Fig. 2: Wavelength-dependent near-field modes measured by PEEM with x-polarization at normal incidence.

a Hyperbolic plasmon polariton modes from 1.03 eV (1200 nm) to 1.38 eV (900 nm) supported by a ~ 37 nm MoOCl₂ flake, excited from SiO₂-coated Au nanoparticles (5 nm SiO₂ @ 200 nm Au). b Corresponding iso-frequency contours (IFCs) (fast Fourier transform (FFT) of (a)), fitted with hyperbolas (white dashed lines). c Mode distributions at shorter wavelengths, from 1.77 eV (700 nm) to 2.25 eV (550 nm), showing the transition from hyperbolic mode to elliptical air mode. d Corresponding FFT images of (c). The white dashed cycles in (d) signify k₀. The color scales in (a, b) are also applied to (c, d). e Optical dark-field image of the measured sample, the yellow circle denotes the measured area by PEEM. f Variation of measured open angles (2θ) of hyperbolas in (b) with wavelengths (red dots), and the corresponding calculated curve from the measured permittivity.