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Paoletti M.S.,Institute for Research in Electronics and Applied Physics | Lathrop D.P.,Institute for Research in Electronics and Applied Physics | Lathrop D.P.,University of Maryland University College
Physical Review Letters | Year: 2011

We present measurements of the angular momentum flux (torque) in Taylor-Couette flow of water between independently rotating cylinders for all regions of the (Ω1, Ω2) parameter space at high Reynolds numbers, where Ω1 (Ω2) is the inner (outer) cylinder angular velocity. We find that the Rossby number Ro=(Ω1-Ω2)/Ω2 fully determines the state and torque G as compared to G(Ro=∞)≡G ∞. The ratio G/G∞ is a linear function of Ro⊃-1 in four sections of the parameter space. For flows with radially increasing angular momentum, our measured torques greatly exceed those of previous experiments, but agree with the analysis of Richard and Zahn. © 2011 The American Physical Society.


Probst R.,University of Maryland University College | Cummins Z.,University of Maryland University College | Ropp C.,University of Maryland University College | Waks E.,University of Maryland University College | And 3 more authors.
IEEE Control Systems | Year: 2012

This article is on microscale flow control, on dynamically shaping flow fields in microfluidic devices to precisely manipulate cells, quantum dots (QDs), and nanowires. Compared to prior methods, manipulating microscopic and nanoscopic objects by flow control can be achieved with simpler and easy-to-fabricate devices, can steer a wider variety of objects, and enables entirely new capabilities such as placement and immobilization of specific quantum dots to desired on-chip locations with nanoscale precision. A companion article investigates flow control in the body and develops methods to shape magnetic fields to direct ferrofluids of therapeutic magnetic nanoparticles to disease locations in patients. © 1991-2012 IEEE.


Garrett J.L.,University of Maryland University College | Garrett J.L.,Institute for Research in Electronics and Applied Physics | Somers D.,University of Maryland University College | Somers D.,Institute for Research in Electronics and Applied Physics | And 2 more authors.
Journal of Physics Condensed Matter | Year: 2015

Measurements of the Casimir force require the elimination of the electrostatic force between the surfaces. However, due to electrostatic patch potentials, the voltage required to minimize the total force may not be sufficient to completely nullify the electrostatic interaction. Thus, these surface potential variations cause an additional force, which can obscure the Casimir force signal. In this paper, we inspect the spatially varying surface potential of e-beamed, sputtered, sputtered and annealed, and template stripped gold surfaces with Heterodyne amplitude modulated Kelvin probe force microscopy (HAM-KPFM). It is demonstrated that HAM-KPFM improves the spatial resolution of surface potential measurements compared to amplitude modulated Kelvin probe force microscopy. We find that patch potentials vary depending on sample preparation, and that the calculated pressure can be similar to the pressure difference between Casimir force calculations employing the plasma and Drude models. © 2015 IOP Publishing Ltd.


Garrett J.L.,University of Maryland University College | Garrett J.L.,Institute for Research in Electronics and Applied Physics | Munday J.N.,Institute for Research in Electronics and Applied Physics | Munday J.N.,College Park
Nanotechnology | Year: 2016

Kelvin probe force microscopy (KPFM) adapts an atomic force microscope to measure electric potential on surfaces at nanometer length scales. Here we demonstrate that Heterodyne-KPFM enables scan rates of several frames per minute in air, and concurrently maintains spatial resolution and voltage sensitivity comparable to frequency-modulation KPFM, the current spatial resolution standard. Two common classes of topography-coupled artifacts are shown to be avoidable with H-KPFM. A second implementation of H-KPFM is also introduced, in which the voltage signal is amplified by the first cantilever resonance for enhanced sensitivity. The enhanced temporal resolution of H-KPFM can enable the imaging of many dynamic processes, such as such as electrochromic switching, phase transitions, and device degredation (battery, solar, etc), which take place over seconds to minutes and involve changes in electric potential at nanometer lengths. © 2016 IOP Publishing Ltd.


