Supporting the Scientific Base for Competencies Essential to Sandia Missions
The DOE Office of Science (SC) is the single largest supporter of basic research in the physical sciences in the U.S., providing more than 40 percent of total funding in this area. Sandia has active research programs funded by:
Advances environmental and biomedical knowledge that promotes national security through improved energy production, development, and use; international scientific leadership that underpins the nation’s technological advances; and research that improves the quality of life for all Americans.
Supports world-class, high-performance computing and networking infrastructures as well as supporting fundamental research in mathematical and computational sciences to enable researchers in DOE scientific disciplines to analyze and predict complex phenomena for scientific discovery.
ARPA-E is an innovative and collaborative government agency that brings together America’s best and brightest scientists, engineers, and entrepreneurs.
The focus of Sandia’s ARPA-E program is to establish partnerships with universities, industry and other National Labs to create innovative energy solutions for the Nation through both maturation of industry capabilities and commercialization of our technologies.
Address Stationary and Transportation Energy pillars
Leverage differentiating facilities/capabilities and Research Foundations of Sandia Labs
Akin to constant concentration biological growth, a new ‘Extended LaMer’ method for reproducible and predictable synthesis of nanoparticles was developed. Significance and Impact
This general approach allows systematic production of precise, monodisperse nanoparticles of any size in a scale-independent approach. Applications include quantum dots, metal nanoparticles, and magnetic particles, which all display size-dependent properties. Research Details
Using conventional reaction conditions, but in bio-inspired steady-state growth conditions (constant temperature, constant concentration) reproducible, constant particle growth occurs without ripening.
This is the only method known that allows systematic variation of size and size-dependent nanoparticle properties in a continuous, predictable manner.
Biology produces nanoparticles with exquisite control of size and crystallographic properties by growing crystals in controlled environments where critical parameters are held constant using the complex machinery of life. In contrast, the well-known methods of nanoparticle synthesis use wildly varying temperature (heating-up method) and concentration (hot-injection method) to produce nanoparticles. While these methods can yield highly crystalline, low size dispersity particles, reproducibility is challenging, and systematic size control is nearly impossible. These reactions are typically described by the “LaMer Mechanism” of growth where dramatic changes in concentration of the reactive species leads to extremely complex kinetics that are difficult to predict. We have developed a general methodology to produce nanoparticles that uses a biologically inspired approach, but conventional equipment and reaction conditions. By maintaining temperature constant throughout the reaction, and using a continuous addition of precursor to maintain reagent concentration constant (after a brief initial state of flux) we have demonstrated a dramatic improvement over conventional approaches. During the steady state regime (with constant temperature and concentration) of the “Extended LaMer” mechanism, particle growth is constant in time so particles may be grown to any size desired, in a way that is predictable by linear extrapolation. This approach is inherently scalable, as heat and mass-transport issues are minimized in a system without variations in temperature and concentration. This is a critical step to enable uniform, monodisperse nanoparticle scale-up for a range of electronic, magnetic, optical and thermal applications. Current applications being explored are the production of precisely tailored nanoparticles for biomedical imaging (on the multi-gram scale), as well as magnetic nanoparticles for high frequency, low loss inductors (on the multi-kg scale).
The SH-generating quantum well layer (pink) and the beam-shaping split-ring metamaterial resonators (the “C”-shaped structures). The phase-coherent emission from individual resonators opens the door for many beam-shaping applications, some of which are presented in the insets. Each device type can be achieved via proper design of individual metamaterial resonators and their arrangement on the surface.
Plasmonic phased-array sources* have been the subject of much interest recently. They can perform a many useful optical-beam manipulations such as beam steering, beam splitting, polarization manipulation, and angular momentum generation (see illustration). Usually, these sources operate through simple linear scattering of an incident laser beam. In a paper in Nature Communications, our research team† demonstrates a new, nonlinear phased-array source at infrared frequencies that uses nanocavities coupled to highly nonlinear semiconductor heterostructures‡.
This is the first time that metamaterial nanocavities coupled to semiconductors have been used to generate light at new wavelengths and manipulate the resulting beams. The electric field of the incident fundamental beam drives a nonlinear polarization in the vicinity of the gap of the split-ring resonator. The nonlinear polarization then acts as a source that “feeds” the resonators at the second harmonic (SH) frequency.
