Development Of TrARPES Setup And Its Applications

Time-resolved and angle-resolved photoemission spectroscopy（tr ARPES）is a new technology developed in past ten years.It combines the technique of ultrafast laser and ARPES to obtain a momentum-resolved,energy-resolved and time-resolved ability to study the electronic structures.Tr ARPES has an wide application on the research of unoccupied states,high-_Csuperconductor,excitonic insulator,charge density wave and Floquet band engineering,etc.As a new experimental technology,there is no mature commercial product to choose from and the tr ARPES equipments are always designed and built by the research groups.The main technical difficulty lies in how to design a suitable ultrafast light source and how to combine the light with the ARPES system.At present,there are two methods to achieve the ultrafast light source for tr ARPES,one is the near-infrared（NIR）+ultraviolet（UV）/deep-UV based on the second harmonic generation（SHG）in crystals,and the other is NIR+extreme-UV based on the high harmonic generation（HHG）.Both of them own their advantages and disadvantages in terms of photon energy,repetition rate,photon flux,energy and temporal resolutions and wavelength tunability.We have designed and built the first HHG-based tr ARPES setup with high pho-ton flux and at high repetition rate in China.We solved the problems of HHG efficiency improvement,harmonics isolation,pump and probe pulses overlap,and automatic scan-ning in time domain,etc.The setup can be operated with tunable photon energy selec-tively among 12,16.8 and 21.6 e V at a repetition rate of 400 k Hz and with the photon flux of 10¹⁰-10¹¹photons/s.The energy and temporal resolutions of the three harmonics are measured as 104/111/157 me V and 276/190/154 fs。At the same time,we developed a dual-wavelength DUV ultrafast light source based on the SHG in crystals for tr ARPES.The photon energy of pump pulses can be switched between 1.2 and 2.4 e V,while that of probe pulses can be switched between 6and 7.2 e V.Four different polarizations can be selected for both of the pump and probe pulses.The setup can be operated at the repetition rate of 500 k Hz.The energy and tem-poral resolutions are determined as 13 me V and 300-500 fs.Furthermore,we designed a pulse compressor to compress the pulse duration of the fundamental beam from 320fs to 80 fs,providing a potential to improve the temporal resolution in the future work.We used our tr ARPES setup to measure the band structures of topological ma-terials Zr₂Te₂P,Bi₂Se₃and Mn Sb₂Te₄.At first,we measured the equilibrium band structures of Zr₂Te₂P using the three different photon energies.It showed that higher photon energy enable a larger accessible measuring range of Brillouin zone,which is one of the advantages of HHG.We performed the same measurements on Bi₂Se₃.The results showed a significant dependence on photon energy,highlighting the importance of the tunability of the XUV source.We chose the harmonic of 16.8 e V to measure the non-equilibrium states of Bi₂Se₃.The data quality shows the stability of the XUV source,which warrants its capability in time-resolved experiments.Finally,we use the SHG-based ultrafast laser（1.2 e V+7.2 e V）to study the band topology of hole-doped sample Mn Sb₂Te₄.By measuring the unoccupied states above_F,we directly proved that both of the ferromagnetic and antiferromagnetic samples of Mn Sb₂Te₄are topolog-ical trivial,which partially solved the controversy over the topological property of this system.