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.