Our team’s phased-array source concept is a single active layer (call it a ‘metasurface’) complemented with a semiconductor heterostructure. Each split-ring nanocavity resonator on the metasurface can act as a point source for a higher-order nanocavity emission. Each nanocavity resonator can be individually tailored with a specific optical phase: to manipulate angular momentum, polarization, spin, etc. A benefit of this approach is that the desired output beam is at a different frequency (wavelength) than the pump—and residual unscattered pump radiation can easily be removed, something that is not possible with simple, linear phased arrays.
* Phased antenna arrays comprise ensembles of subwavelength sources, each radiating with a definite phase relationship relative to the other array elements.
† Omri Wolf, Salvatore Campione, Sheng Liu, and Igal Brener (all in Sandia’s Applied Photonic Microsystems Dept.); Ting Luk (in Sandia’s Nanostructure Physics Dept.); Emil Kadlec and Eric Shaner (both in in Sandia’s Laser, Optics & Remote Sensing Dept.); John Klem (in Sandia’s RF/Optoelectronics Dept.); Michael Sinclair (in Sandia’s Electronic, Optical, & Nano Materials Dept.); Alexander Benz (no longer at Sandia); and Arvind Ravikumar (Electrical Engineering Dept., Princeton Univ.).
‡ A semiconductor heterostructure is a stack of very thin (a few nanometers) semiconducting layers having different properties (in this case, bandgaps and doping). When properly designed, these layers can exhibit quantum phenomena due to electrons being confined to specific layers; this subclass of heterostructures is known as quantum wells (QWs) due to the shape of the electronic potential. QWs can be designed to have a multitude of interesting properties. In this work, we design them to support very large second-order optical nonlinearities.
This technology delivers second harmonic generation in a device that is extremely thin, which could lead to a whole new line of very compact infrared sensors and devices such as tunable filters, lenses, polarization devices, beam splitters/steerers, angular-momentum generators, detectors, and/or modulators (and also to advancements in quantum information science and quantum computing). As an example, our team demonstrated two-second harmonic phased-array sources that perform two optical functions at the second harmonic wavelength (~5 µm): a beam splitter and a polarizing beam splitter. Our metamaterial nanocavities coupled to highly nonlinear semiconductor heterostructures enhance second harmonic generation by orders of magnitude. Arrays of these coupled systems act like a phased array second harmonic source. Proper design of the nanocavity and nonlinear heterostructure will enable such phased arrays to span most of the infrared spectrum.
Our approach to phased-array sources at mid-infrared wavelengths extends our team’s past work on metamaterial nanocavities coupled to semiconductors by now including the optical nonlinearities of the quantum well (QW) to create a phase-locked, localized feed to resonators in the array. We also showed that we could manipulate the phase of the radiated second harmonic by changing the design of the metamaterial nanocavity. By spatially varying the nanocavity shape/orientation in the array, we are able to spatially vary the phase and manipulate the second harmonic beam.
We fabricated doubly resonant metamaterial arrays on top of a semiconductor heterostructure designed to have a large second-order nonlinearity (arising from intersubband transitions
[ISTs] in the QW).
Through measurements of the second harmonic radiation far-field pattern, we proved that the metamaterial resonators were emitting in a phase-coherent manner.
We varied the phase across the array at the second harmonic wavelength to create new functionality.
Metamaterial nanocavities were grown on top of an indium-gallium-arsenide/aluminum-indium-gallium-arsenide QW stack. The separations of subbands 1 → 2 and 2 → 3 are resonant with the fundamental beam, thereby causing the 3 → 1 separation to be resonant with the second harmonic.
Exploiting the phase coherence of the second harmonic radiation, we designed and fabricated arrays with multiple resonators per unit cell. By adjusting the relative phase of the resonators within the unit cell we were able to experimentally demonstrate a polarization beam splitter combine with a source. For one polarization, a single output beam is generated at the second harmonic frequency in the broadside direction; and for an orthogonal polarization, two output lobes are produced at a predetermined angle.
The nanocavities were designed to be resonant at both the fundamental and second harmonic frequencies. The second harmonic polarization depends quadratically on the fundamental E-field (left), so that it becomes symmetric with respect to the gap and can couple to the second harmonic resonance (right).
Our preliminary results show that this design can be readily transferred to different wavelengths by changing the QW materials (e.g., III-nitrides have ISTs at near-infrared frequencies while most III–V heterostructures support ISTs in the terahertz range) and the nanocavity design.
Although our example focuses on second harmonic generation, the new concept is general and can be applied to other types of nonlinear frequency generation. For example, large resonant third-order nonlinear susceptibilities have also been demonstrated in QWs and designing a triply resonant cavity is, in principle, achievable. We expect that our structure will serve as a model system for studying resonant nonlinearities in strongly coupled systems.
