Nonlinear Absorption Properties Of MoTe₂ And MoTe₂/MoS₂ Heterostructure Films

Molybdenum dichloride（MoTe₂）and molybdenum disulfide（MoS₂）are typical transition metal dichalcogenides（TMDs）with bandgap tunability and interlayer coupling characteristics.It is of great significance for the research and application of optical switches,optical limiters,logic circuits,and other optoelectronic devices.In order to investigate the influence of the nonlinear characteristics of TMDs on the new generation of nano-optoelectronic devices,it is particularly important to study the nonlinear optical properties of two dimensional TMDs.In this paper,the nonlinear absorption characteristics of MoTe₂ nanofilms and MoTe₂/MoS₂ heterostructure films are investigated by using the open aperture（OA）Z-scan technique.The results show that MoTe₂ can be used as an important material for optical logic devices and MoTe₂/MoS₂ heterostructure films pave the way for the development of type-II heterostructure nonlinear materials.Therefore,they can be used as an important choice in nonlinear optical devices such as laser mode locking and Q-switching.This thesis includes the following parts:（1）Set up a Z-scan system.Based on Z-scan theory,an OA Z-scan system based on1kHz,800 nm femtosecond laser is designed and built,and the collimated alignment and optimization of the optical path is carried out.Using the nonlinear absorption of standard sample carbon disulfide（CS₂）solution is measured to ensure the accuracy of the measurement results of Z-scan system.（2）The preparation of MoTe₂ nano-film and the measurement of the non-linear absorption coefficient.MoTe₂ nano-films with different thickness were obtained by liquid-phase exfoliation（LPE）and vacuum filtration method.The MoTe₂ nanofilms were characterized by scanning electron microscopy（SEM）,atomic force microscopy（AFM）,UV-vis absorbance and Raman spectra.The nonlinear absorption coefficients of these nanofilms were measured by an OA Z-scan system.The results show that the saturable absorption of MoTe₂ nanofilms is converted to reverse saturable absorption with the increase of pump power.The competition between the ground state and the excited state absorption is analyzed by a four energy level model.We have found that the coexistence of two absorption modes depends not only on the pumping intensity,but also on the transition probability between the absorbed cross-section and the energy level.Such characteristics can be used for all-optical logic gates,optical limiter,photonic storage,and so on.This work has been published in the journal of Applied Surface Science.（3）Preparation of MoTe₂/MoS₂ heterostructure films and measurement of nonlinear absorption coefficient.MoTe₂/MoS₂ heterostructure films with different thickness were also obtained by LPE and vacuum filtration method.Using of X-ray photoelectron spectroscopy combined with visible-infrared absorption spectra,we have studied the band alignment of MoTe₂/MoS₂.It is found that the heterojunction belongs to type-II heterostructure.The Z-scan results show that under the same excitation conditions,compared with pure MoTe₂ and MoS₂ nanofilms,the MoTe₂/MoS₂ nanocomposite film show stronger saturable absorption properties.Combined with the energy level rate equation with the Runge-Kutta algorithm,it is show that the main factor to enhance the saturation absorption response of MoTe₂/MoS₂ heterstructure films are that the ground state absorption cross section is larger than the excited state absorption cross section,the interlayer carrier transition,the low electron hole recombination rate,and the narrow band gap in the heterojunction.The first-principle calculation further proves the interlayer carrier transfer effect between MoTe₂ and MoS₂.The saturable absorption characteristics enhanced by MoTe₂/MoS₂ heterostructure film can be used in nonlinear optical devices such as laser mode-locking and Q-switching.The type II heterostructure lays a foundation for the development of optical modulators,ultrafast laser mode lockers and saturable absorbers.This work has been published in the Advanced Materials Interfaces.