Anisotropic Nonlinear Optical Properties of a SnSe Flake and a Novel Perspective for the Application of All‐Optical Switching

Abstract

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...

Giant Nonlinear Optical Activity in Two-Dimensional Palladium Diselenide

“…Importantly, excellent saturable absorption is observed in 1-3 L PdSe2, with their modulation depths (αs) much higher than most of the other 2D materials [27][28][29]. Our observations not only provide profound insights for the NLO features in this innovative nanomaterial, but also open a new avenue for the future nonlinear optoelectronic application.…”

“…As the sample thickens, the saturable intensity gradually increases. The modulation depths 1 are much higher than most of the other 2D materials, such as SnSe [27], MoSe2 [28], MoS2 [29], which can be a promising advantage for the application in ultrafast lasers. The high modulation depths mainly result from the low unsaturated loss, indicating a low defect density in the PdSe2 [7].…”

Polarization-tunable nonlinear absorption patterns from saturated absorption to reverse saturated absorption in anisotropic GeS flake and an application of all-optical switching

“…where, a, b, c, and f are the Raman-tensor parameters. Therefore, the angular-dependent Raman intensity can be expressed as follows [57]: 3to fit the angle-dependent intensities of A g and B 3g modes shown as the red lines in Fig. 1d, we determined the armchair and zigzag directions of our GeS flake in the optical image (marked with red arrows in Fig.…”

“…The origin of polarizationdependent absorption edges and NA mechanisms of GeS can be further explained by the Fermi's golden rule. The transition rate, defined as the probability of a transition per unit time, is mathematically described as [57] op between the final and initial (i) states. The matrix element can be calculated within the dipole approximation, with the optical transition from state i to f [60], which can be expressed as…”

Controllable all-optical modulation speed in hybrid silicon-germanium devices utilizing the electromagnetically induced transparency effect

“…Fano resonance possesses high sensitivity to the local electromagnetic environment of the resonator [17]. And rapid strides have been devoted to the dynamic tuning of metamaterials by external stimuli such as heat [18], light [19][20][21], or voltage bias [22,23]. Among those options, alloptical modulation is of preference, due to their ultrafast optically tunable permittivity and advantage of integration with present microelectronic and optoelectronic device technology platforms [24][25][26][27][28][29][30].…”