Fig. 3: Low temperature nano-imaging of hyperbolic exciton polaritons in CrSBr.

a Collocated \(E=1.378{{{{{\rm{eV}}}}}}\) near-field images of a \(200{{{{{\rm{nm}}}}}}\) CrSBr microcrystal at \(T=295{{{{{\rm{K}}}}}}\) and \(50{{{{{\rm{K}}}}}}\). Images are normalized to the area-averaged substrate signal and displayed on the same color scale. CrSBr becomes significantly brighter at \(50{{{{{\rm{K}}}}}}\) and new subdiffractional fringes appear (black arrows) corresponding to the suspected hyperbolic exciton polariton (HEP). b \(b\)-axis near-field line profile and, (c) corresponding fast Fourier transform (FFT) revealing observed momenta \({k}_{b}^{{obs}}\) of the air mode (green circle) and HEP (orange diamond). Decomposed air mode and HEP FFT peaks (blue dashed lines) with corresponding real-space profiles shown in (b). d Near-field image at \(50{{{{{\rm{K}}}}}}\) with air modes removed via 2D Fourier filtering. The bottom inset indicates the filtered regions of the 2D FFT. e \(b\)-axis line profiles along the white dashed line in (a) at \(T=295{{{{{\rm{K}}}}}},100{{{{{\rm{K}}}}}},50{{{{{\rm{K}}}}}}\), and \(20{{{{{\rm{K}}}}}}\). The subdiffractional fringes only appear in low-temperature profiles and redshift with decreasing temperature (dashed lines). f \(50{{{{{\rm{K}}}}}}\) near-field image outside hyperbolic band and averaged line profile (black line) with, (g) corresponding FFT reveal TM₀ mode inside the material light cone (red dashed line).