A Sandia-led research team† has, for the first time, observed the coherent propagation of thermal phonons‡ in silicon at room temperature—in two-dimensional phononic crystals formed by introducing air holes in a silicon matrix with minimum feature sizes ~250 nm. To separate incoherent from coherent boundary scattering, the team fabricated phononic crystals with a fixed minimum feature size—differing only in unit cell geometry (see Figure 1). This research was presented in a paper in Nature Communications.
Figure 1. Scanning electron microscope image of the fabricated phononic crystal structures. All samples were fabricated to have a periodicity a = 1100 nm, thickness t = 366 nm and a chosen critical dimension Lc = 250 nm. Highlighted in white is the unit cell of each supercell lattice: (a) simple cubic, SC, (b) 1 × 1, (c) 2 × 2, (d) 3 × 3 and (e) 4 × 4.
Almost all physical processes produce heat as a byproduct, making heat one of the most abundant forms of energy. In semiconductors, this thermal energy is carried by quasiparticles called phonons, which are quantized molecular vibrations. Thermoelectric systems are among the few technologies that can convert heat directly into electricity, using the Peltier effect. While silicon-based semiconductors typically have very favorable power factors, making them attractive for chip cooling and ‘heat scavenging’ applications, their large phonon-dominated thermal conductivity has prevented their use in thermoelectric systems. At low temperatures (70 K and below), phonons behave like waves, undergoing constructive and destructive interference (and allowing one to develop methods of harnessing this behavior to harvest energy). However, around room temperature and higher, phonons were believed to always behave more like particles, undergoing purely incoherent scattering. This means that they would propagate from hot to cold randomly, with no hope of controlling their flow.
† Ihab El-Kady, Charles Reinke, and Zayd C. Leseman (in Sandia’s Applied Photonic Microsystems Dept. and at the University of New Mexico) and Seyedhamidreza Alaie, Drew Goettler, and Mehmet Su (all at UNM).
‡ A phonon is a quantum-mechanical description of an elementary vibrational motion in which a lattice of atoms or molecules uniformly oscillates at a single frequency. Due to the bonds between atoms, the displacement of one or more atoms from their equilibrium positions give rise to a set of vibration waves propagating through the lattice. Long-wavelength phonons give rise to sound; shorter-wavelength higher-frequency phonons give rise to heat. Phonons play a major role in many of the physical properties of condensed (solid) matter, like thermal conductivity and electrical conductivity.
In previous journal articles, this research group has proposed that coherent boundary scattering in phononic crystals with relatively large feature sizes (≥100 nm) may hold the key to solving this problem by scattering phonons with minimal influence on electrons. As phonons traverse such a lattice, they can undergo two types of scattering processes:
simple particle-like incoherent scattering as a result of encountering a boundary, and
wave-like coherent scattering due to the periodic geometry of the artificial lattice of air holes.
Figure 2. Normalized thermal conductivities of phononic crystal samples. The measured thermal conductivity of the SC and the 1 × 1, 2 × 2, 3 × 3, and 4 × 4 supercell phononic crystal samples, along with the predicted values using various models described in the text.
Here, coherence implies that the phonon phase is preserved and that scattering from material boundaries exhibits at least some measure of specularity (angle of reflection is equal to the angle of incidence, past a perpendicular). Practically, this can have profound implications because, while incoherent boundary scattering depends only on the shape, size, and separation of the holes, coherent boundary scattering additionally depends on the symmetry and geometry with which these holes are distributed.
In a novel experiment that uses microscale phononic crystals, i.e., periodic arrangements of different materials, our research team was able, for the first time, to distinguish coherent phonon events from incoherent phonon scattering. In addition, our team developed a hybrid thermal conductivity model that accounts for partially coherent and partially incoherent phonon boundary scattering: the concept of a threshold mean-free path in conjunction with a hybrid theoretical model—where the incoherent and coherent contributions to thermal conductivity are weighted according to the fractional portion of the phonon population mean-free paths relative to that threshold. We observe excellent agreement between this model and experimental data (see Figure 2), and the results suggest that significant room-temperature coherent phonon boundary scattering occurs.
Although the periodicity of the phononic crystal samples is large compared to the average phonon wavelength, our results indicate that a significant portion of the phonon population remains coherent even after undergoing several scattering events. From a different perspective, because there is no direct way to measure the coherence length, our approach has enabled us to use the thermal conductivity as a macroscopic metric for inferring the average phonon coherence length in our phononic crystal samples.
