Patterning a hyperbolic metasurface: Structured metasurfaces potentially enable the control of the propagation direction of excitations on the material's surface. However, the high losses associated with the materials used to date has led to relatively short lifetimes for the excitations. Li et al. patterned a subwavelength grating into a layer of hexagonal boron nitride (hBN) and found that the lifetime and propagation length of the excitations could be much longer. Direct imaging of the polariton excitations illustrates that hBN can be a viable platform for nanophotonic circuits. Science , this issue p. 892

Hexagonal boron nitride (h-BN) has been predicted to exhibit an in-plane thermal conductivity as high as ~ 550 W m⁻¹ K⁻¹ at room temperature, making it a promising thermal management material. However, current experimental results (220–420 W m⁻¹ K⁻¹) have been well below the prediction. Here, we report on the modulation of h-BN thermal conductivity by controlling the B isotope concentration. For monoisotopic ¹⁰B h-BN, an in-plane thermal conductivity as high as 585 W m⁻¹ K⁻¹ is measured at room temperature, ~ 80% higher than that of h-BN with a disordered isotope concentration (52%:48% mixture of ¹⁰B and ¹¹B). The temperature-dependent thermal conductivities of monoisotopic h-BN agree well with first principles calculations including only intrinsic phonon-phonon scattering. Our results illustrate the potential to achieve high thermal conductivity in h-BN and control its thermal conductivity, opening avenues for the wide application of h-BN as a next-generation thin-film material for thermal management, metamaterials and metadevices.