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A human being is not attaining his full heights until he is educated

Horace Mann

Colloquia

Talks start at 12:15 PM
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22
Feb '21
Laboratoire de Physique et Modélisation des Milieux Condensés, CNRS
Sergey Skipetrov
Localization of light by disorder
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Abstract:
Propagation of waves can be blocked and wave modes localized by disorder. First discovered for electrons at low temperatures, this “Anderson localization” takes place for other waves as well. Theoretical studies and experimental observations have been reported for sound and matter waves in various space dimensionalities, going from one-dimensional (1D) chains to three-dimensional (3D) bulk disordered materials. Interestingly, electromagnetic waves in general and light in particular seem to stand out from this general picture. First, the common belief is that Anderson localization is promoted by strong disorder. Localization of light, however, can be observed at weak disorder by confining light propagation to (quasi-)1D tubes or 2D planes but difficulties exist in 3D where even the strongest reachable disorder didn’t allow to demonstrate Anderson localization of light convincingly. Second, recent theoretical studies indicate that strong disorder seems to disadvantage localization by opening a new channel of wave transport involving non-propagating longitudinal fields. These fields are specific for electromagnetism and do not exist for scalar waves (sound or matter waves) or other vector waves (elastic waves in solids). Moreover, the point-scatterer model that allows for an exact solution, predicts diffuse scattering of light and exhibits no sign of localization at any disorder strength, even when the widely accepted Ioffe-Regel criterion of localization is obeyed by far. It seems therefore that in contrast to other waves, light can be localized by disorder in low-dimensional systems only, whereas a structured medium – a photonic crystal or a hyperuniform structure – is required to observe localization in 3D. In these latter materials, localization is expected to take place at weak disorder, similarly to low-dimensional systems and in agreement with the idea that strong disorder impedes the localization of light.
 
1
Mar '21
Next Event
Texas Christian University
Yuri Strzhemechny
Microscale ZnO with Controllable Crystal Morphology as a Platform to Study ‎Antibacterial Action on Staphylococcus Aureus
Abstract:
Nanoscale ZnO particles are known to inhibit the growth of bacteria.  The fundamental mechanisms driving this process, however, are not completely understood.  While there are many contributing factors to consider, we hypothesize that the antimicrobial action is most fundamentally derived from the ZnO surface and its interaction with growth media and the bacteria’s extracellular material.  In this work, we implement minimum inhibition concentration and novel comparative assays to evaluate the antibacterial activity of ZnO microcrystals produced by us using a hydrothermal chemical growth method.  The samples were synthesized in the range of sizes from 1µm to 5µm with varying abundances of surfaces with different polarities.  This approach prevents the ZnO particles to be internalized by the bacterial cells with diameters ca. 500 nm, thus allowing one to study correlations between overall surface polarity and antibacterial action.  These experiments were performed in conjunction with optoelectronic studies of ZnO crystals (photoluminescence, surface photovoltage) to characterize electronic structure and dominant charge transport mechanisms as fundamental phenomena, which could potentially govern the processes leading to an antibacterial behavior in our samples.  We report on the results of these comparative studies relating antibacterial properties with surface morphology and electronic behavior.
8
Mar '21
Weizmann Institute
Nir Davidson
Solving computational problems with coupled lasers
Abstract:
Computational problems may be solved by realizing physics systems that can simulate them. Here we present a new system of up to >1000 coupled lasers that is used to solve difficult computational tasks. The well-controlled dissipative coupling anneals the lasers into a stable phase-locked state with minimal loss, that can be mapped on different computational minimization problems. We demonstrate this ability for simulating XY spin systems and finding their ground state, for phase retrieval, for imaging through scattering medium and more.
15
Mar '21
MIT
Sara Seager
Exoplanets and the Search for Atmospheric Biosignature Gases
Abstract:
Thousands of exoplanets are known to orbit nearby stars and small rocky planets are established to be common.  Driving the field is the study of exoplanet atmospheres, with the goal of detecting a gas that might be indicative of life. A suitable “biosignature gas” is not just one that might be produced by life, but one that: can accumulate in an atmosphere against atmospheric radicals and other sinks; has strong atmospheric spectral features; and has limited abiological false positives. Which gases might be potential biosignature gases in an as yet unknown range of exoplanetary environments? New computer simulations and next-generation telescopes coming online means the ambitious goal of searching for “biosignature gases” in a rocky exoplanet atmosphere is within reach.  
22
Mar '21
Ben-Gurion University
Yonatan Dubi
Do Plant use Quantum Mechanics? Probably Not
5
Apr '21
Princeton University
Salvatore Torquato
Hyperuniform States of Matter and Their Novel Characteristics
Abstract:
Hyperuniformity is a new type of long-range order that encompasses all perfect crystals, perfect quasicrystals, and some exotic disordered states of matter. Disordered hyperuniform many-particle systems [1,2] can be regarded to be new states of disordered matter in that they behave more like crystals or quasicrystals in the manner in which they suppress large-scale density fluctuations, and yet are also like liquids and glasses because they are statistically isotropic structures with no Bragg peaks. Thus, these special correlated disordered materials possess a "hidden order" that is not apparent on large length scales. A variety of groups have found that disordered hyperuniform materials possess desirable photonic and electronic bandgap properties. More recently, we have shown that they possess nearly optimal transport and elastic properties. I will review the salient ideas behind the hyperuniformity concept and  procedures to design a variety of different disordered hyperuniform materials as well as their corresponding physical properties, including novel transport, mechanical, electromagnetic and elastodynamic characteristics [3,4,5]. It has been a numerical and experimental challenge to create very large samples that are hyperuniform with high fidelity. I will discuss recent progress that we have made in this direction [6] and its implications for novel physical properties.
 
  1. S. Torquato and F. H. Stillinger, "Local Density Fluctuations, Hyperuniform Systems, and Order Metrics," Phys. Rev. E, 68, 041113 (2003).
  2. S. Torquato, "Hyperuniform States of Matter," Phys. Reports, 745, 1 (2018).
  3. G. Zhang, F. H. Stillinger, and S. Torquato, "Transport, Geometrical, and Topological Properties of Stealthy Disordered Hyperuniform Two-phase Systems," J. Chem. Phys., 145, 244109 (2016).
  4. S. Torquato and D. Chen, "Multifunctional Hyperuniform Cellular Networks: Optimality, Anisotropy and Disorder," Multifunctional Materials, 1, 015001 (2018).
  5. J. Kim and S. Torquato, Multifunctional Composites for Elastic and Electromagnetic Wave Propagation, Proc. Nat. Acad. Sci., 117, 8764 (2020).
  6. J. Kim and S. Torquato, "New Tessellation-Based Procedure to Design Perfectly Hyperuniform Disordered Dispersions for Materials Discovery, Acta Materialia, 68, 143 (2019).
19
Apr '21
Princeton University
Alejandro Rodriguez
Fundamental bounds on optical devices: a mathematical theory of electromagnetic limits
10
May '21
Queens College of CUNY
Yiming Huang
Dynamics in non-Hermitian Random Media