Second Harmonic Generation in Atomically Thin MoTe2

Optical second harmonic generations (SHGs) from atomically thin MoTe2 flakes with 2H and 1T' phases are studied. From 2H-MoTe2 samples with odd (even) numbers of layers, strong (negligible) SHGs are observed due to the layer-dependent broken inversion symmetry. When pumped by a telecom-band laser, SHG from a monolayer 2H-MoTe2 is about one order of magnitude stronger than that from a monolayer WS2; an extremely high second-order nonlinear susceptibility of 2.5 nm V-1 is estimated, presenting the highest value among those reported in 2D materials. SHG measurements in MoTe2 are also demonstrated as an efficient way to distinguish the 2H-to-1T' phase transition. Compared with the SHG in 2H-MoTe2, 1T'-MoTe2's SHG has much lower efficiency and the polarization dependence is changed from a six-fold pattern to a two-lobe pattern.