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Science is a way of thinking much more than it is a body of knowledge.

Carl Sagan


Talks start at 12:15 PM
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If not mentioned otherwise, all online Zoom.us events are accessible via this link or use meeting ID 829 2687 2594 and passcode 866995.
Oct '21
Stanford University
Joseph W. Goodman
Lessons I’ve Learned in Optics: the Roles of Education, Hard Work and Good Luck
Download PDF zoom @12:15 pm
This talk draws from lessons I have learned after nearly 60 years of working in optics.  The material presented is aimed at young faculty and students,  describing both my mistakes, disappointments, and what I did right. Regarding education, I review what appear to be common trends in the advancement of technology, and emphasize my view of the type of education that will help one retain relevance during such changes. Regarding hard work, I have found that if you work hard you can overcome many educational shortcomings and, if you enjoy the work you are doing, it doesn’t seem to be hard at all. Finally, regarding good luck, I review the various kinds of good luck that befell me, in hopes that you will find similar good luck in your careers.
Oct '21
Vienna University of Technology
Stefan Roter
Optimal states of light in disordered media: information-retrieval and scattering-invariance
In my talk I will present recent advances in designing tailor-made states of light with optimal properties in scattering across highly disordered media. First, I will discuss how the concept of Fisher information allows us to distill  from the measurable scattering matrix of a system the unique state of light, which delivers the maximum amount of information about a desired system parameter of interest to an external observer [1]. In a second part, I will discuss so-called "scattering-invariant modes”; these light fields have the unique property that they are transmitted across a disordered medium with the same output profile as when travelling through free space [2]. Both of these concepts were recently implemented together with the group of Allard Mosk in Utrecht using optical wave-front shaping tools. 

[1] D. Bouchet, S. Rotter, and A. P. Mosk, Nature Physics 17, 564 (2021)
[2] P. Pai, J. Bosch, M. Kühmayer, S. Rotter, and A. P. Mosk, Nature Photonics 15, 431 (2021)
Oct '21
City College of CUNY
Sriram Ganeshan
Hamiltonian structure of 2D fluid dynamics with broken parity
Isotropic fluids in two spatial dimensions can break parity symmetry and sustain transverse stresses which do not lead to dissipation. Corresponding transport coefficients include odd viscosity, odd torque, and odd pressure. In this talk, I will discuss conditions on transport coefficients that correspond to dissipationless and separately to Hamiltonian fluid dynamics. The restriction on the transport coefficients will help identify what kind of hydrodynamics can be obtained by coarse-graining a microscopic Hamiltonian system. Interestingly, not all parity-breaking transport coefficients lead to energy conservation and, generally, the fluid dynamics is energy conserving but not Hamiltonian. I will outline how this dynamics can be realized by imposing a nonholonomic constraint on the Hamiltonian system.

