Reconfigurable infrared hyperbolic metasurfaces using phase change materials.

Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, dynamically changing the properties of metasurfaces can be a major challenge. Here we demonstrate a reconfigurable hyperbolic metasurface comprised of a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with the phase-change material (PCM) single-crystal vanadium dioxide (VO2). Metallic and dielectric domains in VO2 provide spatially localized changes in the local dielectric environment, enabling launching, reflection, and transmission of hyperbolic phonon polaritons (HPhPs) at the PCM domain boundaries, and tuning the wavelength of HPhPs propagating in hBN over these domains by a factor of 1.6. We show that this system supports in-plane HPhP refraction, thus providing a prototype for a class of planar refractive optics. This approach offers reconfigurable control of in-plane HPhP propagation and exemplifies a generalizable framework based on combining hyperbolic media and PCMs to design optical functionality.

Reliable passivation of black phosphorus by thin hybrid coating.

Black phosphorus (BP) possesses several extraordinary physical properties, which include in-plane anisotropy, thickness dependent direct bandgap and high carrier mobility. These physical properties make BP highly desirable from the point of view of fundamental science and modern optoelectronics applications. The excitement about this material has always been accompanied by unreserved skepticism due to its extraordinary degradation under ambient conditions. Here we show ambient degradation of exfoliated BP can be effectively suppressed using thin layer of hybrid metal organic chemical vapor deposition coating of boron nitride (BN) followed by atomic layer deposition coating of Al2O3. We have extensively studied the time dependent surface, optical and electrical properties of BP encapsulated by BN and/or Al2O3 using nanoscale infrared imaging and I-V characterizations. Our results show hybrid thin layer (∼5 nm) BN/Al2O3 coated BP exfoliated on SiO2 substrate is protected from degradation in ambient for over 6 months, much longer than those coated only by BN or Al2O3 layers. Our theoretical modeling of the experimental degradation growth pattern shows that the influence of neighboring elements on the degradation of a given element is minimal for BP flakes with hybrid coating. Electrical characterization further confirms the effectiveness of BN/Al2O3 as encapsulation layer and gate dielectrics with minor changes after several weeks.