Ultrafast Spectroscopic Study Based On Spin Polarization Properties Of Two-Dimensional Perovskite Microcavity Strong Coupling System

Perovskite materials have attracted wide attention because of their excellent optoelectronic properties,such as large absorption coefficient,high carrier mobility and high fluorescence quantum yield.It has great application value in solar cells,light-emitting diodes,photodetectors and other fields.As a member of perovskite materials,2D perovskite exhibits better humidity and temperature stability than traditional three-dimensional perovskite,so it is considered to be one of the most potential new optoelectronic materials.The strong coupling interaction between light and matter is an important direction of basic physics research.The polaritons produced in the strong coupling system can exhibit the properties of light and matter at the same time,such as small effective mass,strong interaction and nonlinear effect.Based on its unique photophysical properties,polaritons have been widely used in the field of optoelectronic devices.Because the exciton binding energy of traditionalⅢ-V inorganic semiconductors is small and the structure of organic semiconductors is uneven,it is difficult to study polaritons at room temperature.2D perovskite is considered a high-quality material for the study of room-temperature polaritons because of its large exciton binding energy,stable chemical properties and uniform structure.At present,the research on its strong coupling system has been carried out,but it is mainly focused on the innovation of preparation methods and the improvement of preparation process,which are used to improve the steady-state optoelectronic properties.However,it is not clear whether the strong coupling system inherits the spin polarization characteristics of 2D perovskite materials,which is very important for its application in spin devices.The generation and evolution of polaritons and the relationship between polaritons and perovskite excitons are also not clear.These problems limit the further application of strong coupling systems in polariton devices.Solving the above problems has a very important significance to better understand the strong coupling system of 2D perovskite microcavity,and also lays a theoretical foundation for the preparation of polariton devices with excellent performance.For the above purpose,different organic ligands were selected to prepare（PEA）₂Pb I₄,HDADPb I₄,（EOA）₂Pb I₄ perovskite microcavity strong coupling system.The ultrafast spectral detection of the sample was carried out by the circular polarization pump-probe technique.The spectral characteristics of the strong coupling system and the effects of the length of the microcavity,the temperature and the binding energy of perovskite excitons in 2D perovskite microcavity strong coupling system are studied.The spin polarization characteristics of the strong coupling system,the internal polariton-polariton interaction and the polariton-phonon coupling process are clarified.The specific research results are as follows:1.The coupling system of（PEA）₂Pb I₄ microcavity with different microcavity lengths was prepared.The anticrossing distribution of the peak position dispersion curve in the coupling hybrid state was observed in the steady-state reflection spectrum,which proved that a strong coupling system was formed.Furthermore,the ultrafast spectra of samples with different microcavity lengths were measured by the circular polarization pump-probe technique,and it was observed that the peak dispersion curves of bleaching signals in the upper and lower polariton branches also showed anticrossing distribution.When the spin-coating speed of the PMMA layer is 6000rpm,the coupling between perovskite and cavity photons occurs in the resonant state.The strong coupling effect of the sample is the strongest and has the characteristic of spin polarization,which shows the inheritance of perovskite properties.Through the circular polarization ultrafast spectrum of the sample at different temperatures,it is confirmed that the spin polarization of the sample always exists at different temperatures.At low temperature,it is observed that the reference perovskite sample has two different phases,the two phases do not share the ground state,and the dynamic of the bleaching signal show non-synchronous oscillation.However,the strong coupling sample produces three new strong coupling hybrid states.The three hybrid states share the same ground state and show synchronous dynamic oscillations with the same oscillation period.This phenomenon reveals that the strong coupling interaction between the two phases and the cavity photons forms the triple strong coupling hybrid state.The amplification of spontaneous emission caused by the triple strong coupling hybrid state can be observed through the steady state spectrum at low temperature.2.The 2D perovskite microcavity sample with the best strong coupling effect was used to study the properties of the internal polaritons.The anticrossing distribution dispersion curve of the strong coupling hybrid state and the Rabi splitting energy of0.137e V were observed by steady-state angle-resolved reflectance spectroscopy.The circular polarization ultrafast spectrum was detected by the pump-probe technique.Under the lower energy hybrid state near-resonant excitation,the photo-induced absorption signal with spin polarization based on band gap renormalization is detected.The energy blue shift of the lower energy hybrid state appears under the same circularly polarized detection,and the offset increases with the increase of pump power.This process can be attributed to the repulsive between polaritons,which can be detected only in the presence of spin polarization,and reflects the inheritance of polaritons to excitons.Under higher energy hybrid state resonant excitation,the spin polarization of the two hybrid states can be observed.Compared with the uncoupled perovskite,the microcavity strong coupling system has more energy states that reflect spin polarization.Dynamic oscillations were observed in the bleaching and photo-induced absorption signals.This phenomenon is caused by the coupling of polariton and phonon,and it is also the inheritance of polariton from exciton.3.（PEA）₂Pb I₄、HDADPb I₄、（EOA）₂Pb I₄ microcavity coupling systems with different exciton binding energies were prepared.In the steady-state reflection spectrum,the reflection peaks of samples showed obvious splitting,which proved that the system was in a strong coupling range.Circular polarization ultrafast spectra of different samples were measured by the pump-probe technique.It is observed that the spin lifetime of polaritons in the strong coupling system is the same as that of the corresponding perovskite.It will increase with the decrease of exciton binding energy,which proves that polaritons inherit the properties of excitons.By changing the pump power,it is observed that the spin lifetime of the strong coupling system is modulated.The spin relaxation of different samples is mainly affected by the exchange of electrons and holes under high pump power.At low pump power,the spin relaxation of the（EOA）₂Pb I₄ microcavity with lower exciton binding energy is mainly affected by the DP mechanism.The spin relaxation of the（PEA）₂Pb I₄ microcavity with higher exciton binding energy is dominated by the MSS mechanism.The spin relaxation of HDADPb I₄microcavity is affected by the two mechanisms.Furthermore,the dynamic oscillations based on polariton-phonon coupling in different samples are observed.The smaller the exciton binding energy of perovskite material is,the longer the dynamic oscillation period and duration of the strong coupling hybrid state is.