Ref:  arXiv:2105.01655
Nov '21
The Institute of Optics / University of Rochester
Miguel Alonso
The structure of light: tailoring and measuring intensity, phase and polarization
Download PDF zoom @12:15 pm
In this talk I will describe some theoretical aspects of the degrees of freedom of light fields such as amplitude, phase, coherence and polarization. The description of these properties often employs geometric representations that highlight not only their properties but also their analogies with other physical phenomena. I will also present several experimental techniques based on these theoretical methods, for applications such as multicore fiber endoscopes, beam shaping, and superresolution fluorescence microscopy.
Nov '21
University of Chicago
Edwin Kite
Mars and the Problem of Planetary Habitability
Download PDF Zoom @12:15 pm
Can we identify simple rules for planetary evolution? Are the processes that generate planetary habitability common or rare? To answer these questions, Mars exploration is vital. What allowed rivers and lakes on Early Mars, which received just 1/3 of the modern Earth’s insolation? And why did Mars’ surface become uninhabitable? I will explain how data from spacecraft missions reveal a rich stratigraphic record of climate-sensitive deposits, allowing models to be tested. We have found that the greenhouse effect of high-altitude water ice clouds is a possible explanation for the warm climates – but only if the surface was arid, consistent with the geologic record. While lake-forming climates on Mars occurred over a time span of >1 Gyr, now Mars’s surface is too cold and dry for life. I will discuss ways in which Martian surface habitability could be re-enabled.
Finally, I will preview planned observations with James Webb Space Telescope to check for atmospheres on rocky exoplanets and - for the first time - take a spectrum of a rocky exoplanet's surface.
Nov '21
Queens College of CUNY and Ames Lalboratory
Joshua Aftergood
Developing dynamical probes of quantum spin liquids inspired by techniques from spintronics
Download PDF Zoom @12:15 pm
This talk is broken into two sections. In the first, we propose an experimental method utilizing a strongly spin-orbit coupled metal-to-quantum magnet bilayer as a probe of quantum magnets lacking long range magnetic order, e.g., quantum spin liquids, via examination of the voltage noise spectrum of the metal layer. The bilayer is held in thermal and chemical equilibrium, and spin fluctuations arising across the interface are converted into voltage fluctuations in the metal as a result of the inverse spin Hall effect. We elucidate the theoretical workings of the bilayer system and provide precise predictions for the frequency characteristics of the enhancement to the ac electrical resistance measured in the metal layer for three candidate quantum spin liquid models. Application to the Heisenberg spin-1/2 kagome lattice model should allow for the extraction of any spinon gap present. A quantum spin liquid consisting of fermionic spinons coupled to a U(1) gauge field should show subdominant Ω^{4/3} frequency scaling of the resistance of the coupled metal. Finally, if the magnet is well-captured by the Kitaev model in the gapless spin liquid phase, then the proposed bilayer can extract the two-flux gap energy which arises in spite of the gapless spectrum of the fermions. We therefore show that spectral analysis of the ac resistance in the metal in a single interface, equilibrium bilayer can test the relevance of quantum spin liquid models to a given candidate material.

In the second section, we examine the temperature dependence of the dc conductivity of a disordered 2d metal when it is interfaced with an insulating paramagnet. The metal contains short-range, isotropic impurities, we consider quantum interference effects, and we include Rashba spin-orbit interactions that arise due to structural inversion symmetry breaking at the interface. The paramagnet acts as a bath of spin fluctuations from the perspective of the metal layer, and the presence of spin fluctuations renormalizes the metallic conductivity. If the affixed insulator is a gapless quantum spin liquid, we show that a lnT correction arises in the absence of spin-orbit interactions while accounting for spin-orbit interactions induces a more divergent T^{-1/2} correction. For temperatures T > T^* ~ 10mK, the lnT correction dominates, whereas for T < T^* the T^{-1/2} correction arising due to spin-orbit interactions takes over. We therefore show that a bilayer as here explicated can act as an all-electrical probe of quantum spin liquid ground states.
Nov '21
University of Maryland
Rajarshi Roy
Seeing the Light: Photons, Complexity and Randomness
Download PDF zoom @12:15 pm
Dec '21
Next Event
University of Science and Technology of Hong Kong
Zhao-Qing Zhang
Anisotropic exceptional points of arbitrary order and some experimental demonstrations
In non-Hermitian systems, an exceptional point of order N (EPN) is characterized by the coalescences of N eigenvalues and the corresponding eigenvectors. In the vicinity of an EPN, both eigenvalues and phase rigidity can have a variety of critical behaviors depending on how the singularity is approached.  In this talk, I will first discuss a class of non-Hermitian systems with asymmetric hoppings. The EPs in such systems can form continuous loops. They can be in the form of multiple ellipses of EP2s or a circle of EPNs.  For any EP on a loop, the critical behavior is anisotropic. It has different critical exponents when the EP is approached from two orthogonal directions in the parameter space. We study the critical exponents of the eigenvalues and phase rigidity of an EP of arbitrary order N. I will then show an experimental realization of anisotropic EP2 in a system of two coupled acoustic cavities with different losses. I will also show an experimental realization of EP3 in a system of three acoustic cavities. The EP3 is found to be an intersection point of multiple EP arcs of order 2 exhibiting a cusp-like singularity. The EP arcs segment the parameter space and give rise to a hybrid topological invariant. Different winding numbers are found when the EP3 is encircled along a loop in different parameter planes. 

[1] Y-X Xiao et al., Phys. Rev. B 99, 241403 (R) (2019).
[2] K. Ding et al., Phys. Rev. Lett. 121, 085702 (2018).
[3] W. Tan et al., Science 370, 1071 (2020).