Ultrafast nonlocal collective dynamics of Kane plasmon-polaritons in a narrow-gap semiconductor

The observation of ultrarelativistic Dirac, Weyl, and Kane fermions in condensed-matter systems has uncovered a cornucopia of novel phenomenology as well as a potential for effective ultrafast light engineering of novel states of matter. While the non-equilibrium properties of two- and three-dimensional (3D) hexagonal crystals have been studied extensively, our understanding of the photoinduced dynamics in 3D singlevalley ultrarelativistic materials is lacking. Here, we employ ultrafast scanning near-field optical spectroscopy to access and control non-equilibrium finite-momentum plasmon-polaritons in thin films of a prototypical narrow-band-gap semiconductor Hg0.81Cd0.19Te. We demonstrate that these collective excitations exhibit distinctly non-classical scaling with electron density characteristic of the ultrarelativistic Kane regime and experience mode splitting due to the coupling of charge oscillations at the top and bottom surface of the thin film. Our observation and ultrafast control of Kane plasmon-polaritons in a semiconducting material using light sources in the standard telecommunications fiber-optics window open a new avenue towards high-bandwidth coherent information processing in next-generation plasmonic circuits.