Construction And Application Of Ultrafast Terahertz Time-domain Spectroscopy Platform

The terahertz time-domain spectroscopy has been widely used in basic scientific research and real applications.In specific its non-destructive detection,,simultaneous acquisition of time-domain and frequency-domain spectrum have outstanding advantages in studying the physical and chemical properties of materials.Therefore,it,can be complementary to the infrared spectroscopy and Raman spectroscopy for investigating the molecular dynamics,collective lattice vibration modes,and the ultrafast carrier transport behavior.However,new improvements are urgently needed in order to satisfy more emerging applications,i.e..higher signal acquisition speed,stronger terahertz sources,and broader spectrum in the frequency domain.The traditional terahertz time-domain spectroscopy system uses a low-speed moving linear displacement stage to generate time delay,and combines lock-in amplification technology for signal acquisition,which is inefficient.or has relative low signal-to-noise ratio In this thesis,the voice coil motor and the data acquisition card,essential to advancements of the terahertz system,air used for the fast time delay,and efficient signal collection,respectively.At the same time,the optical setup with broadband spintronic terahertz emitter for pressure calibration is also coupled into the setup to realize the in-situ terahertz experiment under high pressure conditions.For demonstration,two type of experiments have been done using the upgraded system.(1)In order to optimize the spintronic terahertz emitter,we have systematically investigated the transport behavior of ultrafast spin currents in the ferromagnetic(FM)/nonferromagnetic(NM)heterostructure via inserting the wedge-like Cu or Ag between the FM and NM layers.The decay length and transport velocity of the spin current are?_Cu=4.38±0.18nm and?_Ag=3.71±0.14nm,V_Cu=1.6×105±0.01m/s and V_Ag=2.3×105±0.01m/s,respectively.In Cu and Ag,the decay length of spin current decreases with the increase of frequency.The speed of spin current increases with the increase of frequency in Cu and remains basically stable in Ag.Based on the terahertz signals in the time and frequency domains as a function of the thickness of inserted layer.where the shunting effect and the light intensity influence on the terahertz signal were excluded,the ballistic transport behavior of the ultrafast spin current is revealed and confirmed.(2)The terahertz transmission spectroscopy of water under high pressure was investigated.By using the in-situ calibration system in a limited space,the absorption intensity and spectrum of water at different pressures were obtained.The experimental data show clear change in the amplitude of the transmitted terahertz signals when the phase transition in the water appears,and the related absorption features reveals some information about how the collective vibration of hydrogen bonds in response to the phase transition as well.Therefore,this thesis demonstrates how the ultrafast terahertz time-domain system is upgraded and what associated experiments can be done.This system behaves quite well,especially in the research of terahertz spintronics and terahertz spectroscopy at high pressure.,The new system is believed to be very helpful to explore the ultrafast spin transport and collective excitations in the near future.