Hexagonal boron nitride (hBN) with a single boron isotope have many enhanced physical, thermal and optical properties compared to the most common hBN with the natural distribution of boron (19.9 at. ...% 10B and 80.1 at. % 11B). These property differences can significantly improve the device performance in applications, such as neutron detectors, nanoscale electronics, and optical components. In this study, a new method for the growth of large-scale, high-quality monoisotopic hBN single crystals, i.e., h10BN and h11BN, was developed. hBN single crystals were grown using a nickel–chromium solvent and pure boron and nitrogen sources at atmospheric pressure. The clear and colorless crystals have a maximum domain size of around 1 mm. Raman measurements demonstrate that the crystals produced with this method are pure hBN phase with low defect density, and the spectral peaks vary with the boron isotope concentrations. X-ray photoelectron spectroscopy spectra show that the B–N bond in h11BN is slightly stronger than that in h10BN. The ability to produce crystals in this manner opens the door to isotopically engineering the properties and performance of hBN devices.
Metasurfaces with strongly anisotropic optical properties can support deep subwavelength-scale confined electromagnetic waves (polaritons), which promise opportunities for controlling light in ...photonic and optoelectronic applications. We developed a mid-infrared hyperbolic metasurface by nanostructuring a thin layer of hexagonal boron nitride that supports deep subwavelength-scale phonon polaritons that propagate with in-plane hyperbolic dispersion. By applying an infrared nanoimaging technique, we visualize the concave (anomalous) wavefronts of a diverging polariton beam, which represent a landmark feature of hyperbolic polaritons. The results illustrate how near-field microscopy can be applied to reveal the exotic wavefronts of polaritons in anisotropic materials and demonstrate that nanostructured van der Waals materials can form a highly variable and compact platform for hyperbolic infrared metasurface devices and circuits.
The limited memory retention for a ferroelectric field-effect transistor has prevented the commercialization of its nonvolatile memory potential using the commercially available ferroelectrics. Here, ...we show a long-retention ferroelectric transistor memory cell featuring a metal-ferroelectric-metal-insulator-semiconductor architecture built from all van der Waals single crystals. Our device exhibits 17 mV dec
operation, a memory window larger than 3.8 V, and program/erase ratio greater than 10
. Thanks to the trap-free interfaces and the minimized depolarization effects via van der Waals engineering, more than 10
cycles endurance, a 10-year memory retention and sub-5 μs program/erase speed are achieved. A single pulse as short as 100 ns is enough for polarization reversal, and a 4-bit/cell operation of a van der Waals ferroelectric transistor is demonstrated under a 100 ns pulse train. These device characteristics suggest that van der Waals engineering is a promising direction to improve ferroelectronic memory performance and reliability for future applications.
Abstract
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
10
B 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
10
B and
11
B). 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.
Polaritons formed by the coupling of light and material excitations enable light-matter interactions at the nanoscale beyond what is currently possible with conventional optics. However, novel ...techniques are required to control the propagation of polaritons at the nanoscale and to implement the first practical devices. Here we report the experimental realization of polariton refractive and meta-optics in the mid-infrared by exploiting the properties of low-loss phonon polaritons in isotopically pure hexagonal boron nitride interacting with the surrounding dielectric environment comprising the low-loss phase change material Ge
Sb
Te
. We demonstrate rewritable waveguides, refractive optical elements such as lenses, prisms, and metalenses, which allow for polariton wavefront engineering and sub-wavelength focusing. This method will enable the realization of programmable miniaturized integrated optoelectronic devices and on-demand biosensors based on high quality phonon resonators.
Abstract
Polaritons – coupled excitations of photons and dipolar matter excitations – can propagate along anisotropic metasurfaces with either hyperbolic or elliptical dispersion. At the transition ...from hyperbolic to elliptical dispersion (corresponding to a topological transition), various intriguing phenomena are found, such as an enhancement of the photonic density of states, polariton canalization and hyperlensing. Here, we investigate theoretically and experimentally the topological transition, the polaritonic coupling and the strong nonlocal response in a uniaxial infrared-phononic metasurface, a grating of hexagonal boron nitride (hBN) nanoribbons. By hyperspectral infrared nanoimaging, we observe a synthetic transverse optical phonon resonance (strong collective near-field coupling of the nanoribbons) in the middle of the hBN Reststrahlen band, yielding a topological transition from hyperbolic to elliptical dispersion. We further visualize and characterize the spatial evolution of a deeply subwavelength canalization mode near the transition frequency, which is a collimated polariton that is the basis for hyperlensing and diffraction-less propagation.
Hyperbolic phonon polaritons (HPhPs) are generated when infrared photons couple to polar optic phonons in anisotropic media, confining long-wavelength light to nanoscale volumes. However, to realize ...the full potential of HPhPs for infrared optics, it is crucial to understand propagation and loss mechanisms on substrates suitable for applications from waveguiding to infrared sensing. We employ scattering-type scanning near-field optical microscopy (s-SNOM) and nano-Fourier transform infrared (FTIR) spectroscopy, in concert with analytical and numerical calculations, to elucidate HPhP characteristics as a function of the complex substrate dielectric function. We consider propagation on suspended, dielectric and metallic substrates to demonstrate that the thickness-normalized wavevector can be reduced by a factor of 25 simply by changing the substrate from dielectric to metallic behavior. Moreover, by incorporating the imaginary contribution to the dielectric function in lossy materials, the wavevector can be dynamically controlled by small local variations in loss or carrier density. Counterintuitively, higher-order HPhP modes are shown to exhibit the same change in the polariton wavevector as the fundamental mode, despite the drastic differences in the evanescent ranges of these polaritons. However, because polariton refraction is dictated by the fractional change in the wavevector, this still results in significant differences in polariton refraction and reduced sensitivity to substrate-induced losses for the higher-order HPhPs. Such effects may therefore be used to spatially separate hyperbolic modes of different orders and for index-based sensing schemes. Our results advance our understanding of fundamental hyperbolic polariton excitations and their potential for on-chip photonics and planar metasurface optics.
Abstract
Polaritons in two-dimensional materials provide extreme light confinement that is difficult to achieve with metal plasmonics. However, such tight confinement inevitably increases optical ...losses through various damping channels. Here we demonstrate that hyperbolic phonon polaritons in hexagonal boron nitride can overcome this fundamental trade-off. Among two observed polariton modes, featuring a symmetric and antisymmetric charge distribution, the latter exhibits lower optical losses and tighter polariton confinement. Far-field excitation and detection of this high-momenta mode become possible with our resonator design that can boost the coupling efficiency via virtual polariton modes with image charges that we dub ‘image polaritons’. Using these image polaritons, we experimentally observe a record-high effective index of up to 132 and quality factors as high as 501. Further, our phenomenological theory suggests an important role of hyperbolic surface scattering in the damping process of hyperbolic phonon polaritons.
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