The discovery of fractional charge excitations in new platforms offers crucial insights into strongly correlated quantum phases. While a range of fractional quantum anomalous Hall (FQAH) states have recently been observed in two-dimensional twisted moire systems, the theoretically anticipated filling factor $\nu =-1/3$ FQAH state has remained elusive, with debates centering on its nature of charge density wave or a topological Chern insulator. Here, we report the optical detection of a $\nu =-1/3$ FQAH state in twisted ${\mathrm{MoTe}}_2$ bilayers. Using photoluminescence and reflective magnetic circular dichroism techniques, we identify ferromagnetic states at filling factors $\nu =-1$, $-2/3$, and $-1/3$, all tunable by a vertical electric field. The corresponding Curie temperatures are approximately 11, 3.5, and 2.4 K, respectively. The $-1/3$ state emerges over a narrower electric field range and a lower temperature compared to the integer and other fractional states, indicating a fragile nature that may have led to its absence in previous reports. Notably, the photoluminescence spectra at $\nu =-1/3$ disperse as the out-of-plane magnetic field increases, consistent with a nontrivial topological origin. Theoretical calculations based on the exact diagonalization method further support the interpretation of this topologically nontrivial state.

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• Flat bands
• Topological materials
• Twistronics
• 2-dimensional systems
• Transition metal dichalcogenides
• Optical techniques