Ross J.S.,Lawrence Livermore National Laboratory | Ross J.S.,University of California at San Diego | Glenzer S.H.,Lawrence Livermore National Laboratory | Palastro J.P.,Institute for Research in Electronics and Applied Physics | And 5 more authors.
Review of Scientific Instruments | Year: 2010

We present simultaneous Thomson-scattering measurements of light scattered from ion-acoustic and electron-plasma fluctuations in a N2 gas jet plasma. By varying the plasma density from 1.5× 1018 to 4.0 × 1019 cm-3 and the temperature from 100 to 600 eV, we observe the transition from the collective regime to the noncollective regime in the high-frequency Thomson-scattering spectrum. These measurements allow an accurate local measurement of fundamental plasma parameters: electron temperature, density, and ion temperature. Furthermore, experiments performed in the high densities typically found in laser produced plasmas result in scattering from electrons moving near the phase velocity of the relativistic plasma waves. Therefore, it is shown that even at low temperatures relativistic corrections to the scattered power must be included. © 2010 American Institute of Physics.


Pagan V.R.,College Park | Haas B.M.,College Park | Murphy T.E.,Institute for Research in Electronics and Applied Physics
Proceedings - 2010 IEEE International Topical Meeting on Microwave Photonics, MWP 2010 | Year: 2010

We demonstrate a technique for optically relaying and downconverting microwave signals. The system uses phase modulation in the transmitter and re-modulation and optical filtering in the receiver. Intermodulation distortion is suppressed by adjusting the amplitude of the local oscillator. ©2010 IEEE.


News Article | September 9, 2016
Site: phys.org

Powerful laser beams, given the right conditions, will act as their own lenses and "self-focus" into a tighter, even more intense beam. University of Maryland physicists have discovered that these self-focused laser pulses also generate violent swirls of optical energy that strongly resemble smoke rings. In these donut-shaped light structures, known as "spatiotemporal optical vortices," the light energy flows through the inside of the ring and then loops back around the outside. The vortices travel along with the laser pulse at the speed of light and control the energy flow around it. The newly discovered optical structures are described in the September 9, 2016 issue of the journal Physical Review X. The researchers named the laser smoke rings "spatiotemporal optical vortices," or STOVs. The light structures are ubiquitous and easily created with any powerful laser, given the right conditions. The team strongly suspects that STOVs could explain decades' worth of anomalous results and unexplained effects in the field of high-intensity laser research. "Lasers have been researched for decades, but it turns out that STOVs were under our noses the whole time," said Howard Milchberg, professor of physics and electrical and computer engineering at UMD and senior author of the research paper, who also has an appointment at the UMD Institute for Research in Electronics and Applied Physics (IREAP). "This is a robust, spontaneous feature that's always there. This phenomenon underlies so much that's been done in our field for the past 30-some years." More conventional spatial optical vortices are well-known from prior research—chief among them "orbital angular momentum" (OAM) vortices, where light energy circulates around the beam propagation direction much like water rotates around a drain as it empties from a washbasin. Because these vortices can influence the shape of the central beam, they have proven useful for advanced applications such as high-resolution microscopy. "Conventional optical vortices have been studied since the late 1990s as a way to improve telecommunications, microscopy and other applications. These vortices allow you to control what gets illuminated and what doesn't, by creating small structures in the light itself," said the paper's lead author Nihal Jhajj, a physics graduate student who conducted the research at IREAP. "The smoke ring vortices we discovered may have even broader applications than previously known optical vortices, because they are time dynamic, meaning that they move along with the beam instead of remaining stationary," Jhajj added. "This means that the rings may be useful for manipulating particles moving near the speed of light." Jhajj and Milchberg acknowledge that much more work needs to be done to understand STOVs, including their physical and theoretical implications. But they are particularly excited for new opportunities that will arise in basic laser research following their discovery of STOVs. "All the evidence we've seen suggests that STOVs are universal," Jhajj said. "Now that we know what to look for, we think that looking at a high-intensity laser pulse propagating through a medium and not seeing STOVs would be a lot like looking at a river and not seeing eddies and currents." Eventually, STOVs might have useful real-world applications, like their more conventional counterparts. For example, OAM vortices have been used in the design of more powerful stimulated emission depletion (STED) microscopes. STED microscopes are capable of much higher resolution than traditional confocal microscopes, in part due to the precise illumination offered by optical vortices. With the potential to travel with the central beam at the speed of light, STOVs could have as-yet unforeseen advantages in technological applications, including the potential to expand the effective bandwidth of fiber-optic communication lines. "A STOV is not just a spectator to the laser beam, like an angel's halo," explained Milchberg, noting the ability of STOVs to control the central beam's shape and energy flow. "It is more like an electrified angel's halo, with energy shooting back and forth between the halo and the angel's head. We're all very excited to see where this discovery will take us in the future." More information: DOI: 10.1103/PhysRevX.6.031037 The research paper, "Spatio-temporal optical vortices," Nihal Jhajj, Ilia Larkin, Eric Rosenthal, Sina Zahedpour, Jared Wahlstrand, and Howard Milchberg, appears in the September 9, 2016 issue of the journal Physical Review X.


