Terahertz (THz) radiation of layered molybdenum disulfide (MoS2) crystal under femtosecond laser irradiation was observed using THz surface emission spectroscopy under variable-angle transmission configuration. Although MoS2 demonstrates inversion symmetry, surface-symmetry breaking will introduce the resonant optical rectification, which is consistent with the incident polarization and azimuthal angle dependences of the THz radiation from MoS2. However, the surface depletion field induced THz radiation will make an important contribution under oblique incidence, which is consistent with the radiation saturation due to the electrostatic screening effect by photoexcited carriers. This pump-dependent saturable THz radiation can be fitted well by the calculation from Maxwell equations with electromagnetic boundary conditions. The maximum of the surface depletion field is estimated to be 1.45 × 104 V/cm with 130 nm in depth under −40° incidence. Interestingly, when the incident angle is tuned from −40° to 0°, the optical rectification contribution varies from 40% to 90%. In addition, MoS2 is diagnosed to be p-type from THz waveforms by comparison with GaAs(100). The results not only afford a comprehensive understanding of THz radiation from layered materials like MoS2 but also put forward THz emission spectroscopy for characterizing the surface and interface properties of two-dimensional materials.

MoS 2 /graphene nanocomposite films are fabricated by vacuum filtering with liquid-phase exfoliated MoS 2 / graphene suspension. The nanocomposite films are characterized by Raman spectroscopy, UV−vis spectroscopy, and atomic force microscopy, indicating the optical films with a large scale and high optical homogeneity. The enhanced saturable absorption of MoS 2 /graphene nanocomposite films compared with pristine MoS 2 film and graphene film is investigated using an open-aperture Z-scan technique with a femtosecond laser at 800 nm. The nonlinear absorption coefficient of MoS 2 /graphene nanocomposite film is ∼ −1217.8 cm/GW, which is larger than that of MoS 2 film (∼ −136.1 cm/GW) and graphene film (∼ −961.6 cm/GW) at the same condition. The imaginary part of the third-order nonlinear optical susceptibility of the nanocomposite film can reach Imχ (3) ∼ 10 −9 esu with a figure of merit ∼10 −14 esu cm, low saturable intensity (∼157.0 GW/cm 2 ), and high modulation length (∼32%). A coupling model is considered in order to understand the nonlinear absorption properties of MoS 2 /graphene nanocomposite films, which suggest the enhancement can be attributed to charge transfer between MoS 2 and graphene. The results pave the way for the design of nonlinear optical properties with two-dimensional materials for good performance of optical switches or mode lockers based on saturable absorbers.

We have developed a Monte Carlo program that can account for Raman scattering in the ocean when polarization effects are not considered. The program is capable of coupling an inhomogeneous atmosphere to an inhomogeneous ocean through a dielectric interface. We have studied the filling in of both the 486-nm H(beta) and the 518-nm Mg Fraunhofer lines caused by Raman scattering in the ocean. The amount of fill varies with solar zenith angle, angle of view, depth in the ocean, and magnitude of the cross sections. By monitoring the shapes of Fraunhofer lines we can learn a great deal about the relative importance of this inelastic process in oceanic optics.