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The direction-dependent dual-mechanism refractive index sensor composed of the forward strip graphene and backward composite photonic structure with black phosphorus (BP) is proposed. The linear correlation between the absorption by different mechanisms and the refractive index of the analyte can be realized for two incident directions. The simulated results show that the sensitivities of the forward incidence with the refractive index measurement ranges of 1.2 - 1.6 and 1.6 - 1.9 are 3.135 THz/RIU (refractive index unit) and 1.135 THz/RIU. The sensitivities of the backward incidence with refractive index detection zones 1.2 - 1.5 and 1.6 - 2.0 are 9.500 THz/RIU and 14.650 THz/RIU. In addition, the forward incidence owns more appropriate linearity while the backward incidence possesses better angular stability. These results can find applications in reconfigurable integrated photodetectors for refractive index sensing. [ABSTRACT FROM AUTHOR]