Terahertz Emission Spectroscopy Of Graphene And Tungsten Ditelluride Excited By A Femtosecond Laser

The investigation of high efficient and compact terahertz(THz)sources complies with the development trend of microminiaturization and integration of optoelectronic devices.Therefore,it is of great significance to study the terahertz emission properties of low dimensional materials and further to find new terahertz radiation sources.Two dimensional(2D)materials have a potential to be as a new terahertz radiation source because it can effectively reduce device size.When excited by a femtosecond(fs)laser,the transient dielectric polarization or photocurrent in picoseconds or sub-picoseconds scale will be generated on the surface or interface of 2D materials which will emit THz radiation.The analysis of the THz wave signal can also reflect the ultrafast photoelectric response of the 2D materials.In this thesis,we selected a coup of 2D metallic materials,graphene and tungsten ditelluride(WTe2),as the research objects.We mainly focused on the THz emission spectroscopy from samples excited by fs laser.One aims is to investigate the THz emission performance of materials,which can provide the fundamental investigation for 2D THz sources.The other is to probe the photoresponses of the two metallic materials by THz emission spectroscopy.The main works and innovation points are listed as follows:(1)The construction of THz time-domain spectroscopy system.In this part,we started to introduce the spectroscopy system from the aspects of system design and system optimization,including transmission configuration THz time-domain spectroscopy,time-resolved THz spectroscopy(optical pump-THz probe spectroscopy),and THz emission spectroscopy.Despite,two kinds of THz polarization measurement methods were introduced and the advantages of the two methods are compared.This part lay an experimental basis for the following works and it also provides a reference for the construction of spectral system for others.(2)Enhanced THz emission from graphene.How to enhance the interaction between light and graphene and further enhance the THz radiation intensity of graphene? It is the biggest limitation for graphene as a THz source.Based on this consideration above,we have proposed two methods to enhance the intensity of THz radiation by stacking multiple graphene layers and vertically grown graphene with surface structures.We observed that both the two kinds of graphene samples have enhanced the THz radiation intensity drivenby a linearly polarized fs laser.It is exciting to find that the THz intensity emitted from vertically grown graphene is 10 times stronger than that of a single layer graphene.Considering the advantage of easy preparation and large signal enhancement factor,it may be more appropriate to be used as THz sources.We systematically studied the THz spectroscopy of vertically graphene including the dependence of the THz intensity on the pump power,the polarization angle,the incident angle,and the azimuthal angle of the sample.The comprehensive analysis shows that the physical mechanism of THz radiation from vertically grown graphene is that the photon drag effect induced sub-picosecond scale transient photocurrents emit THz wave outward.The theoretical calculation based photon drag effect fits well with the experimental results,which further prove the mechanism is correct.For the first time,vertically grown graphene is used to enhance graphene THz emission without any other materials assistance,which provides a new thought for the application of graphene THz sources.(3)Photon helicity dependent photocurrent induced elliptically polarized THz radiation from vertically grown graphene.Up to date,all studies on THz emission from graphene are restricted to be excited by linearly polarized fs laser while the case for other polarization states irradiation has not been investigated.Therefore,here a quarter wave plate is used to change the photon helicity of the pump beam and the photon helicity dependent photocurrents as well as the polarization state of THz wave in vertically grown graphene has been studied.By fitting the relationship between the photocurrent and the polarization angle changing with a quarter-wave plate,we obtained the ratio of linear and circular photon drag effect contributed to THz radiation.In addition,the THz polarization states shows that a linearly polarized pump leads to a linearly polarized THz radiation and the elliptically or circularly polarized fs light induced an elliptically polarized THz radiation.And the helicity direction of elliptically polarized THz wave is the same to the fs light.These results show that vertically grown graphene not only can be used as a polarization-sensitive photodetector,but also can be used as a polarization controllable THz wave radiation source.Particularly,elliptically polarized THz radiation has a great potential application in the field of chiral material analysis.(4)THz emission spectrum of tungsten ditelluride.As a metallic material,same to graphene,it is easy to generate photocurrents for WTe2 as it has large photoconductivity and ultrafast relaxation process.Theoretically,WTe2 can emit THz waves under fs laserirradiation.In this part,THz emission spectrum of a 50 nm thick WTe2 material excited by linearly polarized and elliptically polarized fs light were studied.The experimental results show that THz emission from WTe2 is isotropic;and the saturation of THz intensity will appear with increasing the pump power.The THz signal does not show polarity reversal when the sample is excited from the substrate-sample and sample-substate direction,which indicates that the THz radiation process is not wave-vector dependent.The comprehensive analysis confirmed the mechanism of THz radiation is that the photogalvanic effect induced photocurrents emit THz radiation outward.Under the same conditions,THz intensity emitted from WTe2 is twice as large as semi-insulating Ga As(110)under lower power irradiation(5 m W),which indicates that WTe2 has a higher energy conversion efficiency than Ga As.The experimental results can be well fitted by the theory of linear and circular photogalvanic effects,indicating the the mechanism analysis is reasonable.In addition,the THz polarization states from WTe2 are always linearly polarized regardless of linear or circular polarization excitation.This is the first time to report THz wave emission from WTe2,which shows that WTe2 has the possibility to be THz sources materials and the photogalvanic effect of WTe2 provides a theoretical basis for understanding the photoresponse.