Study Of Non-equilibrium Dynamics Of High Temperature Supereconductor Based On Optical Pump Probe System

Optical pump-probe technology is an ultra-fast spectrum technology with a wide range of applications.It can be used to measure the internal transient process of the material under pump light excitation by adjusting the relative delay time between the pump light and the probe light,and more information can be obtained by changing the wavelength or pump power.Superconducting materials have important applications in the field of national defense,medical,information technology,scientific research and so on.Due to the difference in superconducting mechanism between iron-based,copper-based superconductors and other materials,the critical transition temperature of iron-based,copper-based superconductors is really high,so iron-based,copper-based superconductors are called high-temperature superconductors.Optical pump-probe technology is more and more important in the field of superconductivity,and it plays a key role in superconductivity measurement because of its high resolution and expandability.The main research contents are listed as follows:1.The principle and classification of ultra-fast spectroscopy were studied and the types of light sources required for the experiment to determine the experimental scheme of the optical pump-probe system.A femtosecond optical pump probe platform system was constructed,and the causes of noise generation in the optical path construction were analyzed.Improvement measures were presented from the aspects of attenuation system,optical delay line,and chopper modulation.The integrated control program of the lock-in amplifier and the electric displacement platform was designed by the LabVIEW software,which realized the automatic collection of data during the experiment and effectively improved the experimental efficiency.2.Characterize the basic physical properties of iron selenide materials,including X-ray diffraction,scanning electron microscopy,energy dispersive X-ray spectrometry,etc.,Then the information of the lattice structure,surface morphology,and stoichiometry of FeSe materials was obtained.The dispersion curves of the optical parameters such as refractive index,dielectric constant,absorption rate,and photoconductive spectrum were obtained by using the Kramers-Kronig relations through the reflection spectrum of FeSe in the visible light region.The parameters were compared and analyzed with other literature experiments,which were used for further analysis of superconducting properties are of great significance.At the same time,the depth of optical penetration is calculated,providing a reference for the sample selection in transient testing.3.The changes of light-induced reflection of GaAs and FeSe/CaF2 thin film samples were measured by a pump probe system,and their kinetic curves were obtained.The multi-exponential model was used for data fitting,then the function of the relaxation time with temperature was obtained through the model,and the dynamic process of it was analyzed.The collective excitation behavior during the process of transient excitation was also discussed based on above.