Abstract
Hyperbolic phonon polaritons (HPhPs) are hybrid excitations of light and coherent lattice vibrations that exist in strongly optically anisotropic media, including two-dimensional materials (e.g., MoO3). These polaritons propagate through the material's volume with long lifetimes, enabling novel mid-infrared nanophotonic applications by compressing light to sub-diffractional dimensions. Here, the dispersion relations and HPhP lifetimes (up to ≈12 ps) in single-crystalline α-MoO3 are determined by Fourier analysis of real-space, nanoscale-resolution polariton images obtained with the photothermal induced resonance (PTIR) technique. Measurements of MoO3 crystals deposited on periodic gratings show longer HPhPs propagation lengths and lifetimes (≈2×), and lower optical compressions, in suspended regions compared with regions in direct contact with the substrate. Additionally, PTIR data reveal MoO3 subsurface defects, which have a negligible effect on HPhP propagation, as well as polymeric contaminants localized under parts of the MoO3 crystals, which are derived from sample preparation. This work highlights the ability to engineer substrate-defined nanophotonic structures from layered anisotropic materials.
Highlights
The ability to confine and to guide light at length scales smaller than its free-space wavelength (λ0) enables numerous nanophotonic applications, including surfaceenhanced absorption and scattering [1,2,3,4,5], nanoscale waveguides [6,7,8], and non-linear optics [9, 10]
In scattering-type scanning near-field optical microscopy (s-SNOM) and photothermal induced resonance (PTIR) real-space maps, hyperbolic PhPs (HPhPs) appear as periodic fringes with spacing proportional to the flake thickness (Figure S1)
The polariton fringes observed in PTIR measurements result from especially intense local heating and thermal expansion of the sample that occurs in regions where the strong tipenhanced near-field overlaps with the polariton field [36]
Summary
The ability to confine and to guide light at length scales smaller than its free-space wavelength (λ0) enables numerous nanophotonic applications, including surfaceenhanced absorption and scattering [1,2,3,4,5], nanoscale waveguides [6,7,8], and non-linear optics [9, 10]. Phonon polaritons (PhPs), by contrast, couple light with optical phonons in a polar crystal These excitations can exist only within spectral ranges, known as reststrahlen bands, delimited by transverse optical (TO) and longitudinal optical (LO) phonon pairs [14, 15]. In which the real permittivity differs in sign along orthogonal principal axes, support hyperbolic PhPs (HPhPs) within the material’s volume, propagating at angles determined by the wavelength-dependent permittivity [14, 16, 17].
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