Fig. 4: Reversed CR in hBN/MoO₃ heterostructure.

a The schematic illustration of reversed CR in the air/MoO₃ and hBN/MoO₃ heterostructures. b The IFC of hBN/MoO₃ heterostructure shrinks compared to pure MoO₃ (i.e., air/MoO₃), so the \({\theta }_{k}\) is increased. The thickness of hBN is 50 nm. c Real-space images of polaritonic reversed CR in air/MoO₃ and hBN/MoO₃ with an excitation frequency of 977 cm⁻¹ when \({{{{{\rm{\beta }}}}}}\) ≈ 90°. Scale bars: 0.5 μm. d The summarized experimental extracted \({\theta }_{k}\) on air/MoO₃ and hBN/MoO₃ at different excitation frequencies. Experimental data: points. The thickness of MoO₃ and hBN is approximately 280 nm and 7 nm, respectively. Error bars are obtained from different line profiles within each scanned image. e Summary of reversed CR based on varied materials and structures. In the previous experiments, the reversed CR was mainly realized in the microwave band based on the design of artificial left-hand metamaterials (LHM), such as LHM structure¹⁴, transverse magnetic-LHM (TM-LHM) structure¹³ and metallic metamaterial (MTM) structure¹⁵. In the shorter wavelength band, some theoretical works predict that reverse CR can exist in natural hyperbolic vdW materials (vdWs) or vdW heterostructures (vdWHs), such as hBN (type I hyperbolic band)²¹, MoO₃ (type II hyperbolic band)²⁴ and multilayer graphene stacked hyperbolic heterostructures⁴⁹.