Fig. 1: Far-field excitations of hyperbolic phonon polaritons (HPhPs) in in-plane hyperbolic polariton tuners.

a Schemes of the tuners comprised of van der Waals (vdW) α-MoO₃ periodic ribbon patterns and Fourier transform infrared spectroscopic (FTIR) measurements. b Photograph of a typical α-MoO₃ flake grown on a silicon substrate with a 300-nm oxide layer. The largest lateral length of the flake is 1 cm. c Scanning electron microscopy image of periodic ribbon patterns with a fixed width w = 800 nm and different skew angles θ of 0°, 30°, 60°, and 90°. d Polarized reflectance spectra of the pristine α-MoO₃ thin flake shown in (b). The electric fields of the incident light, E_inc, are along [001] (red curve) and [100] (blue curve) crystallographic directions, respectively. e, f Experimental polarized reflectance spectra of one-dimensional periodic tuner patterns with θ = 0° (e) and 90° (f). The polarization of the incident light is paralleled to [001] (red curves) and [100] (blue curves) directions, respectively. Insets: enlarged reflectance spectra in the range of 995 to 1020 cm⁻¹. The ω₀ represents the resonant frequency of polariton tuners. The \({\omega }_{{{{{{\rm{TO}}}}}}}^{001}\) and \({\omega }_{{{{{{\rm{TO}}}}}}}^{100}\) represent the transverse optical (TO) phonon frequencies along the [001] and [100] crystallographic directions, respectively, and the \({\omega }_{{{{{{\rm{LO}}}}}}}^{010}\) indicates the longitudinal optical (LO) phonon frequency along the [010] crystallographic direction. Colored shaded regions in (d), (e), and (f) mark the frequency ranges of different Reststrahlen bands of α-MoO₃.