Genetic algorithm gradient ascent (GAGA) optimization of compact symmetry-breaking photonic crystals

Abstract

Fundamental limits of thermal radiation are imposed by Kirchhoff’s law, which assumes the electromagnetic reciprocity of a material or material system. Thus, breaking reciprocity can enable breaking barriers in thermal efficiency engineering¹. This thesis presents 1D photonic crystals composed of Weyl/Dirac semimetal and dielectric layers, whose structures are optimized to maximize the nonreciprocity of infrared radiation absorptance/emittance in planar and compact designs. Two different mechanisms to enable nonreciprocal infrared absorbers/emitters are simulated and compared – anomalous Hall effect in Weyl semimetals 2 and electric-current-induced Fizeau drag in either Dirac or Weyl semimetals3 . To engineer an ultra-compact absorber structure that does not require gratings or prisms to couple light, a genetic algorithm (GA) was used to maximize nonreciprocity in the design globally, followed by the application of the numerical gradient ascent (GAGA) algorithm as a local optimization to further enhance the design. The first absorber design takes advantage of the intrinsic nonreciprocity of time-reversal symmetry (TRS) breaking Weyl semimetals due to their pseudomagnetic field in momentum space. GAGA methodology is then applied to design and optimize a flat absorber using inversion (IS) breaking Weyl/Dirac semimetals as active layers, in which tunable nonreciprocity is induced through an applied DC current bias. This momentum bias imparts plasmon Fizeau drag, the drag of an electrical current on propagating surface plasmon polaritons (SPPs). A semi-classical theory recently developed is used to model SPP transport along interfaces of 3D semimetals under Fizeau drag3 . Lastly, in both cases the optimization algorithm accounts for both s- and p-polarized absorptance spectra to create a final design suitable for thermal applications, which maximizes the nonreciprocal absorptance of p-polarized light and simultaneously minimizes the parasitic, reciprocal absorptance of s-polarized light.