Long-Lived Phonon Polaritons in Hyperbolic Materials

• DOI: 10.1021/acs.nanolett.1c01562
• Publication Date: 2021-06-18T11:30:05Z
• Authors: Guangxin Ni; Alexander S. McLeod; Zhiyuan Sun; Joseph R. Matson; Chiu Fan Bowen Lo; Daniel A. Rhodes; Francesco L. Ruta; Samuel L. Moore; Rocco A. Vitalone; Ramon Cusco; Luis Artús; Lin Xiong; Cory R. Dean; James C. Hone; Andrew J. Millis; Michael M. Fogler; James H. Edgar; Joshua D. Caldwell; D. N. Basov
• Keywords: Van der Waals heterostructures; Hyperbolic materials; 02 engineering and technology; 0210 nano-technology; Phonon polaritons; Nanoinfrared imaging

Abstract Natural hyperbolic materials with dielectric permittivities of opposite sign along different principal axes can confine long-wavelength electromagnetic waves down to the nanoscale, well below the diffraction limit. This has been demonstrated using hyperbolic phonon polaritons (HPP) in hexagonal boron nitride (hBN) and -MoO3, among other materials. However, HPP dissipation at ambient conditions is substantial and its fundamental limits remain unexplored1,2. Here, we exploit cryogenic nano-infrared imaging to investigate propagating HPP in isotopically pure hBN and naturally abundant -MoO3 crystals. Close to liquid-nitrogen temperatures, the losses for HPP in isotopic hBN drop significantly, resulting in propagation lengths in excess of 25 micrometers, with lifetimes exceeding 5 picoseconds, thereby surpassing prior reports on such highly-confined polaritonic modes. Our nanoscale, temperature-dependent imaging reveals the relevance of acoustic phonons in hyperbolic polariton damping and will be instrumental in mitigating such losses for miniaturized middle infrared technologies operating at the liquid-nitrogen temperatures.