Creation and Control of Scattering Singularities in non-Hermitian Systems

Abstract

The ability to controllably manipulate non-Hermitian wave scattering environments has been used to discover exotic scattering phenomena such as scattering exceptional points and coherent perfect absorption, and create numerous applications including signal routing, filtering, imaging, sensing, wireless power transfer, etc. We introduce the concept that many of these applications and phenomena are fundamentally governed by singularities of the scattering matrix. With this understanding, our demonstrated ability to control the location of scattering singularities can be used to enhance current applications and develop new ones. In generic complex scattering systems, there is an abundance of topologically protected scattering singularities corresponding to complex zeros of various functions of the scattering matrix. We show that with just three tunable parameters we are able to create the conditions for nearly any scattering singularity at arbitrary frequencies in such systems. With five tunable parameters, we demonstrate that more complex scenarios can be accomplished, such as making disparate singularities coincident or placing singularities at different frequencies. A benefit of systems with tunable parameters is that generic complex systems can be re-purposed into exhibiting many different useful properties solely by reconfiguring their tunable parameters. A particularly interesting scattering phenomenon is coherent perfect absorption, where a specific wavefront injected into the system is completely absorbed, with no energy reflected or transmitted through any channel. After creating the conditions for this singularity at an arbitrary preselected frequency, we can find and inject the coherent perfect absorption wavefront and demonstrate output-to-input power ratios as low as 1x10^{-10}, 2x10^{-8}, and 3x10^{-8} in a quarter bowtie billiard with two, three, and four ports, respectively.