Terahertz Carrier Dynamic Property And Terahertz Emission Spectra Of Tungsten Disulfide

Layered transition-metal dichalcogenides（TMDs）are typical graphene-like two-dimentional（2D）materials with adjustable band gap and optically selective spin-valley,creating a new direction for the research on new 2D optoelectronic devices.Tungsten disulfide（WS₂）is typical layered TMDs,which exhibits strong second harmonic generation,layer-dependent photoluminescence,large conduction-band spin splitting and large exciton binding energy,and emerging high-properties in nano-integrated electronic devices.Ultrafast carriers on the order of sub-picoseconds make it possible to produce high-performance optoelectronic devices in the terahertz（THz）region.THz spectroscopy works as a contactless and efficient research method for the detection and study on THz properties of semiconductor materials.However,few researches are reported about the excellent optoelectronic properties of WS₂ materials in the THz region.In this thesis,we will discuss the characteristics of the carrier dynamics and THz emission spectrum for WS₂in the THz region.It provides a good foundation for the research and development of THz functional devices based on layered TMDs,and promotes the development of THz technology.In this thesis,the main work and innovations are listed as follows:（1）Static optical parameters and free carriers dynamics of WS₂ materials.The THz time-domain signal of the WS₂ materials were tested by THz time-domain spectrum system（THz-TDS）in a transmission configuration.The optical parameters such as the refractive index and absorption coefficient were obtained by the parameter extraction methods in THz region.Carrier relaxation process on the surface of WS₂ materials were obtained by optical-pump terahertz-probe（OPTP）system.The relationship between the THz photoconductivity and the change of incident pump light was analyzed.（2）THz time-domain emission spectroscopy in both transmission and reflection configurations has been employed to understand the THz radiation property and surface properties of WS₂ under the 800 nm femtosecond laser excitation.We observed only one polarization of THz radiation under different polarization of pump beam and a saturation effect with the increasing of pump power.The results are different from that of molybdenum disulfide（MoS₂）based on optical rectification in spite of similar physical and optoelectronic properties of them.The nonlinear optical coefficient calculation based on the first-principle method combined with the azimuthal angle dependence of THz radiation implies that THz radiation is insensitive to the azimuthal angle in WS₂.From the pump polarization angle dependence of THz radiation,we find that the contribution due to the nonlinear effect is only 12%approximately.All these suggest the main THz mechanism from WS₂ is due to the surface depletion field induced by the surface states.We also analyzed the surface field features of WS₂ with the maximum surface depletion field of approximate 1.2×10⁵ V/cm.Fresnel law combined with the dipole radiation model is also used to analyze the angular dependence of THz radiation.The results can not only afford a fundamental THz radiation property of layered materials,but also promote the development of THz devices based on the layered materials.（3）Layered TMDs with fantastic spin polarization property pave a new way in spintronic and valleytronic devices.Herein,polarized THz emission spectroscopy is used to analyze the spin polarization of the multilayer WS₂ under linear and circular polarized excitation.Interface built-in field verified by linear polarized excitation make a change of the layered potential gradient to induce additional Rashba Spin splitting aroundΓ-point,where an asymmetric charge distribution in momentum space resulting in in-plane spin-polarized current,which generated the circular THz radiation.These results not only provide a new contactless detect method in spin polarization,but also afford a fundamental spin/valley properties in layered TMDs.（4）Recent advances in the development of polarized THz emission from nanomaterials has not only opened up a new“TeraNano”interdiscipline but also provided a new tool for nonlinear optical process research.Herein,THz radiation mechanisms of monolayer WS₂are first investigated by both linear and circular polarization laser excitation at room temperature.The results reveal that polarized THz emission is dominated by the optical rectification based on in-plane nonlinear dipoles under the 800 nm excitation,which is totally different from that of bulk WS₂.The mechanism is verified by the azimuthal angle and pump polarization angle dependence of THz emission in both experiment and theory.Furthermore,we observed controllably elliptically polarized THz emission with the maximum ellipticity e≈0.52 based on non-resonant nonlinear process under the circularly polarized excitation.A clear understanding of THz radiation mechanism of 2D materials will facilitate further design,optimization,and polarization control of integrable 2D THz optoelectronics.