Extracting the Infrared Permittivity of SiO2 Substrates Locally by Near-Field Imaging of Phonon Polaritons in a van der Waals Crystal

Abstract

Layered materials in which individual atomic layers are bonded by weak van der Waals forces (vdW materials) constitute one of the most prominent platforms for materials research. Particularly, polar vdW crystals, such as hexagonal boron nitride (h-BN), alpha-molybdenum trioxide (α-MoO3) or alpha-vanadium pentoxide (α-V2O5), have received significant attention in nano-optics, since they support phonon polaritons (PhPs)―light coupled to lattice vibrations― with strong electromagnetic confinement and low optical losses. Recently, correlative far- and near-field studies of α-MoO3 have been demonstrated as an effective strategy to accurately extract the permittivity of this material. Here, we use this accurately characterized and low-loss polaritonic material to sense its local dielectric environment, namely silica (SiO2), one of the most widespread substrates in nanotechnology. By studying the propagation of PhPs on α-MoO3 flakes with different thicknesses laying on SiO2 substrates via near-field microscopy (s-SNOM), we extract locally the infrared permittivity of SiO2. Our work reveals PhPs nanoimaging as a versatile method for the quantitative characterization of the local optical properties of dielectric substrates, crucial for understanding and predicting the response of nanomaterials and for the future scalability of integrated nanophotonic devices.

Manipulating the Self‐Trapped Excitons in the Lead Iodide/Hexagonal Boron Nitride van der Waals Heterostructures

“…As optical properties of propagating phonon polaritons are extremely sensitive to the dielectric environment, [ 17 ] the variation of the dielectric environment can be experimentally inferred by near‐field imaging of the propagation of phonon polaritons. Using scattering‐type scanning near‐field optical microscopy, the phonon polaritons behaviors of hBN in PbI 2 /hBN heterostructures are investigated at nanoscale resolution.…”

“…The dielectric function can be derived from the relationship of the wavelength (𝜆 p ) and propagation length (L p ) of phonon polaritons. [17,18] The extracted 𝜆 p and L p in the heterostructure region are smaller than the corresponding 𝜆 p and L p in the bare hBN region (Figure 2D), indicating an enlarged value for the dielectric function for the PbI 2 /hBN heterostructures when compared with hBN. [19] It has been widely studied that the band structure and excitonic energy of 2D vdW materials strongly correlate with the surrounding dielectric environment due to dielectric screening induced modulation of the behavior of charge carriers.…”

Focusing of Hyperbolic Phonon Polaritons by Bent Metal Nanowires and Their Polarization Dependence

“…The simulation results in Figure show the E z distribution obtained after changing the polarization direction of the incident light, where the direction of polarization is indicated in Figures –. The permittivity of α-MoO 3 crystals was calculated by the Lorentz model, , and the permittivity of Ag and the permittivity of the silicon dioxide substrate were taken from references , .…”

Guided Polaritons along the Forbidden Direction in MoO3 with Geometrical Confinement

“…The SiO 2 is treated as the substrate (infinite thick) in the analytical calculations, and the dielectric function is taken from ref . The dielectric function of MoO 3 is taken from ref .…”