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Figure 4: Negative refraction of hyperbolic polaritons supported by naturally anisotropic 2D materials. (a) Imaginary part of surface conductivity of anisotropic 2D material in the left region. In (2), we set , , , , , , , and , where and is the free-electron mass. (b) Distribution of electric field excited by a dipole source at 60 THz. The red dashed line represents the boundary between left/right regions. For the right region, we have and .