heat sources to convert thermal radiation into electricity, at an efficiency higher than photovoltaics. Moving to lower temperature range, we show that fabrics can be made to radiate out human body heat while remain opaque to visible light. We also demonstrate that by localizing solar energy on water surface, we can boil water and even achieve superheated steam under one sun. The talk will end with a discussion of the entropy of light and how we exploit the understanding to design better thermal-to-electrical energy converters.
We report on optical design and applications of hybrid meso-scale devices and materials that combine optical and thermal management functionalities owing to their tailored resonant interaction with light in visible and infrared frequency bands. We outline a general approach to designing such materials, and discuss two specific applications in detail. One example is a hybrid optical-thermal antenna with sub-wavelength light focusing, which simultaneously enables intensity enhancement at the operating wavelength in the visible and reduction of the operating temperature. The enhancement is achieved via light recycling in the form of whispering-gallery modes trapped in an optical microcavity, while cooling functionality is realized via a combination of reduced optical absorption and radiative cooling. The other example is a fabric that is opaque in the visible range yet highly transparent in the infrared, which allows the human body to efficiently shed energy in the form of thermal emission. Such fabrics can find numerous applications for personal thermal management and for buildings energy efficiency improvement.
KEYWORDS: Electrons, Gold, Solar energy, Photons, Metals, Energy efficiency, Semiconductors, Data conversion, P-type semiconductors, Energy conversion efficiency
We evaluate the limiting efficiency of full and partial solar spectrum harvesting via the process of internal photoemission in Au-semiconductor Schottky junctions. Our results based on the ab initio calculations of the electron density of states (e-DOS) reveal that the limiting efficiency of the full-spectrum Au converter based on hot electron injection is below 4%. This value is even lower than previously established limit based on the parabolic approximation of the Au electron energy bands. However, we predict limiting efficiency exceeding 10% for the hot holes collection through the Schottky junction between Au and p-type semiconductor. Furthermore, we demonstrate that such converters have more potential if used as a part of the hybrid system for harvesting high- and low-energy photons of the solar spectrum.
The Photon-Trap Structures for Quantum Advanced Detectors (PT-SQUAD) program requires MWIR
detectors at 200 K. One of the ambitious requirements is to obtain high (> 80 %) quantum efficiency over
the visible to MWIR spectral range while maintaining high D* (> 1.0 x 1011 cm √Hz/W) in the MWIR. A
prime method to accomplish the goals is by reducing dark diffusion current in the detector via reducing
the volume fill ratio (VFR) of the detector while optimizing absorption. Electromagnetic simulations
show that an innovative architecture using pyramids as photon trapping structures provide a photon
trapping mechanism by refractive-index-matching at the tapered air/semiconductor interface, thus
minimizing the reflection and maximizing absorption to > 90 % over the entire visible to MWIR spectral
range. InAsSb with bandgap appropriate to obtaining a cutoff wavelength ~ 4.3 μm is chosen as the
absorber layer. An added benefit of reducing VFR using pyramids is that no AR-coating is required.
Compound-barrier (CB) detector test structures with alloy composition of the InAsSb absorber layer
adjusted to achieve 200 K cutoff wavelength of 4.3 μm (InAsSb lattice-matched to GaSb). Dark current
density at 200 K is in the low 10-4 A/cm2 at Vd = -1.0 V. External QE ~ 0.65 has been measured for
detectors with a Si carrier wafer attached. Since illumination is through the Si carrier wafer that has a
reflectance of ~ 30 %, this results in an internal QE > 0.9.
High performance thermoelectric materials in a wide range of temperatures are essential to broaden the application
spectrum of thermoelectric devices. This paper presents experiments on the power and efficiency characteristics of lowand
mid-temperature thermoelectric materials. We show that as long as an appreciable temperature difference can be
created over a short thermoelectric leg, good power output can be achieved. For a mid-temperature n-type doped
skutterudite material an efficiency of over 11% at a temperature difference of 600 °C could be achieved. Besides the
improvement of thermoelectric materials, device optimization is a crucial factor for efficient heat-to-electric power
conversion and one of the key challenges is how to create a large temperature across a thermoelectric generator
especially in the case of a dilute incident heat flux. For the solar application of thermoelectrics we investigated the
concept of large thermal heat flux concentration to optimize the operating temperature for highest solar thermoelectric
generator efficiency. A solar-to-electric power conversion efficiency of ~5% could be demonstrated. Solar
thermoelectric generators with a large thermal concentration which minimizes the amount of thermoelectric
nanostrucutured bulk material shows great potential to enable cost-effective electrical power generation from the sun.
We examine light-trapping in thin crystalline silicon periodic nanostructures for solar cell applications. Using group
theory, we show that light-trapping can be improved over a broad band when structural mirror symmetry is broken. This
finding allows us to obtain surface nanostructures with an absorptance exceeding the Lambertian limit over a broad band
at normal incidence. Further, we demonstrate that the absorptance of nanorod arrays with symmetry breaking not only
exceeds the Lambertian limit over a range of spectrum but also closely follows the limit over the entire spectrum of
interest for isotropic incident radiation. These effects correspond to a reduction in silicon mass by two orders of
magnitude, pointing to the promising future of thin crystalline silicon solar cells.
