Fig. 4: Far-field excitations of tunable THz HPhPs in α-MoO₃ one-dimensional periodic tuner patterns.

a Experimental transmittance spectra of tuner patterns with different ribbon widths, showing redshifting behaviors of the two resonance valleys with changing w. The long axes of the ribbons are paralleled to the [100] (left panel) and [001] (right panel) crystallographic directions, respectively. The colored spheres indicate the resonance frequencies. The dashed lines are guides to the eye. b HPhP dispersion relations in the THz regime, showing the consistency between the results of calculation and experiment. The wavevector is normalized by the thickness of the α-MoO₃ flake. The colored solid lines are the calculated results according to the analytical dispersion relations of HPhPs propagating in the α-MoO₃ flake. The colored spheres show the experimental results extracted from (a). c Experimental transmittance spectra of tuner patterns with different θ, showing that as θ increase, the HPhP_[001] and HPhP_[100] resonances shift monotonically to higher and lower frequencies, respectively. The ribbon widths are fixed as w = 1200 nm. The colored spheres indicate the resonance frequencies. The dashed lines are guides to the eye. d Dependence of Q-factor on resonance frequency ω₀, showing that in Band 1 and 3, the Q-factors increase monotonically as the ω₀ increases. The solid spheres are extracted from the transmittance spectra in (a). The solid lines are a guide for eyes. Colored shaded regions in (d) mark the frequency ranges of different Reststrahlen bands of α-MoO₃.