applications, [4,[17][18][19] and therefore have aroused enthusiasm in researchers. Due to their high crystal structure symmetry, graphene, which has been well studied, and most TMDCs exhibit an inplane isotropic feature. However, a few 2D materials with a surprisingly low lattice symmetry, such as BP, tin selenide (SnSe), [20][21][22][23] gallium telluride (GaTe), [24] and rhenium disulfide (ReS 2 ), [17,[25][26][27][28] can also show significant anisotropic in-plane optical, electrical, and thermal properties. [17,20,[23][24][25]28] For instance, the charge carrier mobility, [18,29] photoemission, [30] and thermoelectric figure of merit (ZT) [31][32][33] of BP along the armchair direction are larger than those in the zigzag direction. In addition to anisotropy, BP also has a tunable thickness-dependent direct bandgap ranging from 0.3 to 1.5 eV, [12,13] filling the space between zero-gap graphene [1,2] and large-gap (1-2.5 eV) TMDCs. [4,34] Additionally, BP has a high carrier mobility (≈10 3 cm 2 V −1 s −1 ) [18] compared with TMDCs (10-200 cm 2 V −1 s −1 ). [4] Therefore, BP has been viewed as an alluring and ideal candidate for applications in field-effect transistors (FETs), [18,35] fast-response optical switches, [36] photovoltaic devices, [37] mid-infrared polarizers, and polarization sensors, [29] owing to its distinguished physical properties. Although BP exhibits great potential for various anisotropic optical, electronic, and optoelectronic high-performance devices, it degrades within a short time when exposed to oxygen and water vapor in air, causing difficulties in practical applications. Consequently, it is highly important to explore BP-like materials with appropriate properties including a narrow bandgap, high carrier mobility, air stability, and low cost.Regarded as a promising alternative to BP, SnSe also consists of a puckered honeycomb layered crystal structure similar to that in BP, exhibiting highly anisotropic valence bands, [23] a crystal-orientation-dependent high charge carrier mobility (≈10 3 cm 2 V −1 s −1 ), [38] and linear optical absorption. [39] Additionally, due to the narrow-bandgap semiconductor nature, the indirect bandgap of SnSe is ≈0.9 eV, [39][40][41][42] whereas its direct bandgap is ≈1.3 eV, [40,42] leading to optical transitions of SnSe The deceptively simple tin selenide (SnSe) film has emerged as an appealing 2D material with a narrow bandgap, high charge carrier mobility, and significant thermoelectric figure of merit. In particular, compared with most commonly investigated 2D materials, SnSe with a puckered honeycomb structure possesses a lower lattice symmetry, resulting in prominent in-plane anisotropy. Herein, with polarization-dependent Raman spectroscopy and polarization-dependent nonlinear absorption measurements, pronounced polarization-dependent nonlinear optical properties of a SnSe flake are demonstrated originating from the anisotropic optical transition probability of SnSe, which is confirmed by ultrafast polarization-dependent pump-probe experiments. Furt...
