Control of Nonreciprocal Surface Plasmon Polariton Transport in Weyl Semimetals via Optical Pumping and Strain Engineering

Weyl semimetals (WSMs) are a material of emerging interest due to their distinct electronic band structure, which supports pairs of chiral Weyl nodes formed from intersecting, linearly dispersing bands. The Weyl nodes in a pair each act as a source and a sink of Berry curvature respectively, resulting in a net flux of Berry curvature between them and zero Berry flux outside of them in momentum space. The flux of Berry curvature between the Weyl nodes gives rise to the unique, conductive Fermi arc surface states of WSMs.<br/>Either time-reversal symmetry or inversion symmetry breaking is required for the existence of a WSM. Time-reversal symmetry breaking gives rise to the anomalous Hall effect in WSMs. On the other hand, inversion symmetry breaking WSMs have exhibited an exceptionally large bulk photogalvanic effect (PGE) under illumination from linearly polarized infrared (IR) light. This linear bulk PGE results in part from the Berry curvature induced shirt current, in which an electron excited by linearly polarized light experiences a spatial shift upon transition from the valence to the conduction band, resulting in a photocurrent without the need for a p-n junction or external field. The detection of mid-IR light with conventional noncentrosymmetric semconductors is limited by a lack low energy bandgap materials but is enabled in a WSM by the gapless nature of its bandstructure. The use of the bulk PGE in WSMs has great potential for IR imaging and energy harvesting applications and for surpassing the efficiency limits inherent to p-n junction type detectors [1,2].<br/>In addition to exploiting the bulk PGE, WSM surfaces can be engineered to control the <i>surface</i> photogalvanic effects, which opens the door to manipulating both bulk and surface photocurrents in WSMs for enhanced IR detection and devices. Surfaces of WSMs with broken time-reversal symmetry also support non-reciprocal surface plasmon-polariton modes (SPP) propagation, that enable directional energy transfer, Kirchhoff’s law violation, and tunable near-field radiative heat transfer [3].<br/>A key step in the development of WSM SPP engineering is to establish means to control them by external stimuli, especially optical stimuli. In this work, we model the impact of the bulk and surface photogalvanic effects as well as material strain on the WSMs surface states. Recent works have shown the Lorentz reciprocity of materials is broken by applying dc currents which produce a Doppler frequency shift of the electron plasma (Fizeau drag) [4]. Here, we exploit the fact that spontaneous photocurrent in WSMs acts as a dc current in the illuminated sample and imparts Fizeau drag on the electron plasma, providing an additional mechanism to control the nonreciprocal behavior of WSM SPPs by optical means. We also compare the impact of the PGE-induced Fizeau drag to the impacts of strain [5] on the SPP modes. A combination of these two mechanisms enables effective means for both static and dynamic control of nonreciprocal SPP propagation.<br/>[1] S. Pajovic, et al, Radiative heat and momentum transfer from materials with broken symmetries: opinion, Optical Materials Express, 11(9), 3125-3131, 2021.<br/>[2] S.V. Boriskina, M. Blevins, S. Pajovic, There and Back Again: the nonreciprocal adventures of light, Opt. Photon. News, Sept. 2022.<br/>[3] S. Pajovic, et al, Intrinsic nonreciprocal reflection and violation of Kirchhoff’s law of radiation in planar type-I magnetic Weyl semimetal surfaces, Phys. Rev. B 102(16), 165417, 2020.<br/>[4] K. Y. Bliokh, et al, Electric-current-induced unidirectional propagation of surface plasmon-polaritons, Opt. Lett. 43, 963-966, 2018.<br/>[5] Bugaiko, O. & Gorbar, E. & Sukhachov, Pavlo. (2020). Surface plasmon polaritons in strained Weyl semimetals. Physical Review B. 102. 10.1103/PhysRevB.102.085426.<br/>This research has been supported by the Army Research Office (W911NF-13-D-0001), Lincoln Laboratory, Massachusetts Institute of Technology (ACC-777), and a Draper Fellowship to M.B.