Eric Sembrat's Test Bonanza

Georgia Electronic Design Center Distinguished Lecture Series

Resonances for Spatially Distributed Emitters

Featuring Steven Johnson, Professor of Applied Mathematics and Physics, MIT

Abstract: It’s well known that a resonant cavity can dramatically enhance light emission by a fluorescent particle, via the Purcell effect. A closely related enhancement occurs for ensembles of coherent or incoherent emitters, which arises in many circumstances: lasing, thermal emission, fluorescent media, Raman scattering in fluids, scattering by surface roughness, and even darkmatter axion haloscopes. However, such “distributed” emission problems favor quite different resonant geometries, in part because the role of corner singularities is upended by spatial averaging. Moreover, even though distributed-emission problems tend to be naturally translation invariant, the process of seeking an optimal emission-enhancing geometry leads to spontaneous symmetry breaking. Theoretically, new tools are becoming available to reveal the possible behaviors and upper bounds of light–matter interactions in complex nanostructured geometries. Computationally, the modeling of such systems naively involves an ensemble average of a large number of expensive electromagnetic simulations, but new trace-optimization algorithms now make it possible to perform large-scale “inverse design” of distributed emission over thousands of degrees of freedom.

Biography: Steven G. Johnson is a Professor of Applied Mathematics and Physics at MIT. He works in the field of nanophotonics—electromagnetism in media structured on the  wavelength scale, especially in the infrared and optical regimes—where he works on many  aspects of the theory, design, and computational modeling of nanophotonic devices, both  classical and quantum. He is coauthor of over 200 papers and over 25 patents, including the  second edition of the textbook Photonic Crystals: Molding the Flow of Light. In addition to  traditional publications, he distributes several widely used free-software packages for  scientific computation, including the MPB and Meep electromagnetic simulation tools and  the FFTW fast Fourier transform library (for which he received the 1999 J. H. Wilkinson Prize  for Numerical Software).

Pizza and soda will be available post seminar.

Pumpkins will be dropped from the top of the Howey Physics Building as part of a fundraiser for the Society of Physics Students.

This year’s event will be from 4:15-5:15 on 10/31/22 at the Howey Physics Building, following a bake sale by the Society of Women in Physics from 2:15-4:15 in the Howey Courtyard! This year we also have some new decorating options including pumpkins filled with fake blood.

Purchase a pumpkin for the pumpkin drop

The IceCube Neutrino Observatory has reported a diffuse flux of TeV-PeV astrophysical neutrinos in three years of data. The observation of tau neutrinos in the astrophysical neutrino signal is of great interest in determining the nature of astrophysical neutrino oscillations. Tau neutrinos become distinguishable from other flavors in IceCube at energies above a few hundred TeV, when the particle shower from the initial charged current interaction can be separated from the cascade from the tau decay: the two cascades are called a "double bang" signature. I will discuss the search for tau neutrinos in IceCube, including an analysis which uses the digitized signal from individual IceCube sensors to resolve the two showers, in order to be sensitive to taus at as low an energy as possible. This is the first IceCube search to be more sensitive to tau neutrinos than to any other flavor. I will present the results and prospects for future high energy tau neutrino searches in IceCube and beyond.

In this talk, I will summarize recent results from the South Pole Telescope 2500 deg^2 survey. This mass-limited survey has discovered hundreds of new galaxy clusters at 0 < z < 1.7, allowing an unprecedented view of galaxy cluster evolution. Using follow-up observations from Spitzer, Hubble, Chandra, XMM-Newton, Magellan, VLT, ALMA, ATCA, and Gemini, we are able to study the evolution of the stars, gas, and dark matter in these massive systems. Based on these data, we constrain the evolution of cluster galaxies, the central AGN, the cooling ICM, the heavy metal abundance of the ICM, the dynamical state of the cluster, and various other cluster properties. Looking forward, I will present several new and ongoing surveys which will dramatically change the landscape of galaxy cluster research in coming years.

The field of active matter is the result of applying statistical physics to the motion of biological and biomimetic systems, from animal flocks to the cell's cytoskeleton and from robotic swarms to self-propelled colloids. Unlike bird flocks, which can move around freely, cells inside an organism or filaments inside a cell move in a very confined space bounded by curved walls. What is more, the shape of the boundaries can affect the dynamics in dramatic ways. Recently my focus has been on building a theoretical framework to study such problems by combining the concepts of active matter with those of the geometry of curved surfaces. I will discuss what such an approach can teach us about the way active systems respond to the geometry of their environment and what I hope it can teach us about the way such systems deform their environment and regulate their own shape.

