| Terahertz(THz)waves have many excellent characteristics due to their frequency band,and have applications in materials science,biology,medicine,security imaging and communications.THz technology is inseparable from the THz source.With the development of THz technology,the demands for broadband THz source are increasing day by day.However,traditional THz sources,including photoconductive antennas,electro-optical(EO)crystals,and laser-indued air plasmas,have their own defects on broadband THz waves radiation.In recent years,the emergence of spintronic THz emitters has solved these shortcomings.Spintronic THz emitters have attracted the attentions of researchers due to their advantages of high strength,simple fabrication,more flexible and cheaper.Most spintronic THz emitters are based on ferromagnetic(FM)and nonmagnetic(NM)metallic bilayers that can convert the spin-polarized currents into transient charge currents,radiating THz waves.The research on spintronic terahertz mainly focuses on enhancing the performance of THz emitters including the effects of layer materials,layer thicknesses,layer numbers,sample geometries,sample annealing,and sample magnetization with different components,cascade structures,and magnetic field shaping of the intensity and polarization of spintronic emitters.However,the most effective means of controlling spintronic THz devices is to adjust the external pemanent magnets or electromagnets,which is disadvantageous for compact or onchip integration.In addition,for non-ferromagnetic materials,the most materials used are heavy metals like Pt,but the light metals with weak spin-orbit coupling(SOC)are used less.On the other hand,there are also lack of researches on spintronic THz emitters based on FM/FM bilayer structures.Therefore,it is of grreat significance to electrically control spintronic THz emission,look for liht metal materials which could realize the spin-to-charge conversion,and study new systems of spintronic THz emission based on ferromagnetic structures.In view of the above problems,firstly,we design a THz wave emitter based on Pt/CoFeB/PMN-PT(011)structure,and study the electrically modulation of spintronic THz emission via magnetoelectric(ME)coupling.We can use electric fields to modulate the intensity and polarization of THz waves radiated from the emitter.By applying different voltages,THz waves of different intensity and polarization can be generated.Moreover,the THz modulation depth(|ΔS/Smax|)can reach 69%,and the THz polarization angle can be rotated up to 28 degrees.In addition,the electrically modulation of THz wave is non-volatile,and the emitter can be encoded by applying different excitation voltages.Compared with traditional spintronic THz emitters controlled by magnetic field,the Pt/CoFeB/PMN-PT(011)structure controlled by electric field is more flexible and has more powerful modulation effect It has the potential to be developed into a programmable array-type THz source that can be applied to terahertz near-field ghost imaging.This research provides a powerful means for the miniaturization and chipization of spintronic THz wave emitting devices,and it can broaden the application of multiferroic materials,and can also develop chip THz sources and THz memory devices.Secondly,we use the 3d magnetic phase transition light metal alloy NiCu instead of NM heavy metal materials in traditional spintronic THz emitters.The THz emission spectroscopy of CoFeB/NiCu heterostructures were systematically studied.We find the THz intensity in the CoFeB/Ni70Cu30 heterostructure can reach half that of the CoFeB/Pt bilayer structure.And THz emission is generated by the inverse spin Hall effect rather than the interfacial effect In addition,we study the effect of temperature on the THz emission and proved that terahertz emission is independent of the magnetic state of NiCu.The results of this work show that light metals such as 3d transition metals can also achieve efficient THz emission.And ferromagnetic materials may also replace the traditional NM layer,which is of great significance for the development of new spintronic THz devices.Finally,based on the above work,we study the spin charge conversion of ferromagnetic Ni films and their heterojunctions.In different structures,the Ni film has different functions and roles.In the single layer ofNi film,the THz waves are generated by the co-effect of anomalous Hall effect and ultrafast demagnetization.In the FM/Ni structure,the Ni film acts as a spin converter to convert the spin currents into charge currents via the inverse spin Hall effect.In the Ni/NM structure,Ni film acts as a spin injector,and THz emission is achieved through the inverse spin Hall effect of NM materials.It can be seen that Ni can be used as both a spin injector and a spin converter.This work shows that pure ferromagnetic structures have great potential applications in spin charge conversion devices.In addition,we extended the material system to 3d metals,studied the THz emission spectroscopy of single 3d metal layers and CoFeB/3d metal heterostructures.In the experiment,we find the single layers of paramagnetic Ti antiferromagnetic Cr film and ferromagnetic films can emit THz waves.Moreover,in heterostructurs,the ferromagnetic Ni metal has the best efficiency of THz spin-to-charge conversion.Those researches is important to the mechanisms of THz emission and development of spintronic THz emitters. |
