Acceleration of Subwavelength Polaritons by Engineering Dielectric-Metallic Substrates

F. H. Feres, R. A. Mayer, I. D. Barcelos, R. O. Freitas and F. C. B. Maia

ACS Photonics 7, 1396 (2020)
By synchrotron infrared nanospectroscopy, we demonstrate modulation of momentum and group velocity of subdiffractional hyperbolic phonon-polaritons (HP2), in hexagonal boron nitride nanocrystals, by varying the SiO2 film thickness in the hBN/SiO2/Au heterostructure. We reveal acceleration of the HP2 pulse in a hBN/(SiO2 wedge)/Au heterostructure with gradient of the SiO2 thickness. The acceleration is explained by a semiclassical modeling considering the polariton pulse as a free quantum particle with effective mass dependent on its group velocity. In quantitative agreement with simulations and semiempirical analysis, the modeling predicts an average acceleration of 1.5 × 1018 m·s–2 close to that of ∼ 1.45 × 1018 m·s–2 obtained from experimental inputs. From the fundamental aspect, the polariton acceleration allows discussing the undulatory-corpuscular behavior of polariton quasi-particles. The acceleration induced by the wedge is a general effect that can provide for control of the polariton pulse dynamics, which is compelling for future polaritonic devices.