Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, all recent designs have exhibited concepts using geometrically fixed structures, or used materials with excessive propagation losses, thereby limiting potential applications. Here we show how to overcome these limitations using a reconfigurable hyperbolic metasurface comprising a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with a phase-change material (PCM), single crystal vanadium dioxide (VO2). Metallic and dielectric domains in VO2 provide spatially localized changes in the local dielectric environment to tune the wavelength of hyperbolic phonon polaritons (HPhPs) supported in hBN by a factor of 1.6. This contrasts with earlier work using surface phonon polaritons, where propagation could only be observed above a low-loss dielectric phase. We demonstrate the first realization of in-plane HPhP refraction, which obeys Snell’s law and the means for launching, reflecting and transmitting HPhPs at the PCM domain boundaries. To demonstrate practical applications of this platform, we show how hBN could be combined with either VO2 or GeSbTe glasses to make refractive nanophotonic waveguides and lenses. This approach offers control of in-plane HPhP propagation at the nanoscale and exemplifies a reconfigurable framework combining hyperbolic media and PCMs to design new optical functionalities including resonant cavities, beam steering and waveguiding.
Conventional optical components are limited to size-scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called “flat” optical components that beget abrupt changes in these properties over distances significantly shorter than the free space wavelength. While high optical losses still plague many approaches, phonon polariton materials have demonstrated long lifetimes for localized modes in comparison to plasmon-polariton based nanophotonics. Our work predicts a further 14-fold increase in the optic phonon lifetime and we experimentally report a ~3-fold improvement through isotopic enrichment of hexagonal boron nitride (hBN). We establish commensurate increases in the phonon polariton propagation length via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach towards realizing the loss control necessary for the development of phonon polariton based nanophotonic devices.
We report on the Raman analysis of the phonon lifetimes and decay channels of the A<SUB>1</SUB>(LO) and E<SUB>2</SUB>(high) phonons of single-crystalline bulk AlN grown using the sublimation- recondensation method. The temperature dependence of the phonon lifetimes was investigated from 10 K to 1275 K. Lifetimes of the A<SUB>1</SUB>(LO) phonon and the E<SUB>2</SUB>(high) phonon of 0.75 ps and 2.9 ps, respectively, were measured at 10 K. Our experimental results show that the A<SUB>1</SUB>(LO) phonons of AlN decay primarily into two phonons of equal energy (Klemens' decay channel), most likely longitudinal- acoustic phonons. AlN is therefore in great contrast to GaN, where a symmetric decay of the A<SUB>1</SUB>(LO) phonon is not possible due to a large energy gap between the acoustic and optical phonon branches. For the E<SUB>2</SUB>(high) phonon, we find an asymmetric phonon decay. Contributions from two- and three-phonon decay channels were used for the modeling of the temperature dependence of the E<SUB>2</SUB>(high) phonon lifetime. Phonon lifetimes and decay channels of the E<SUB>1</SUB>(LO), A<SUB>1</SUB>(TO) and E<SUB>1</SUB>(TO) phonons of AlN were also investigated.