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Abstract  Strong two‐photon absorption (TPA) in monolayer MoS 2  is demonstrated in contrast to saturable absorption (SA) in multilayer MoS 2  under the excitation of femtosecond laser pulses in the near‐infrared region. MoS 2  in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO 2 /Si by employing the seeding method through chemical vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS 2  possesses a nonsaturation TPA coefficient as high as ∼(7.62 ±0.15) ×10 3  cm/GW, larger than that of conventional semiconductors by a factor of 10 3 . As a result of TPA, two‐photon pumped frequency upconverted luminescence is observed directly in the monolayer MoS 2 . For the multilayer MoS 2 , the SA response is demonstrated with the ratio of the excited‐state absorption cross section to ground‐state cross section of ∼0.18. In addition, the laser damage threshold of the monolayer MoS 2  is ∼97 GW/cm 2 , larger than that of the multilayer MoS 2  of ∼78 GW/cm 2 .

Giant two‐photon absorption in monolayer MoS 2