News Article | February 15, 2017
Site: phys.org

The concept also applies to the design of stellarators, which are complex nuclear fusion experiments meant to explore fusion's potential as an energy source. Stellarators work by confining a ring of blazing-hot plasma inside a precisely shaped magnetic field generated by external electromagnetic coils. When the plasma gets to several million degrees—as hot as the interior of the sun—atomic nuclei begin to fuse together, releasing massive amounts of energy. Before turning a single bolt to build one of these rare and expensive devices, engineers create exacting plans using a series of algorithms. However, a wide variety of coil shapes can all generate the same magnetic field, adding levels of complexity to the design process. Until now, few researchers have studied how to choose the best among all potential coil shapes for a specific stellarator. University of Maryland physicist Matt Landreman has made an important revision to one of the most common software tools used to design stellarators. The new method is better at balancing tradeoffs between the ideal magnetic field shape and potential coil shapes, resulting in designs with more space between the coils. This extra space allows better access for repairs and more places to install sensors. Landreman's new method is described in a paper published February 13, 2017 in the journal Nuclear Fusion. "Instead of optimizing only the magnetic field shape, this new method considers the complexity of the coil shapes simultaneously. So there is a bit of a tradeoff," said Landreman, an assistant research scientist at the UMD Institute for Research in Electronics and Applied Physics (IREAP) and sole author of the research paper. "It's a bit like buying a car. You might want the cheapest car, but you also want the safest car. Both features can be at odds with each other, so you have to find a way to meet in the middle." Researchers used the previous method, called the Neumann Solver for Fields Produced by External Coils (NESCOIL) and first described in 1987, to design many of the stellarators in operation today—including the Wendelstein 7-X (W7-X). The largest stellarator in existence, W7-X began operation in 2015 at the Max Planck Institute of Plasma Physics in Germany. "Most designs, including W7-X, started with a specifically shaped magnetic field to confine the plasma well. Then the designers shaped the coils to create this magnetic field," Landreman explained. "But this method typically required a lot of trial-and-error with the coil design tools to avoid coils coming too close together, making them infeasible to build, or leaving too little space to access the plasma chamber for maintenance." Landreman's new method, which he calls Regularized NESCOIL—or REGCOIL for short—gets around this by tackling the coil spacing issue of stellarator design in tandem with the shaping of the magnetic field itself. The result, Landreman said, is a fast, more robust process that yields better coil shapes on the first try. Modeling tests performed by Landreman suggest that the designs produced by REGCOIL confine hot plasma in a desirable shape, while significantly increasing the minimum distances between coils. "In mathematics, we'd call stellarator coil design an 'ill-posed problem,' meaning there are a lot of potential solutions. Finding the best solution is highly dependent on posing the problem in the right way," Landreman said. "REGCOIL does exactly that by simplifying coil shapes in a way that the problem can be solved very efficiently." The development of nuclear fusion as a viable energy source remains far off into the future. But innovations such as Landreman's new method will help bring down the cost and time investments needed to build new stellarators for research and—eventually—practical, energy-generating applications. "This field is still in the basic research stage, and every new design is totally unique," Landreman said. "With these incompatible features to balance, there will always be different points where you can decide to strike a compromise. The REGCOIL method allows engineers to examine and model many different points along this spectrum." The research paper, "An improved current potential method for fast computation of stellarator coil shapes," Matt Landreman, was published February 13, 2017 in the journal Nuclear Fusion. Explore further: Physicists confirm the precision of magnetic fields in the most advanced stellarator in the world


