Figure 3: Experimental demonstration of super-resolution imaging with tunable hyperbolic polaritons in the Type I band.

(a) Sketch of the experimental setup. The right inset is the normalized laser spectrum of the used mid-infrared broadband laser. The gold nanodiscs are with 0.3-μm diameter and 1.3-μm centre-to-centre separation. (b) The AFM topography taken at the top surface of the 0.15-μm-thick hBN flake. (c) The 2D infrared optical amplitude (s₂) images taken with the broadband laser. (e) The control infrared amplitude image taken with a CO₂ laser at ω=952 cm⁻¹ that is out of the hyperbolic region of the hBN. The small black dots in the image are caused from topographic features (corresponding topographic image shown in (d). Scale bars, 0.5 μm. (f) Detailed profiles of the s-SNOM signals across two neighbouring discs (along the line marked in e, averaged over five scan lines) for the cases using the broadband laser (red line) and the CO₂ laser (black line), respectively. Both profiles are normalized to their respective minimum values outside the discs. The broadband imaging shows much stronger contrasts for the discs. (g) Detailed Nano-FTIR line profiles at various frequencies. Dashed line marks the position variations of the peak of edge-launched hyperbolic polaritons. (h) Optical widths and corresponding directional angles of the hyperbolic polaritons evaluated from the experimental results (dots) in comparison with the calculated results (solid lines). The error bars result from the spatial pixel size (50 nm) in nano-FTIR measurements.