Improvement Of Ultrafast Time-resolved Spectroscopic System And Its Applications

The traditional semiconductor material made a great contribution to the development in the photoelectric field.Somehow with the advent of post-Moore era,it has been put forward a higher demand for the photoelectric information systems.Optoelectronic devices are developing towards the direction of smaller volume,larger bandwidth,lower power consumption,integration and intelligence,which have prompted the researchers to explore new materials,new structures and new mechanisms.In recent years,new optoelectronic materials like perovskite,transition metal dichalcogenides（TMDCs）and superstructure materials have shown great application potential in the optoelectronic field,and continuously update the performance in various of optoelectronic devices.However,the mechanism of the physical properties of these new materials has not been fully explored,especially on the optical physical process of the micro or nano scale,which limits the performance optimization and wider application of the optoelectronic devices with these new materials.The physical processes of the interaction of photons,phonons,excitons and electric charges usually occur over very short periods of time from nanosecond to femtosecond.Until the advent of ultrafast spectroscopy,there were very few methods to study these processes.The ultrafast spectroscopy technology is suitable for ultrashort optical pulse sequences ranging from femtosecond to picosecond.It can be used to study the photoinduced dynamic process of meta-excitation which is closely related to the performance of the nano devices.It provides a solution for understanding the interaction mechanism between light and matter in optoelectronic materials and revealing the transient and steady-state properties of excited state of optoelectronic materials.In view of the above,we build and improve the ultra-fast time resolution system,and use the ultrafast pump-probe technology and transient absorption technology to study the photophysical process of new photoelectric material systems with different dimensions.With the function expansion of the ultrafast spectral system,the interaction mechanisms between high-energy pump light and lithium niobate（LN）crystals,TMDCs,lead halide perovskite quantum dots are deeply studied.The research results are obtained as the following.1.The functions of polarization resolution detection,variable temperature detection,microscopy detection and in-situ detection are expanded based on the ultrafast time-resolved spectroscopic system in the laboratory.The polarization resolution detection can realize the modulation of different polarization states of pump or probe light.The spin characteristics of carriers in materials can be studied by analyzing the change of polarization states of the probe signal.The variable temperature detection system is integrated with precision temperature control platform.The limit temperature is less than 25 K,which makes it possible to systematically study the relationship between the photoexcitation transient process and the temperature.The microscope detection enables the system to maintain the time resolution of 100 fs and improve the spatial resolution of the system to the order of 10μm,making it possible to detect single particle and dynamic process of unit functional structure.The in-situ detection system combines the ultrafast pump-probe system with the femtosecond laser machining system,which makes it possible to in-situ monitor the femtosecond laser modification,and provides an effective means for the research of the mechanism and technology improvement of the femtosecond laser.2.The transient characteristics of Cherenkov-type phonon polaritons（Ph Ps）in LN crystals are studied by in situ pump-probe system with high time resolution.By identifying the locally excited Ph Ps,our findings identifies the time-domain and THz-domain features of Cherenkov-type Ph Ps in pristine LN crystals,which has honeycomb-like structures in THz domain（or k-space）and a Cherenkov cone in real space,analogues to the inversion case of honeycomb lattices in real space and Dirac cones in k-space for the novel materials with honeycomb lattices like graphene.In combination with femtosecond laser direct written（Fs LDW）techniques,we demonstrate that the effects of Fs LDW on LN crystals are the modulations of Ph Ps,which is helpful for the establish of Ph P threshold criterion guiding for ultrasoomth laser nanofabrication.It can offer opportunities to achieve an in-situ,real-time,nondestructive and quantificational examination to the three-dimensional femtosecond laser modification inside solids.3.The electronic state evolution of TMDCs（monolayer WSe₂）modified by femtosecond laser has been studied by using the in situ and microscope ultrafast pump-probe system.We have analyzed the steady-state and transient spectra of the monolayer WSe₂ and presented key stages of electronic state evolution for intense femtosecond laser irradiations treated.We find the femtosecond-laser-induced oxidation mechanism,which can induce the formation of lateral type-I WSe₂/WO₃ heterostructures by the replacement of the Se atoms near the monolayer WSe₂ into O atoms and the formation the WO₃ clustes.Both X-ray photoelectron spectroscopy measurements and first-principle calculations further confirm the femtosecond-laser-induced oxidation mechanism.In addition,the control experiments on monolayer WSe₂ covered by graphene not only indirectly demonstrate the oxidation reaction,but also directly reveal an efficient extraction for the band-edge exciton states in monolayer WSe₂.This study provides a solution for real-time in-situ monitoring of femtosecond laser modification of monolayer TMDCs materials by directly observing the electronic state near the modification threshold.This primary monitoring method can be applied to TMDCs materials in which the sulfur group elements are easily oxidized under environmental conditions4.The dynamics of spin polarization of excitons in Cs Pb Br₃ quantum dots is studied by using polarization resolution and variable temperature ultrafast pump-probe system.The Cs Pb Br₃ quantum dots with a particle size of about 8 nm are prepared by thermal injection method.The transient absorption of Cs Pb Br₃ quantum dot films is measured at variable temperature,and the transient absorption dynamic characteristics of co-polarization and oppo-polarization detection were analyzed.It shows that the ground state bleaching signal of Cs Pb Br₃ quantum dots will split at low temperature and the Rashba splitting caused by the center inversion asymmetry of Cs Pb Br₃ quantum dots may be one of the main reasons for the level splitting.It is also found that the spin relaxation time of Cs Pb Br₃ quantum dots is gradually shortened with the decrease of temperature,which is more consistent with D′yakonov-Perel′spin relaxation mechanism.The research provides a physical basis for the research of spin devices based on perovskite quantum dots.In this thesis,we have researched the new physical phenomena and complex physical mechanism of photoelectric materials under different dimension system through the function expansion of ultrafast time-resolved spectroscopy system.The results of the physical processes study on the same kind of material have a certain universality,which provide a theoretical basis for the improvement of photoelectric performance and the wider applications of novel optoelectronic materials.It also provides technical routes and solutions for the research of more complex and diverse physical process and mechanism.