Nanowires are interesting candidates for thermoelectric applications because of their potentially low thermal
conductivity and high power factor. However, measurements at the single-wire level are challenging and tend to lack
detailed information about the atomic-level structure of the sample and contacts. We are modifying a high-resolution
transmission electron microscope (HRTEM) with integrated scanning tunneling microscope (STM) for in-situ
measurements of the thermoelectric properties of individual nanowires and nanotubes. A slender hot-wire probe is used
to make electrical and thermal contact to the free end of a nanowire or nanotube. The electrical conductance of the
nanowire/nanotube can be measured with the usual STM mode of operation. The Seebeck coefficient can be extracted
from the transient response to a step change in the joule heating of the hot-wire probe. The thermal conductance can be
calculated from the temperature and heat leakage of the hot-wire probe. These measurements are combined with
detailed HRTEM observations.
The Reflection Grating Spectrometer (RGS) on Constellation-X will require thousands of large gratings with very exacting tolerances. Two types of grating geometries have been proposed. In-plane gratings have low ruling densities (~500 l/mm) and very tight flatness and assembly tolerances. Off-plane gratings require much higher ruling densities (~5000 l/mm), but have somewhat relaxed flatness and assembly tolerances and offer the potential of higher resolution and efficiency. The trade-offs between these designs are complex and are currently being studied. To help address critical issues of manufacturability we are developing a number of novel technologies for shaping, assembling, and patterning large-area reflection gratings that are amenable to low-cost manufacturing. In particular, we report results of improved methods for patterning the sawtooth grating lines that are required for efficient blazing, including the use of anisotropic etching of specially-cut silicon wafers to pattern atomically smooth grating facets. We also report on the results of using nanoimprint lithography as a potential means for replicating sawtooth grating masters. Our Nanoruler scanning beam interference lithography tool allows us to pattern large area gratings up to 300 mm in diameter. We also report on developments in grating assembly technology utilizing lithographically patterned and micromachined silicon metrology structures ("microcombs") that have achieved submicron assembly repeatability.
The proposed Reflection Grating Spectrometer (RGS) on the Constellation-X mission is designed to provide high-resolution x-ray spectroscopy of astrophysical sources. Two types of reflection grating geometries have been proposed for the RGS. In-plane gratings have relatively low-density rulings (~500 lines/mm) with lines perpendicular to the plane of incidence, thus dispersing x-rays into the plane. This geometry is similar to the reflection grating spectrometer flown on the X-ray Multi-Mirror (XMM) mission. Off-plane, or conical, gratings require much higher density rulings (>5000 lines/mm) with lines parallel to the plane of incidence, thus dispersing x-rays perpendicular to the plane. Both types present unique challenges and advantages and are under intensive development. In both cases, however, grating flatness and assembly tolerances are driven by the mission's high spectral resolution goals and the relatively poor resolution of the Wolter foil optics of the Spectroscopy X-ray Telescope (SXT) that is used in conjunction with the RGS. In general, to achieve high spectral resolution, both geometries require lightweight grating substrates with arcsecond flatness and assembly tolerances. This implies sub-micron accuracy and precision which go well beyond that achieved with previous foil optic systems. Here we present a progress report of technology development for the precision shaping, assembly and metrology of the thin, flat grating substrates.
We are developing a Scanning Beam Interference Lithography (SBIL) system. SBIL represents a new paradigm in semiconductor metrology, capable of patterning large-area linear gratings and grids with nanometer overall phase accuracy. Realizing our accuracy goal is a major challenge because the interference fringes have to be locked to a moving substrate with nanometer spatial phase errors while the period of the fringes has to be stabilized to approximately one part per million. In this paper, we present a review of the SBIL design, and report recent progress towards prototyping the first-ever SBIL tool.
KEYWORDS: Interferometers, Lithography, Beam splitters, Diffraction gratings, Digital signal processing, Mirrors, Optics manufacturing, Signal processing, Optical lithography, Electron beam lithography
The fabrication of large high-quality diffraction gratings remains one of the most challenging tasks in optical fabrication. Traditional direct-write methods, such as diamond ruling or electron-beam lithography, can be extremely slow and result in gratings with undesired phase errors. Holographic methods, while generally resulting in gratings with smoother phase, frequently require large aspheres and lengthy optical setup in order to achieve desired period chirps. In this paper we describe a novel interference lithography method called scanning-beam interference lithography (SBIL) that utilizes small phase-locked scanning beams to write general periodic patterns onto large substrates. Small mutually coherent beams are phase controlled by high-bandwidth electro-optic components and caused to overlap and interfere, generating a small grating image. The image is raster-scanned over the substrate by use of a high-precision interferometer-controlled air bearing stage, resulting in large grating patterns. We will describe a prototype system in our laboratory designed to write gratings with extremely low phase distortion. The system is being generalized to pattern gratings with arbitrary period progressions (chirps). This technology, with extensions, will allow the rapid, low cost patterning of high-fidelity periodic patterns of arbitrary geometry on large substrates that could be of great interest to astronomers.