There is a strong desire, often driven by real or perceived pressures, to publish research in a top tier journal like Science.&nbsp; However, with a rejection rate above 90%, it is a difficult process.&nbsp; When a paper gets rejected without referee comments, it is hard to know why the paper failed to get past the initial screening process.&nbsp; In this talk, I will describe the publication process at Science, within the broader context of developing skills for more effective scientific communication.&nbsp; Aside from publishing in high impact journals, good communication tools are essential for forming scientific collaborations, bypassing research obstacles, avoiding conflicts during scientific presentations and explaining scientific research to funding bodies and the public at large, who are the primary source of financial support for scientific research.

DNA nanotechnology, especially scaffolded DNA origami, has emerged into a field that fabricates well-defined nanostructures with unprecedented geometric complexity and precision. This technology is proposed to eventually provide integral components for complex nanomachines and nanofactories. The power of DNA as a nanoscale building material is that it can be designed to self assemble into complex nanostructures that are held together by numerous k_BT-scale (0.025 eV) interactions. This allows DNA-based structures to be both globally stable and locally dynamic. Currently, DNA nanotechnology has a number of applications, including drug delivery, single molecule sensing, and templating of crystalline nanoparticles. However, applications rely largely on static nanomaterial properties. I will discuss the overall current state of the DNA nanotechnology field and our work on developing DNA based nanosensors, whose functionality relies on structural dynamics.&nbsp;

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Recent work from Marten Scheffer and colleagues has made bold claims.

"Complex dynamical systems, ranging from ecosystems to financial markets and the climate, can have tipping points at which a sudden shift to a contrasting dynamical regime may occur [1]. Although predicting such critical points before they are reached is extremely difficult, work in different scientific fields is now suggesting the existence of generic early-warning signals that may indicate for a wide class of systems if a critical threshold is approaching." In a paper, now in Press in Critical Care Medicine, Scheffer and colleagues (including me) argue that these results may be applicable in medicine [2].&nbsp; I will discuss this work from the context of my own interest in bifurcations, problems associated with alternans rhythms [3], and transition to and risk stratification for sudden cardiac death.

&nbsp;[1] "Early-warning signals for critical transitions" Nature 461, 53&nbsp;(2009) by Marten Scheffer et al.

&nbsp;[2] "Slowing down of recovery as generic risk marker for acute transitions in chronic diseases" by Olde-Rikkert et al. Critical Care Medicine (2016)

&nbsp;[3] "Predicting the onset of period-doubling bifurcations in noisy cardiac systems" T. Quail, A. Shrier, L. Glass, PNAS 112, 9358-9363 (2015)

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The nascent technique of 4D printing has the potential to revolutionize manufacturing in fields ranging from organs-on-a-chip to architecture to soft robotics. By expanding the pallet of 3D printable materials to include the use stimuli responsive inks, 4D printing promises precise control over patterned shape transformations. With the goal of creating a new manufacturing technique, we have recently introduced a biomimetic printing platform that enables the direct control of local anisotropy into both the elastic moduli and the swelling response of the ink.

We have drawn inspiration from nastic plant movements to design a phytomimetic ink and printing process that enables patterned dynamic shape change upon exposure to water, and possibly other external stimuli. Our novel fiber-reinforced hydrogel ink enables local control over anisotropies not only in the elastic moduli, but more importantly in the swelling. Upon hydration, the hydrogel changes shape accord- ing the arbitrarily complex microstructure imparted during the printing process.

To use this process as a design tool, we must solve the inverse problem of prescribing the pattern of anisotropies required to generate a given curved target structure. We show how to do this by constructing a theory of anisotropic plates and shells that can respond to local metric changes induced by anisotropic swelling. A series of experiments corroborate our model by producing a range of target shapes inspired by the morphological diversity of flower petals.

The enjoyment of music and art are uniquely human experiences. Yet we still do not understand the attributes that lead us to appreciate some artistic works and not others. In this talk I will address how concepts in mathematics and physics can help us to think about these matters. Chaos refers to irregular time series that are generated following a definite set of deterministic rules. A fractal is an image, in which magnification of a small region is similar to the whole. I will give examples of how the concepts of chaos and fractals can be exploited to propose simple computer algorithms that can be used to generate sequences of sounds and images. I will also show how random patterns of dots can be manipulated to generate displays that are visually interesting, and that can be used as an input to probe the physiological processes underlying visual perception. The talk will challenge you to think about what you hear and see, and how you do it.

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