Science Advances

The PDF file includes:

• Note S1. Theoretical model for calculating the isofrequency surface of the PhPs
• Note S2. Simulation of the dipole-launched PhP waves on the α-MoO₃ surface
• Note S3. Simulation of the nanoantenna launched PhPs on the α-MoO₃
• Note S4. Determination of the crystalline direction of a typical α-MoO₃ flake
• Note S5. Simulation of the s-SNOM image using the phenomenological cavity model
• Note S6. Calculations on the complex reflectivity of the multilayer structure α-MoO₃/SiO₂
• Fig. S1. Schematic showing the isofrequency curves of the PhPs in the biaxial α-MoO₃ flake.
• Fig. S2. Simulations of the dipole-launched hyperbolic PhPs on the α-MoO₃ surface.
• Fig. S3. Propagation directions of the in-plane hyperbolic PhPs.
• Fig. S4. Silver nanoantenna-launched PhPs at various incidence frequencies.
• Fig. S5. Determination of the crystalline directions of the α-MoO₃ by Raman spectroscopy.
• Fig. S6. Near-field optical images showing in-plane anisotropic PhPs characteristics of the α-MoO₃.
• Fig. S7. Comparison of the experiment and simulation near-field images of the PhPs distributions illuminated by 986 cm⁻¹ (Band 3).
• Fig. S8. Scheme of the multi-layered structure consisted of air/α-MoO₃/SiO₂.
• Fig. S9. Optical image of the α-MoO₃ flake used for conducting the hyperspectral PiFM movies.
• Legends for movies S1 and S2.
• Table S1. Parameters used in calculating the relative permittivities (Eq. S1).

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Other Supplementary Material for this manuscript includes the following:

• Movie S1 (.mp4 format). Hyperspectral PiFM movie of the edge perpendicular to the 100 direction, which is adjacent to the corner 1 shown in fig. S9.
• Movie S2 (.mp4 format). Hyperspectral PiFM movie of the edge perpendicular to the 001 direction, which is in proximity of the corner 1 shown in fig. S9.