Engineering phonon polaritons in van der Waals heterostructures to enhance in-plane optical anisotropy

Abstract

Van der Waals (vdW) heterostructures assembled from layers of two-dimensional materials have attracted considerable interest due to their novel optical and electrical properties. Here, we report a scattering-type scanning near-field optical microscopy study of hexagonal boron nitride on black phosphorus (h-BN/BP) heterostructures, demonstrating the first direct observation of in-plane anisotropic phonon polariton modes in vdW heterostructures. Notably, the measured in-plane optical anisotropy along the armchair and zigzag crystal axes exceeds the ratio of refractive indices of BP in the x-y plane. We explain that this enhancement is due to the high confinement of the phonon polaritons in h-BN. We observe a maximum in-plane optical anisotropy of αmax = 1.25 in the frequency spectrum at 1405 to 1440 cm−1. These results provide new insights into the behavior of polaritons in vdW heterostructures, and the observed anisotropy enhancement paves the way to novel nanophotonic devices and to a new way to characterize optical anisotropy in thin films.

Symmetry‐Reduction Enhanced Polarization‐Sensitive Photodetection in Core–Shell SbI3/Sb2O3 van der Waals Heterostructure

“…For example, the photocurrent anisotropy ratio of polarization‐sensitive photodetector based on pure BP was 3.5, while the photocurrent anisotropy ratio of the device increased to 6 and even 10.76 by constructing the BP/ReSe 2 and BP/InSe heterostructure photodetectors, respectively . Further results indicated that fabricating heterostructure could be a desirable and effective method to improve the in‐plane anisotropy by symmetry‐reduction in low‐dimensional materials . However, BP, BP‐based heterostructures and numerous 2D anisotropy semiconductors have relatively narrow bandgap, and complicated optical systems are required to regulate them within the desirable short spectral range.…”

Functional Mid‐Infrared Polaritonics in van der Waals Crystals

“…Interestingly, it is also possible to induce anisotropy to the dispersion of polaritons in an isotropic 2D material by stacking it with an anisotropic one. For example, in a very recent experimental study, Chaudhary et al demonstrated h‐BN phonon polaritons exhibiting an in‐plane anisotropy induced by the BP substrate . The study demonstrates the anisotropy of 1.25 for the phonons in 40 nm‐thick layer of h‐BN at 1405–1440 cm −2 , caused by the strong in‐plane anisotropy of BP, as discussed in Section .…”

“…Furthermore, since the unique properties of 2D crystals stem from the periodicity of their atomic structure, free carriers and polariton species populating such materials are highly susceptible to the perturbation of the crystalline atomic structure. Therefore, properties of polaritons in 2D materials can be engineered at the quantum level via: (1) vertical stacking of vdW crystals, which leads to a hybridization of polariton species or control over the level of their anisotropy; (2) imposing a superlattice periodicity on 2D materials, which causes significant alteration on the polariton characteristics in nontrivial manner …”

Phonon Polaritons and Hyperbolic Response in van der Waals Materials

“…Permittivity engineering of the substrate also affects the damping/propagation length of phonon polaritons (line profiles in Figure a): a lower damping was reported for phonon polaritons from suspended hBN (blue curve) compared with that from hBN on SiO 2 substrate (red curve) due to the elimination of substrate material loss for propagating polaritons (Im ε air << Im ε SiO2 ). Based on the same physics, the wavelength of phonon polaritons exhibit orientation dependence ( a < b ) from hBN on black phosphorus (Figure c), which possesses an anisotropic permittivity in the mid‐IR.…”

“…c) The s‐SNOM image of hyperbolic phonon polaritons from hBN on black phosphorus at frequency of 1410 cm −1 . Reproduced with permission . Copyright 2019, American Association for the Advancement of Science.…”