Two-dimensional Materials And The Related Van Der Waals Heterostructures For THz Generation And Modulation

The designation and optimization of terahertz?THz?optoelectronic functional devices show great contribution to the development of THz technology.Two-dimensional?2D?materials with atomic thickness and related unique physical properties,were regarded as the outstanding materials in miniaturized and integrated functional devices.Van der Waals heterostructures by mixing 2D and three-dimensional?3D?materials were employed to improve the performance of traditional optoelectronic devices.In THz regime,2D materials and 2D/3D van der Waals heterostructures have been demonstrated in high performance THz modulator,and show great potential in intense THz generation.In this thesis,2D semimetal graphene and 2D insulator h-BN were employed as covering materials on Si surface to realize the van der Waals heterostructures.The physical processes for enhancing THz modulation and generation from these 2D/3D interfaces were investigated.In addition,the combination of graphene and metamaterial design enables a graphene metamaterial,in which the graphene surface plasmon enhance the THz modulation efficiency in graphene.The main contributions and innovations of the author are listed as followed:?1?Graphene/semiconductor heterostructures demonstrate the improvement of traditional electronic and optoelectronic devices due to their outstanding charge transport properties inside and at the interfaces.However,very limited information has been accessed from the interfacial properties by traditional measurement which is destructive.Herein,we present an active THz surface emission spectroscopy for the interface built-in potential and charge detrapping time constant evaluation from interface of graphene on SiO₂/Si?Gra/SiO₂/Si?.The active THz generation presents an intuitive insight into the depletion case,weak inversion case,and strong inversion case at the interface in the van der Waals heterostructure.By analyzing the interface electric field induced optical rectification in a strong inversion case,the intrinsic built-in potential is identified as-0.15V at Gra/SiO₂/Si interface.The interface depletion layer presents 44%positive THz intrinsic modulation by the reverse bias voltage and 70%negative THz intrinsic modulation by the forward bias voltage.Moreover,a time dependent THz generation measurement has been used to deduce the charge detrapping decay time constant.The investigation will not only highlight the THz surface emission spectroscopy for the graphene-based interface analysis,but also hold a potential for the efficient THz intrinsic modulation as well as the enhancement of THz generation by the heterostructures.?2?Even though Si is the cost efficient and extensively existed semiconductor in modern optoelectronics,it is not considered as an efficient THz emitter due to the low carrier drift velocity and small saturated build-in electric field from inversion layer and low drift velocity.Herein,we present an effective way to enhance THz generation from graphene/Si Schottky junction?GSSJ?excited with femtosecond laser under electrical gating without rapid saturation and with high carrier drift velocity.This mixed-dimensional van der Waals interface demonstrates large saturation pump fluence without an inversion layer by removing the native oxide layer on Si surface.The THz generation from GSSJ effectively increases with the bias voltage.The THz generation from GSSJ under the same excitation condition is stronger than that from the traditional THz source GaAs?100?and InAs?100?based on surface field effect and photo-Dember effect respectively.The results not only present an efficient THz generation from GSSJ but also demonstrate the ability for THz generation in probing the mixed-dimensional van der Waals interface.?3?Interfacial effect in mixed-dimensional van der Waals heterostructure can improve the performance of THz components such as THz emitter and THz modulator,which highlight the development of THz technology.However,this improvement has only been demonstrated in heterostructure based on graphene or small bandgap two-dimensional materials.Herein,we present enhancement for THz generation and modulation from Si by insulating hexagonal boron nitride?h-BN?layer coating.The h-BN surface adsorption charges induced localized electric dipoles lower the Fermi energy on Si surface,leading to an enlarged build-in electric field.Consequently,THz generation from h-BN/Si demonstrated approximately two times enhancement compared with that by bare Si surface.Furthermore,the enlarged build-in electric field separates the photocarriers at the interface efficiently.The resulting charge accumulation raises up the photoconductivity and enlarges the THz modulation depth at h-BN/Si interface after optical illumination,which is approximately ten times higher than that by Si surface.These results not only present an efficient THz generation and modulation from h-BN/Si interface,but also pave the way for the deep understanding of interfacial effect in mixed-dimensional van der Waals heterostructures.?4?Plasmonic response in graphene-based metamaterials show great potential for THz wave manipulation.In this work,we study the tunable surface-plasmon-polariton-like modes based on graphene complementary split ring resonators in THz region.Our study suggests that these modes can be generated by graphene metamaterials due to the diffraction coupling of surface plasmon and propagating electromagnetic wave.Furthermore,the modes can be tuned by stacking graphene metamaterials layers or by changing the graphene Fermi level with electric field.Our results present the physical understanding in graphene metamaterials and pave the way for 2D materials in novel applications in THz region.