Fig. 1. Conceptual design of planar waveguide of HPhPs in α-MoO3. (a) Experimental schematic for launching HPhPs of α-MoO3 on gold slit by PiFM setup. (b) Simulation of the 2D near-field distribution of HPhPs in α-MoO3 on gold and air substrate, launched by a dipole. (c), (d) 3D simulation; the width of the gold slit is 6 μm in (c) and 1.5 μm in (d). (e), (f) Real-space propagation of HPhPs on gold substrate (e) and air substrate (f) launched by a dipole. (g), (h) IFC of HPhPs in α-MoO3 on gold substrate (g) and air substrate (h). The α-MoO3 thickness for all simulation settings is 242 nm. The incident light and dipole frequency for all simulation settings is 890  cm−1.

Fig. 2. Controlling the waveguide mode of HPhPs in α-MoO3 by varying the gold slit width. (a) Real space near-field nano-images of the waveguide mode of HPhPs with varying widths of gold slit. The frequency of the incident light is ω=890  cm−1. (b) PiFM signal amplitude fringe profiles extracted along the dashed lines in (a). (c) FFT on the near-field fringe profiles in (b). The abscissa is the incident light’s wave vector (k0=2πω). (d)–(f) Near-field images recorded with three representative excitation frequencies. The thickness of α-MoO3 measured by AFM is 242 nm.

Fig. 3. Near-field manipulation of nanoscale ultra-narrow waveguides. (a) Near-field imaging of nanoscale waveguides with and without extended ports. (b) PiFM signal amplitude fringe profiles extracted along the center of waveguides in (a). (c), (d) Near-field images recorded with two representative excitation frequencies. (e) Comparison of the contrast of waveguides with and without extended ports. The geometry of all the extended ports is 2  μm×1  μm. The thickness of α-MoO3 is 242 nm.

Fig. 4. Dispersion, propagation length, FOM, and vg of the waveguide mode of HPhPs. (a) Dispersion relation of HPhPs in the α-MoO3 flake/gold slit composite structure. Colored squares indicate the experimental results obtained from PiFM images. Dashed colored lines correspond to the analytical dispersion relation of different waveguide widths. Pseudo-colored images represent the calculated Im rp of the air/α-MoO3/air or gold structure. (b) Theoretical (dashed colored lines) and experimental (colored marks) results of propagation length of HPhPs on different-width waveguide and gold substrate. (c) Theoretical (dashed colored lines) and experimental (colored marks) results of FOM of HPhPs on different-width waveguide and gold substrate. (d) Theoretical curves of group velocity of HPhPs on air substrate, 1.5 μm waveguide, and gold substrate.

Fig. 5. Real-space visualization of Y-shaped routing waveguide. (a) Experimental near-field images of HPhPs propagating in a 305 nm thick α-MoO3 flake on a Y-shaped gold slit at ω=890  cm−1. (b) Simulation result using the same parameters as those in (a). The red dashed arrows in (a) and (b) represent the waveguide’s energy flow direction. (c) Analytic IFC of α-MoO3/air at ω=890  cm−1. (d) Experimental near-field images of HPhPs propagating in a 305 nm thick α-MoO3 flake on a Y-shaped gold slit at ω=785  cm−1. (e) Simulation results using the same parameters as those in (d). The red dashed arrows in (d) and (e) represent the waveguide’s energy flow direction. (f) Analytic IFC of α-MoO3/air at ω=785  cm−1.

Fig. 6. Real-part permittivities of α-MoO3, where three different Reststrahlen bands of α-MoO3 are shaded in different colors.

Fig. 7. Characterization of α-MoO3 flake/gold slits composite structure. (a) SEM image of gold slits. (b) Optical microscopy image of α-MoO3 flake/gold slits composite structure.

Fig. 8. Fringe profiles in the 220 nm waveguide with extended port at 890  cm−1 [Fig. 3(a) in the main text].

Fig. 9. Near-field images of 650 nm and 1 μm waveguides at different frequencies.

Fig. 10. (a) The optical microscopy image of waveguide structure with a rotation angle. θ is the angle between the longitudinal axis of the waveguide and the x axis. The thickness of the α-MoO3 flake is 305 nm. (b) The experimental near-field images of HPhPs propagating in waveguides with different θ (from left to right, the values of θ are 0°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, and 80°) at 890  cm−1. Due to the limitations of the scanning range in the near-field experiment, the figure was divided into two scans, referred to as the left image and the right image, respectively. (c) The same as (b) except that the excitation frequency of incident light is 785  cm−1.

Fig. 11. 2D simulation of the electric field distribution in the cross-section of a waveguide with a slit width of 1.5 μm at 930  cm−1.