Fig. 2: Tunable polaritonic reversed CR.

a Experimental setup for scattering near-field microscopy, where infrared light (polarization field E₀) is shining on the metal nanowire in a controlled direction. b Experimental MoO₃ phonon polaritons with reversed CR (polarization field E₀ is along the x-axis), and c, no CR (polarization field E₀ is along the y-axis) with an excitation frequency of 971 cm⁻¹, respectively. The red dashed arrows are guidelines. The thickness of MoO₃ is about 380 nm. \({\theta }_{k}\) is equal to the angle between the longitudinal axis of metal nanowire and interference fringes of phonon polariton in MoO₃. SA is the near-field amplitude. Scale bars: 0.5 μm. d Extracted interference fringes of phonon polaritons at the same position (1.6 \(\mu\) m from the top of Ag nanowire) from b, and c. e The theoretically calculated IFCs of MoO₃ show that the wave vectors of polariton are shrunken as the excitation frequency increases (i.e., increased \({\theta }_{k}\)). f The summarized experimental and simulated extracted \({\theta }_{k}\) of MoO₃ phonon polariton at different excitation frequencies. Experiment: red points. Simulation: grey line. Each error bar corresponds to a different line profile within a single scan image. Note: In the rectangular coordinate system, the x direction is the direction of the nanowire, and [1 0 0] of MoO₃ in Fig. 2 is parallel to the nanowire.