Concept of diffusion is widely used to describe propagation of light through multiple scattering media such as clouds, interstellar gas, colloids, paint, biological tissue, etc. Such media are often called random. This terminology is, however, misleading. Notwithstanding its complexity, the process of wave propagation is entirely deterministic – uniquely defined by the exact positions of scattering centers and the shape of the incident wavefront – making it possible to deduce the precise pattern of wave field throughout the system. Technological advances over the last decade enabled one to synthesize an arbitrary wavefields opening new frontier in light control inside strongly scattering media.

Feasibility of the coherent control necessitates a general framework for predicting and understanding the ultimate limit for a targeted energy delivery into a
diffusive system. In this talk, we will discuss such scientifically and technologically important questions as “How can one systematically find the incident wavefront that optimally deposits energy into a target region of arbitrary size and shape, deep inside a diffusive medium?” and “What is the ultimate limit on the energy  enhancement in a region?” Predictable energy delivery opens the door to numerous  applications, e.g., optogenetic control of cells, photothermal therapy, as well as probing and manipulating photoelectrochemical processes deep inside nominally opaque media.

Alexey Yamilov (QC alum, PhD 2001) is a professor of Physics at Missouri S&T. His research activities are in the areas of theoretical and computational condensed matter physics and optics, he employs a variety of analytical and numerical techniques to study transport of the electromagnetic, electronic and other types of waves in the inhomogeneous media, where a line-ofsight propagation is hindered by scattering. The purpose of the research is to uncover and exploit physical phenomena caused by wave interference with the goals of: (i) understanding new behaviors originating not only from the fundamental laws of physics but also from complexity of the system itself; (ii) developing techniques for coherent control of wave propagation; (iii) designing artificial structures/systems with a set of desired properties.
 

A crucial chapter in our cosmic history remains almost entirely unobserved. During the first several hundred million years after the big bang, known as the “Cosmic Dawn”, our Universe was transformed from a cold, dark, and uniform expanse of hydrogen and helium into the vibrant, chemically enriched realm of stars and galaxies that we observe today. The drivers of this transformation were the first stars, galaxies, and black holes which chemically enriched, heated and ionizing material from which future generations of stars and galaxies like our own Sun and Milky Way would eventually form. These first sources set the stage for the cosmos we live in today. 

While infrared telescopes like the James Webb promise to observe the very brightest of these early galaxies, the vast majority are too faint to ever detect directly. To get around this, radio astronomers are attempting to constrain the properties of these first sources by observing their statistical impact on 21cm emission from intergalactic hydrogen gas during the Cosmic Dawn. By the time these radio waves reach earth, they are redshifted to two meter wavelengths. Using similar techniques, we can also use measurements of large-scale Hydrogen emission to better understand the nature of Dark Energy and fundamental physics. 

Unfortunately, foreground radio emission from nearby galaxies and our own is five orders of magnitude brighter. I will describe recent advances in our understanding of how to design close packed radio interferometers and data analysis that will allow us to distinguish the faint cosmological signal from the vastly brighter foregrounds; focusing on the Hydrogen Epoch of Reionization Array, a next generation 21cm experiment being commissioned in South Africa. I will finish by discussing areas where I expect significant improvements can be made in the coming years.

97% of stars in the Milky Way will conclude their evolution as compact white dwarf stars, so white dwarfs serve as observational boundary conditions to constrain theories of stellar evolution.  Compressing the mass of the Sun into the volume of the Earth, these objects also provide remote laboratories for probing extreme physics.  As astronomy enters a “big data” era characterized by large surveys recording movies of the dynamic universe, the field of stellar astronomy is experiencing a renaissance.  In this talk, I share my vision for two key projects that utilize the newest survey data to advance the state of white dwarf research: uncovering the population of planets that orbit white dwarf stars, and using stellar pulsations to achieve the first reliable seismic constraints of white dwarf interior structures.