This work could profoundly impact thermoelectrics by allowing for an additional mechanism for reducing the thermal conductivity of a material without affecting its electrical conductivity—by simply arranging the pores in an optimal phononic crystal geometry. Coherent phonon effects can be amplified and engineered for applications such as thermal management, heat scavenging, and energy harvesting, opening the door for guiding heat on-chip and thermal isolation of sensitive microelectronics. While the coherent phononic crystal effects seen here represent a small yet significant fraction of the overall thermal conductivity reduction, their presence at room temperature is extremely encouraging. Given that a more pronounced effect is expected at lower temperatures, the impact on thermoelectric cooling could be profound.
As part of Sandia’s core geochemistry program funded by DOE Office of Basic Energy Sciences (BES), Sandia’s Randall Cygan (in Sandia’s Geoscience Research and Applications Dept.) and his collaborators, W. Crawford Elliott and his PhD student Laura Zaunbrecher from Georgia State University, have been examining the adsorption mechanisms of metal cations onto soils and sediments. This effort is especially important for the environmental treatment of chemical and radioactive contaminants, including the fission product cesium-137—which is a significant contaminant at the Savannah River Site in South Carolina and in the region near the disabled Fukushima reactors in Japan. The article describing their work, “Molecular models of cesium and rubidium adsorption on weathered micaceous minerals,” was chosen by the editors of Journal of Physical Chemistry A to be the cover feature of the June 4, 2015, issue.
Equilibrated molecular structure of interlayer Cs+ (yellow), Rb+ (purple), and K+ (blue) in mica-vermiculite hybrid structure (HIV-mica wedge model) as derived from molecular dynamics. Frayed edges are represented by wedge zone in periodic structure. Wedge, vermiculite, and mica zones are indicated for the lower left section of the simulation cell.
Mechanism-based adsorption models for the long-term interaction of chemical and radionuclide species with clay minerals are needed to improve the accuracy of groundwater reaction and flow models, and related simulations for performance assessment of waste sites and repositories. The nanoscale nature of clay minerals often limits their characterization by traditional analytical methods. Therefore, molecular simulation using geometry-optimization and molecular-dynamics methods have been used to investigate the adsorption behavior of Cs+ and Rb+ cations at frayed-edge wedges (a proxy for frayed-edge sites) and in the interlayer region formed as a result of the transformation of muscovite to interlayered vermiculite during weathering and soil formation/development. Our results indicate that Cs+ binds more strongly than Rb+ in the vermiculite interlayer, while Rb+ binds more strongly than Cs+ in the vermiculite-mica wedge region. This is the first study to explore the detailed structure and energetics of molecular binding mechanisms at important wedge sites associated with soils and sediments.
Ultimately, these models and results will guide further determination of adsorption capacities associated with complex natural materials, especially those that can impact the attenuation and sequestration of radioactive contaminants in the environment. This work is also critical to performance assessment research activities sponsored by US Nuclear Regulatory Commission in their evaluation of the suitability of nuclear materials storage and disposal.
Sandia and the Laboratory for Laser Energetics (LLE) at the University of Rochester will investigate the compression and heating of high energy-density, magnetized plasmas at fusion-relevant magneto-inertial
[confinement] fusion (MIF) conditions, building on the recent Magnetized Liner Inertial Fusion (MagLIF) concept successes. The SNL-LLE team will conduct focused experiments based on the MagLIF approach at both SNL and LLE facilities, targeting key physics challenges in the intermediate plasma density regime. The team will also exploit and enhance a suite of simulation and numerical design tools validated by these experiments. Through this project, the team will provide critical information for improved compression and heating performance as well as insights on loss mechanisms and instabilities for hot, dense, magnetized plasmas. This information will help accelerate MagLIF concept development, and will also inform the continued development of intermediate-density approaches across the DOE’s Advanced Research Projects Agency–Energy (ARPA-E) ALPHA (Accelerating Low-cost Plasma Heating and Assembly) program portfolio.
ARPA-E’s ALPHA program funds developing the tools to build foundations for new pathways toward fusion power. ALPHA is focused on approaches that exploit magnetic fields to reduce energy losses in the intermediate-ion-density regime between lower-density magnetic-confinement fusion (MCF) and higher-density inertial-confinement fusion (ICF). This intermediate-density regime is not as well explored as the more mature MCF and ICF approaches, and it may offer new opportunities for more attractive fusion reactors with energy and power requirements that are compatible with low-cost technologies.