News Article | December 11, 2015
Site: www.rdmag.com

A new diagnostic imaging technique developed by a University of Maryland-led team of researchers promises to boost efficiencies of solar cells by making it possible to find and correct previously undetected ways that solar cells fall far short of theoretical efficiencies. Theory indicates that current solar cell technologies should able to convert solar energy to electrical energy with at least 30 percent efficiency, but the actual efficiencies of current cells is only around 20 percent. Thus solar panels produce one third less power than the theoretical maximum of these devices. “With the new imaging technique our team has developed, academic and industry researchers will be able to diagnose where solar cells lose efficiency and close the gap between theory and the actual efficiencies experienced by consumers who install solar panels on their homes and businesses,” said University of Maryland (UMD) Assistant Professor Marina Leite,  in the UMD Institute for Research in Electronics and Applied Physics (IREAP) and the department of materials science & engineering in the A. James Clark School of Engineering. Solar cell efficiencies depend on the maximum achievable open-circuit voltage generated by the device under illumination.  Open-circuit voltage determines how well any photovoltaic device operates, and researchers must be able to measure and image it in order to diagnose which processes are adding to or subtracting from cell efficiency. The new, ambient temperature imaging technique presented by Leite and her team [Tennyson, et al] is a variation of illuminated Kelvin Probe Force Microscopy, which is a non-contact, non-destructive imaging technique used to determine the composition and electronic state of a surface. Traditionally, this technique uses a laser diode to scan the surface of a solid and measure the potential difference between the tip of the probe and the surface of that material. Tennyson, et al.  takes this conventional imaging method further to demonstrate a “direct correlation between Kelvin Probe Force Microscopy measurements (light- minus dark-KPFM) and the open-circuit voltage of photovoltaic devices through the measurement of the quasi-Fermi level splitting”. This indirect measurement allows the UMD-led team to observe precisely [at nanoscale resolution] where the open-circuit voltage is changing. The researchers say that previous imaging techniques for determining solar cell efficiencies had to be performed under vacuum at very cold temperatures (-333 Fahrenheit or 70 Kelvin). Their new technique fills an important gap in the literature surrounding solar cell efficiencies, providing a “straightforward, universal, and accurate method to measure the open-circuit voltage... with high spatial resolution,” they say.


News Article | December 11, 2015
Site: phys.org

Theory indicates that current solar cell technologies should able to convert solar energy to electrical energy with at least 30 percent efficiency, but the actual efficiencies of current cells is only around 20 percent. Thus solar panels produce one third less power than the theoretical maximum of these devices. "With the new imaging technique our team has developed, academic and industry researchers will be able to diagnose where solar cells lose efficiency and close the gap between theory and the actual efficiencies experienced by consumers who install solar panels on their homes and businesses," said University of Maryland (UMD) Assistant Professor Marina Leite, in the UMD Institute for Research in Electronics and Applied Physics (IREAP) and the department of materials science & engineering in the A. James Clark School of Engineering. Solar cell efficiencies depend on the maximum achievable open-circuit voltage generated by the device under illumination. Open-circuit voltage determines how well any photovoltaic device operates, and researchers must be able to measure and image it in order to diagnose which processes are adding to or subtracting from cell efficiency. The new, ambient temperature imaging technique presented by Leite and her team [Tennyson, et al] is a variation of illuminated Kelvin Probe Force Microscopy, which is a non-contact, non-destructive imaging technique used to determine the composition and electronic state of a surface. Traditionally, this technique uses a laser diode to scan the surface of a solid and measure the potential difference between the tip of the probe and the surface of that material. Tennyson, et al. takes this conventional imaging method further to demonstrate a "direct correlation between Kelvin Probe Force Microscopy measurements (light- minus dark-KPFM) and the open-circuit voltage of photovoltaic devices through the measurement of the quasi-Fermi level splitting". This indirect measurement allows the UMD-led team to observe precisely [at nanoscale resolution] where the open-circuit voltage is changing. The researchers say that previous imaging techniques for determining solar cell efficiencies had to be performed under vacuum at very cold temperatures (-333 Fahrenheit or 70 Kelvin). Their new technique fills an important gap in the literature surrounding solar cell efficiencies, providing a "straightforward, universal, and accurate method to measure the open-circuit voltage... with high spatial resolution," they say. The findings of Leite and her team are published in, and featured on the cover of, the December 9 issue of Advanced Energy Materials. More information: Solar Cells: Nanoimaging of Open-Circuit Voltage in Photovoltaic Devices. Adv. Energy Mater. DOI: 10.1002/aenm.201570123

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