Among the overall stellar population, stars with masses several times larger than that of the Sun (roughly 8 times and above) are rare but play a significant role in shaping their environments. They are the progenitors of spectacular supernova explosions and exotic compact objects, namely neutron stars and black holes. One of the most important physical processes characterizing these stars and shaping their evolution is their strong, radiatively-driven winds. In this talk, I will briefly introduce the winds of massive stars and their main characteristics. Then, I will discuss the various structures that arise within the wind and their observable consequences. Finally I will focus on the interaction between magnetic fields and winds in a subset of these stars, and how multi-wavelength observations can help us decode the mysteries of these objects.

Major technological advances have enabled the discovery of a small number of directly imaged exoplanets. These imaged worlds can be studied in far greater detail than exoplanets detected by indirect methods such as transit and radial velocity techniques. Next-generation telescopes such as the recently launched James Webb Space Telescope and the upcoming 30-m telescopes (e.g. ELT, TMT, GMT) will enable direct exoplanet characterization. Based on the handful of exoplanets studied to date, it is clear that the interpretation of future observational data hinges on a thorough understanding of their atmospheric processes. In this talk I will discuss our past, current and future efforts to investigate the atmospheres of extrasolar worlds. In particular, I will discuss how a combination of observational and computational techniques will reveal three critical atmospheric processes: clouds, winds and aurorae. Each of these processes are well-studied in our own Solar System and we can now begin to study them on worlds beyond our own.

It is widely believed that the ability to predict, manage, and exploit light-matter interactions could produce a major conceptual and practical impact on various applications utilizing electromagnetic waves. In this respect our understanding of resonant cavity-enhanced phenomena has resulted in fundamentally new predictive models for light-matter interactions. Simply put, light-matter interactions depend on the strength of the electric or magnetic field components of the electromagnetic wave, which can be very non-uniform in complex media. Our approach is thus the following: we first engineer complex media with useful spatial distributions of electric and magnetic fields, and then introduce optically active components (magnetic, nonlinear, phase-change, etc.) at specific locations in the medium, to enhance desired responses and suppress unwanted ones. Multilayer photonic structures (a.k.a. 1-D photonic crystals) are especially suitable for controlling light-matter interactions. They are simpler to design, easier to fabricate and optimize. Here we illustrate our approach to the design of multilayer structures by the example of wide-aperture omnidirectional isolator and reflective optical limiter. The former is a nonreciprocal device that transmits light in only one direction, thereby eliminating adverse effects of back reflection in optical systems [1]. The latter is a photosensitive device protecting optical systems and components from damage caused by intense optical radiation [2]. 

[1] R. Kononchuk, C. Pfeiffer, I. Anisimov, N. Limberopoulos, I. Vitebskiy, and A.A. Chabanov, Wide-aperture layered-sheet isolator, Phys. Rev. Applied 12, 054046 (2019). 
[2] R. Kononchuk, S. Suwunnarat, M.S. Hilario, A.E. Baros, B.W. Hoff, V. Vasilyev, I. Vitebskiy, T. Kottos, A.A. Chabanov, A reflective mm-wave photonic limiter, Sci. Adv. 8, abh1827 (2022). 

To characterize scattering resonances, a useful tool is the complex resonance spectrum corresponding to eigenmodes (QNM) able to leak energy. However, for reflection/transmission problems (e.g. waveguides, gratings or screens), the complex resonance spectrum does not directly quantify transmission efficiency, and the question of good or perfect transmission is of great importance in many topics of wave physics: extraordinary optical transmission, topological states immune to backscattering, perfect transmission resonances, transmission eigenchannels through disordered media, reflectionless metamaterials or metasurfaces. 

We present here an alternative spectrum allowing one to identify situations where perfect transmissions occur [1]. The operator yielding the spectrum of reflectionless modes is non-hermitian, and it is PT-symmetric for systems with spatial mirror symmetry. Eigenmodes (reflectionless modes) and eigenvalues (reflectionless complex frequencies) will be presented in various scattering situation, from the simplest 1D setup to several 2D waveguide geometries.

[1] A.-S. Bonnet-Ben Dhia, L. Chesnel, V. Pagneux. Trapped modes and reflectionless modes as eigenfunctions of the same spectral problem. Proc. R. Soc. A, 474(2213), 20180050 (2018)