Terahertz Wave Radiation And Terahertz Wave Detection Of Spin Polarized Electrons

Since the discovery of the ultrafast demagnetization of the ferromagnetic metals,the spin degree of electrons have been gradually applied to the generation of terahertz radiation.The principle of generating terahertz wave by the use of electron spin is to optically pump the ferromagnetic metal to generate spin current,and then convert the ultrafast spin current to ultrafast charge current to rediate terahertz waves.Metallic heterojunction is expected to be a coherent wide spectrum terahertz radiation source because it doesn’t have the phonon absorption in the terahertz frequency as the common polar semiconductors.In addition,the terahertz emission spectroscopy provides the tool to study the ultrafast electron spin dynamics,which is helpful to promote the study of ultrafast manipulation of electron spin.The research on terahertz sources based on electron spin properties mainly involves the generation and transformation of ultrafast spin.The spectrum width of terahertz wave radiation depends on the time scale of the spin current duration,and the spin current duration is closely related to the electron spin dynamics.Therefore,the development of detection methods for the electron spin dynamics is conducive to the search of new spin generation materials and the optimization of the spectrum of terahertz sources.On the other hand,to improve the intensity of terahertz waves,it is necessary to improve the conversion of ultrafast spin current to charge current.The physical mechanism of the conversion mainly includes the inverse spin Hall effect and the inverse Rashba-Edelstein effect.The ultrafast spin current to charge current conversion ability of the inverse spin Hall effect has been demonstrated by extensive experiments,and metal thin films with strong inverse spin Hall effect have been developed as commercial terahertz sources.The inverse Rashba-Edelstein effect mainly exists at the interface of heterojunction with broken central symmetry,and its spin current conversion ability was first demonstrated in Ag/Bi heterojunction.However,recently,whether the spin current conversion in this heterojunction originates from the inverse Rashba-Edelstein effect has even been controversial.Therefore,the ultrafast spin current conversion ability of this effect needs to be proved experimentally.To verify the ability of the inverse Rashba-Edelstein effect to convert the ultrafast spin current to charge current,we investigate the La Al O₃//Sr Ti O₃ heterojunction.This interface has strong Rashba spin orbit coupling,and its DC spin current charge current conversion ability has been proved by spin pumping experiments.Moreover,its conversion efficiency is one order of magnitude higher than Ag/Bi heterojunction,and there is no interference from inverse spin Hall effect.Therefore,it is suitable to study the application of inverse Rashba-Edelstein effect in terahertz wave source.In this paper,the Ni Fe/La Al O₃//Sr Ti O₃ series samples with the Ni Fe layer as the spin injection layer have been studied.Terahertz radiation generation and its dependence on the applied magnetic field have been observed under femtosecond laser pumping.We exclude the ultrafast demagnetization by comparing the radiation signals of Ni Fe//Sr Ti O₃ samples in experiment and calculating the terahertz radiation intensity by the vacuum magnetic diople in theory.The effectiveness of the interface inverse Rashba-Edelstein effect in terahertz frequency is confirmed.In order to further optimize terahertz radiation and verify the corresponding optical model,the refractive index and reflectance of the Sr Ti O₃ substrate were measured,and the correlation between the reflectance spectrum of the Sr Ti O₃ and emission spectrum of the Ni Fe/La Al O₃//Sr Ti O₃ was confirmed,which provided the support for further optimization of terahertz radiation efficiency.In order to further enrich the detection methods of electron spin dynamics and promote the spectral optimization of the spin terahertz radiation sources,we develop a terahertz time-domain spectral detection method with high sensitivity and time resolution.Although the current optical pumping detection method can also characterize the spin dynamics of electrons,the low photon energy of terahertz wave can effectively reduce the influence of other effects on the spin state of electrons.Ultrafast spin polarization in semi-insulated Ga As was investigated by optically pumped terahertz detection experiments at room temperature.After non-equilibrium carriers were injected into Ga As using circulally polarized and linearly polarized femtosecond laser pulses,different dynamical properties of the two polarized light excitation were observed in both transmitted and reflected terahertz signals.This should be due to the production of spin polarized electrons and spin relaxation.The electron spin relaxation time can be obtained from the experiment,which indicates that terahertz time-domain spectroscopy can be effectively applied to the study of the spin dynamics of semiconductors.