First Principles Simulations Of Ultrafast Laser-induced Phase Transition In Condensed Matters

Ultrafast laser can regulate the structure and properties of condensed matter,inducing materials to achieve hidden states that cannot be achieved under thermal equilibrium.The wide spectrum and time range characteristics of lasers allow them to be coupled with different degrees of freedom in materials,providing more possibilities for precise manipulation of material properties,and have wide applications in basic physics,information science,biomedicine,and other fields.Although many cases of ultrafast laser-induced phase transitions have been found experimentally,their intrinsic mechanism and dynamics on the ultrafast time scale remain ambiguous.For example,by measuring second harmonic generation,researchers have found that ultrafast lasers can induce metastable ferroelectricity in paraelectric materials,but this macroscopic measurement is difficult to directly reflect the electron excitation and the microscopic motion process of ions.Based on nonlinear phonon control,ultrafast lasers can drive structural transformation in materials,but the specific conversion path is unknown.In addition,as a new method,whether ultrafast laser can induce new ordered phases in metal materials is very interesting and worth exploring.Theoretical research can not only improve our basic understanding of laser-induced material phase transition,but also help us find new methods to manipulate the macroscopic properties of condensed matter and guide the synthesis of new quantum materials.Using the non-adiabatic molecular dynamics method,we can simulate the non-equilibrium process of the system and explore the photoexcitation phenomena.In this work,we will introduce the ultrafast laser-induced ferroelectricity,photoinduced sp²-sp³ structural transformation in graphite,and photoinduced solid-solid phase transition in metals in turn.The main contents are summarized as follows:1.We simulate the ultrafast dynamic process of laser-induced ferroelectric phase transition in Sr Ti O₃ and explore the electronic origin of photo-excited ferroelectricity.We find that the paraelectric-to-ferroelectric transition in Sr Ti O₃ originates from the spatial inversion symmetry breaking induced by photoexcitation.The femtosecond laser pulse drastically changes the charge density between the top O and Ti ions along the laser polarization direction,which induces a transient uniaxial tensile deformation.We track the ion motion and charge transfer during the ferroelectric phase transition.The results show that the formation of ferroelectricity is closely related to the charge changes of different atomic orbitals.At the same time,we discuss the effects of laser intensity and wavelength on photoinduced polarization.In general,long-wavelength intense laser excitation is more conducive to polarization formation in Sr Ti O₃.The optical manipulation method of ferroelectric polarization provides great potential in developing ferroelectric devices with greater degrees of freedom.2.Experiments have found that graphite materials can be transformed into a sp³hybrid structure under ultrafast laser irradiation,but the microscopic transformation path is unclear.Using non-adiabatic molecular dynamics calculations,we reveal the pathway and mechanism of laser-induced structural transformation of graphite.We identify three consecutive stages for the laser-induced sp²-sp³ bond transformation in graphite and analyze electron excitation and phonon excitation in detail.We find that the structural transformation process,including interlayer compression and bonding,is driven by the cooperation of charge carrier multiplication and selective phonon excitation through electron-phonon interactions.High laser fluence is a necessary condition for the irreversible compression of graphite layers.In addition,we believe that the hot electrons scattered intoσconduction bands maybe play a key role in the introduction of in-plane instability in graphite.3.In addition to simulating and analyzing the existing photoinduced phase transitions that have been found in experiments,we also explore the feasibility of solid-solid phase transition in metals induced by femtosecond laser pulses.We find that femtosecond laser excitation induces two different ordered phases in ruthenium metal.In a constrained system,the lattice structure is transformed into a face-centered-orthorhombic structure by interlayer slip.The slip is driven by the Coulomb force generated by the light-induced spatially inhomogeneous charge distribution.The material undergoes an ultrafast hcp-to-fcc phase transition within 400 fs under unconstrained boundaries.Finally,we conduct a photoexcitation test on the magnesium with the same hcp structure and find that the lattice structure follows the Burgers path to transform into an unstable bcc phase.Obviously,femtosecond laser-induced phase transition in the metal is a nonthermal process indirectly driven by electronic excitation.Our exploration embodies the potential of ultrafast laser applications in metal materials.Based on the non-adiabatic molecular dynamics method,we explore the non-equilibrium process of ultrafast laser-induced phase transition in condensed matter from the perspective of electrons and phonons.This ultrafast optical control method provides a new idea for material design and has potential applications in material processing and optical memory.