We report on progress in developing low-cost methods for shaping thin-foil glass x-ray optics. Such optics might serve as substrates for reflection gratings or as foil mirrors in high-throughput missions such as Constellation-X. Novel thermal shaping to lithographically defined pin chucks leads to the desired shape with high accuracy, thereby avoiding the need for replication. To demonstrate this method we have produced 200 micron-thick glass sheets with sub-micron flatness and half power diameter below 10 arc seconds. We also present a process for depositing low-stress metallic coatings that provides high x-ray reflectivity without significant foil distortion.
Achieving arcsecond angular resolution in a grazing-incidence foil optic X-ray telescope, such as the segmented mirror approach being considered for the Constellation-X Spectroscopy X-Ray Telescope (SXT), requires accurate placement of individual foils. We have developed a method for mounting large numbers of nested, segmented foil optics with sub- micrometer accuracy using lithographically defined and etched silicon alignment micro-structures. A system of assembly tooling, incorporating the silicon micro-structures, is used to position the foils which are then bonded to a flight structure. The advantage of this procedure is that the flight structure has relaxed tolerance requirements while the high accuracy assembly tooling can be reused. A companion paper by Bergner et al. discusses how our process could be used for the SXT. We have built an assembly truss with a simplified rectilinear geometry designed to experimentally test this alignment and mounting technique. We report results of tests with this system that demonstrate its ability to provide sub- micrometer alignment of rigid test optics.
We describe recent progress toward producing a segmented mirror that meets the mass and angular resolution requirements for the Constellation-X Spectroscopy X-ray Telescope (SXT). While the segmented approach has its heritage in conical thin foil X-ray mirrors pioneered at GSFC, the Constellation-X implementation introduces innovations in nearly all components. The baseline configuration uses thermally formed glass for reflector substrates; thermally formed Be is being investigated as an option. Alignment is performed using etched Si microstructures that locate reflectors to submicron accuracy. The only aspect preserved from previous mirrors is epoxy replication of the X-ray reflecting surface. Thus far, all developments have been at the component level. Nonetheless, we have made substantial progress toward meeting the Constellation-X SXT angular resolution goal.
Segmented mirrors are one of the two approaches being investigated for both the Spectroscopy X-ray Telescope (SXT) and the Hard X-ray Telescope (HXT) on Constellation-X. Mirrors based on the grazing incidence foil optics pioneered by GSFC will meet the stringent Constellation-X SXT weight requirement, but the currently achieved resolution falls short of the 15 inch half-power diameter (HPD) required for Constellation-X. Significant contributions to the blur arise from the figure of individual reflectors and from inaccurate mounting. Only a small contribution to the HPD of the existing mirrors arises from the conical approximation. In this paper, we describe our program for improving the spatial resolution of segmented mirrors to meet the COnstellation-X requirement. Our effort incorporates accurately figured replication mandrels, mechanically more robust reflector substrates, high accuracy alignment, and ultimately a transition from conical to curved reflecting surfaces.
BNL has been developing a remote sensing technique for the detection of atmospheric pollutants based on the phenomenon of resonance Raman LIDAR that has also incorporated a number of new techniques/technologies designed to extend its performance envelope. When the excitation frequency approaches an allowed electronic transition of the molecule, an enormous enhancement of the inelastic scattering cross- section can occur, often up to 2 to 4 orders-of-magnitude, and is referred to as resonance Raman, since the excitation frequency is in 'resonance' with an allowed electronic transition. Exploitation of this enhancement along with new techniques such as pattern recognition algorithm to take advantage of the spectral fingerprint and a new laser frequency modulation technique designed to suppress broadband fluorescence, referred to as frequency modulated excitation Raman spectroscopy and recent developments in liquid edge filter technology, for suppression of the elastic channel, all help increase the overall performance of Raman LIDAR.
BNL has been developing a remote sensing technique for the detection of atmospheric pollutants using resonance Raman LIDAR that has also incorporated a number of new techniques/technologies designed to extend it performance envelope. Chief among these new techniques is the use of pattern recognition to take advantage of the spectral fingerprint and a new laser frequency modulation technique, referred to as Frequency Modulated Excitation Raman Spectroscopy, designed to suppress broadband fluorescence. In the laboratory, broadband fluorescence suppression approaching 3 orders-of-magnitude has been achieved. In addition, the application of a BNL designed knife-edge Rayleigh filter has also bee demonstrated using our LIDAR system where spectral features as close as 200 cm-1 from the excitation line were observed. How all these features help increase the overall performance of Raman LIDAR will be discussed.
We introduce an adaptive mixing algorithm for estimating the relative ratios of chemicals in a mixture spectrum. This procedure is particularly well suited to mixtures with a large dynamic range of mixture weights. It has the advantage of being able to be used in conjunction with a band-pass (difference-to-Gaussian or DOG) filter, and a correction of baseline off-set and tilting of the spectrum. Output of these filtering techniques is a cleaner signal retaining most of the relevant Raman spectral signature while minimizing artifacts due primarily to Rayleigh, dust, and atmospheric aerosols. We will describe the results of applying these algorithm to mixture spectra with both real and simulated additive noise.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.