Giant two-photon absorption in monolayer MoS2

Strong two-photon absorption (TPA) in monolayer MoS2 is demonstrated in contrast to saturable absorption (SA) in multilayer MoS2 under the excitation of femtosecond laser pulses in the near-infrared region. MoS2 in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO2/Si by employing the seeding method through chemical vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS2 possesses a nonsaturation TPA coefficient as high as approximate to(7.62 +/- 0.15) x10(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 MoS2. For the multilayer MoS2, the SA response is demonstrated with the ratio of the excited-state absorption cross section to ground-state cross section of approximate to 0.18. In addition, the laser damage threshold of the monolayer MoS2 is approximate to 97 GW/cm(2), larger than that of the multilayer MoS2 of approximate to 78 GW/cm(2).