Optical Spectroscopy And First-Principle Simulations Of Nonlinear Excitation Processes Of Quantum Materials

Ultrafast optical spectroscopy is developing rapidly and can be used to investigate the ultrafast dynamics of non-equilibrium states of quantum materials,even down to the realization of non-thermal melting and controlled phase transitions in materials.However,its atomic processes and microscopic mechanisms remain elusive due to complex interaction between many degrees of freedom of photon,charge,spin,lattice,and orbital.Using the ultrafast optical spectroscopy methods and time-dependent density functional theory simulations,the interaction mechanisms of light and matter in the excited dynamics can be deeply understood by studying the nonlinear optical effects in the system.In this work,we will introduce the nonlinear optics effects with the order of second harmonic generation（SHG）,third harmonic generation（THG）,High harmonic generation（HHG）and laser melting.The following is the majority of the research:1.We report the generation and control of SHG from a series of LaTiO₃/SrTiO₃superlattices under weak laser pulses.We demonstrate that the nonlinear optical susceptibilityχ^（2）of the monolayer interface superlattice is 24.3 pm/V,which is even 6times greater than the conventional ones（BBO）.The samples have good optical properties for potential optoelectronic devices.Our results exemplify the insight“interface is the device”.By controlling the temperature,interface layer,well or barrier,we can design and optimize the SHG on the nanometer scale.Therefore,our new method provides better versatility and flexibility for designing better frequency doubling optical devices.2.As the laser intensity increase,we further modulate the THG and HHG of two-dimension and oxides materials.Together with the time-dependent density functional theory simulations and experiments,we reveal the modulation process in graphene and Mo S₂.Here,we report that an optical pump can dynamically modulate the THG with a relative modulation depth above 90%at femtosecond time scale for a broad frequency ranging from near-infrared to ultraviolet.The results stem from the nonlinear dynamics of the photoexcited carriers.In addition,in graphene and Zn O,we find that the HHG increases with the laser ellipticity,which is related to the photoexcited electron scattering at the boundary of brillouin region.3.We investigate ultrafast dynamics of laser-driven non-thermal melting of the ceramics Mg O and semiconductor Silicon based on accurate real-time time-dependent density functional theory under the strong laser pulses.We report here that laser melting is greatly accelerated by high-order nonlinear dynamics of uniformly distributed photocarriers in an archetypal ceramic Mg O.Ultrafast energy transfer between the lattice and uniformly distributed electrons is resulted from fast carrier relaxation and strong electron-phonon couplings.In addition,we show that a coupled carrier multiplication and phonon generation process during carrier relaxation in silicon is a dominant force driving ultrafast structural changes.Universal abnormal damping by Pauli Exclusion Principle is predicted,consistent with existing experiments and hinting at new perspective in ultrahigh intensity laser applications.The combination of experimental techniques and theoretical calculations allows for a deep and clear understanding of the physical processes in the excited state and the use of optical methods to ultrafast"control"quantum materials.This will provide a direction for the realization of ultrafast optical switching,preparation of new materials for reading and writing optical data storage,and integrated optoelectronic circuits.