Illustration of the stages of the MagLIF concept on Z. The target is a metal cylinder ~1 cm tall containing fusion fuel. The target is axially magnetized using magnetic field coils (not shown). The Z machine applies a current along the axial direction, which creates an azimuthal magnetic field outside the liner and the resulting magnetic pressure compresses the liner. Early in the implosion, a multi-kJ laser pulse from the Z-Beamlet laser is coupled to the fuel through a thin plastic window on one end. This energy is homogenized over the several tens of nanoseconds it takes to compress the fuel.
As shown in the figure (right), MagLIF is an innovative MIF concept that exploits advances in pulsed-power technology. A MagLIF target is magnetized with an axial field of up to 30 tesla (~60,000 times greater than the Earth’s magnetic field) before the shot. The target is imploded using a 20 million ampere, 100 nanosecond current from the Z pulsed-power accelerator. The liner rapidly becomes plasma. Just as the liner’s inner surface begins to move, the multiple-kilojoule, 1 terawatt, 0.53 mm Z-Beamlet laser is used to heat the deuterium gas.
The axial magnetic field suppresses electron heat transport from the hot fuel to the cold liner wall—enabling electron-ion equilibration and allowing a quasi-adiabatic compression to fusion temperatures (>50 million Kelvin) and fuel pressures of approximately gigabars (~5 billion times atmospheric pressure). The embedded axial magnetic field is compressed to ~13 kilotesla. Target designs on the Z facility are predicted to reach fusion yields that approach and exceed the driver energy invested in the fuel if equimolar deuterium-tritium (DT) fuel is used. In these designs, the fuel is compressed radially by ~25–30 times before the fuel’s plasma pressure stops the implosion (the point of ‘stagnation’). At stagnation, the charged-particle products (alpha particles for DT reactions or 1 MeV tritons for deuterium-deuterium [DD] reactions) are magnetized, meaning that their gyro radii are less than the final fuel radius resulting in their confinement in the plasma.
MIF has long been discussed as a promising fusion approach using plasma densities between those of MCF (~1014 ions and electrons per cubic centimeter) and traditional ICF (>1025 ions and electrons per cubic centimeter), but there remains a paucity of experimental data quantitatively compared to state-of-the-art simulations validating MIF, particularly in fusing plasmas. In late 2013, experiments began on the Z pulsed-power facility studying MagLIF targets. These experiments showed that a 70 kilometers per second cylindrical liner implosion compressing laser-heated and axially magnetized deuterium gas could produce a final ion temperature of up to about 44 million Kelvin and up to 2 × 1012 thermonuclear DD neutrons. In addition, >1010 secondary DT neutrons were observed, which is only possible with significant fuel magnetization. By reducing electron thermal conduction losses and magnetizing the ions, MagLIF relaxes the constraints on traditional inertial fusion designs (final fuel pressure, areal density, convergence, driver power, and intensity).
OMEGA (left) stands 10 meters tall and is approximately 100 meters in length. This system delivers pulses of laser energy to targets in order to measure the resulting nuclear and fluid dynamic events. In this target chamber (center), OMEGA’s 60 laser beams focus up to 40,000 joules of energy onto a target (right) that measures less than 1 millimeter in diameter in approximately one billionth of a second.
It is predicted that yields two orders of magnitude larger are possible on Z using deuterium fuel within the next three years. If equimolar DT were to be used for the fusion fuel instead of pure deuterium, simulations suggest the fusion yields could exceed the total amount of energy delivered to the fuel (fuel gain > 1). Such an achievement—modeled and understood—would clearly demonstrate MIF’s credibility as an ICF/MCF alternative. A key challenge is the Z facility’s relatively low shot rate. Our research will greatly increase MagLIF rate of progress by conducting many more experiments per year on Sandia’s Z-Beamlet laser and the University of Rochester’s OMEGA (shown in the figure above) and OMEGA-EP lasers. Our experiments will target key physics challenges such as laser preheating and fuel magnetization. We will also conduct fully integrated, scaled-down experiments of the MagLIF concept on LLE’s OMEGA facility. Testing MagLIF on OMEGA and leveraging NNSA-funded research on Z will demonstrate key MIF tools on multiple facilities at significantly different energy, time, and spatial scales. Another key project goal is to use state-of-the-art simulations and numerical design tools, tested by these experiments, to validate and improve fusion-target design parameters for magnetized plasmas.
ARPA-E funding is $3.8M over two years. At this point, negotiations are underway to reconcile the elements of Sandia and LLE ’s originally proposed research program to the scope of the ARPA-E funding award.