Highly Efficient and Anisotropic Second Harmonic Generation in BiInSe Alloy Nanowire and ReS₂/BiInSe Heterostructures

Topological insulators (TIs) present huge advantages for both low power electronic devices and quantum computing. However, high dark current induced by topological conductivity restricts their application in micro- and nano-optoelectronic devices. Alloying engineered Bi_1.3In_0.7Se₃ nanowires (NWs) exhibit broadband optical sensitivity and ultrafast photoresponsivity, which provides more possibilities for regulation of their photophysical properties. The Bi_1.3In_0.7Se₃ NWs possess a centrosymmetric crystal structure. Under the electric-dipole (ED) approximation, the second harmonic generation (SHG) effect is prohibited. However, due to the anisotropic morphological structure, the second-order NLO response remains significant because of the nonuniform distribution of the external electric field. Here we report that Bi_1.3In_0.7Se₃ NWs present highly efficient and anisotropic SHG with an optimal second-order nonlinearity of 22.9 pm/V at an average power density of 7.1 × 10⁴ W/cm² under excitation at 900 nm, which is much higher than that of commonly used NLO crystals. Nonuniform distribution of the electric field in the vicinity of Bi_1.3In_0.7Se₃ NWs governed anisotropic second-order polarizability. Theoretical analyses were applied to unveil the variation in ED and electric-quadrupole (EQ) contributions along with the changeable anisotropic morphology of Bi_1.3In_0.7Se₃ NWs. Furthermore, two different types of vertical ReS₂/Bi_1.3In_0.7Se₃ heterojunctions were constructed to modulate the anisotropic SHG response. Our findings present a promising prototype for next-generation all-optical NLO devices featuring excellent wavelength tunability and prominent polarization sensitivity.