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In this work, we report an intrinsic indefinite permittivity in crystalline MgB₂ that exhibits a negative group refraction for the blue-purple light. Based on the ellipsometry data of a crystalline MgB₂ sample, the equifrequency contour (EFC) is mapped and verified to be hyperbolic which enables a negative refraction for all incident angles. The indefinite dielectric tensor is attributed to the two Drude dispersions with different plasma frequencies between two crystal directions, which originates from the strong anisotropic crystal structure. Numerical simulations are also performed to demonstrate the negative refraction behaviour and the focusing property as a hyperlens. It is, for the first time, demonstrated that an all-angle negative refraction is able to occur in natural materials at visible frequency range.

indefinite medium, negative refraction, ellipsometry