RQC Seminar

225th RQC Seminar

• Speaker

  Dr. Steven Mark Anlage
  ( Quantum Materials Center, University of Maryland )

• Date

  16:00-17:00 (4:00 p.m. - 5:00 p.m.), September 22, 2025 Monday)

• Venue

  Hybrid(ZOOM・ Wako Main Research Bldg. 3F 345-347 Seminar Room / 研究本館3階 セミナー室 (345-347) (C01))

• Title

  Creation and Manipulation of Topological Scattering Singularities in Generic non-Hermitian Scattering Systems

• Inquiries

  norilab_rqc_assist[at]ml.riken.jp

Abstract
The dream of completely controlling scattering in complex non-Hermitian settings, such as strongly scattering media and chaotic enclosures, has intrigued researchers for decades and motivated many studies in wavefront shaping, perfect absorption, metasurface development, etc. Scattering singularities, such as diverging complex time delays, and loss of a complete basis to describe scattering, provide an organizing principle for control of complex scattering systems. By taking active control over the boundary conditions in complex scattering environments which lack artificially-imposed geometric symmetries, we demonstrate via microwave experiments the ability to manipulate the spectrum of the scattering operator. Motivated by the perspective of Michael Berry about complicated light scattering, we generalize the concept of singularity speckle patterns to arbitrary two-dimensional parameter spaces and any complex scalar function that describes wave phenomena involving complicated scattering. In wave scattering systems specifically, we are often concerned with singularities associated with complex zeros of the scattering matrix S. Some examples are Coherent Perfect Absorption (CPA), Reflectionless Scattering Modes (RSMs), Transmissionless Scattering Modes (TSMs), and S-matrix Exceptional Points (S-EPs). All scattering singularities empirically share a universal statistical property: the tail of the probability distribution function of a quantity that diverges only at a singularity has the form of a -3-power law. The heavy tail of the distribution provides an estimate for the likelihood of finding a given singularity in a generic scattering system. We use these universal statistical results to determine that homogeneous system loss is the most important parameter determining singularity density in a given parameter space. Finally, we discuss events where distinct singularities coincide in parameter space, which result in higher order composite singularities that have new applications. These higher order singularities are not topologically protected, and we do not find universal statistical properties for them. The experimental work is carried out in one-dimensional complex microwave graphs, two-dimensional microwave billiards, and three-dimensional enclosures. We support our empirical results from microwave experiments with Random Matrix Theory simulations and conclude that all results presented are found in all generic scattering systems.