We investigate the quasiparticles of a single nodal ring semimetal ${\text{SrAs}}_3$ through axis-resolved magneto-optical measurements. We observe three types of Landau levels scaling as $ϵ\sim \sqrt{B}$, $ϵ\sim B^{2/3}$, and $ϵ\sim B$ that correspond to Dirac, semi-Dirac, and classical fermions, respectively. Through theoretical analysis, we identify the distinct origins of these three types of fermions present within the nodal ring. In particular, semi-Dirac fermions—a novel type of fermion that can give rise to a range of unique quantum phenomena—emerge from the end points of the nodal ring where the energy band disperses linearly along one direction and quadratically along the perpendicular direction, a feature not achievable in nodal point or line structures. The capacity of the nodal ring to simultaneously host multiple fermion types, including semi-Dirac fermions, establishes it as a valuable platform to expand the understanding of topological semimetals.

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• Optical conductivity
• Nodal-line semimetals
• Fourier transform infrared spectroscopy
• Optically detected magnetic resonance