Phonon And Spin Ultrafast Dynamics Of Low Dimensional Functional Materials

With the continuous development of science and technology,information storage and communication devices are gradually moving towards miniaturization,high frequency,and integration.How to achieve higher frequency optoelectronic modulation and faster information read-write has become the primary issue that hinders the further development of information communication technology.Traditional three-dimensional functional materials and conventional physical property control methods can no longer meet the requirements of the rapidly developing modern information technology industry.In recent years,thanks to the growing maturity of femtosecond laser technology and the constant emergence of various low-dimensional functional materials,humans have been able to achieve the manipulation of the physical properties of low-dimensional functional materials on the femtosecond time scale,providing new directions and opportunities for the exploration and development of high-frequency low-dimensional communication devicesAlthough researchers have conducted extensive research on the ultrafast dynamic behavior of three-dimensional functional materials,there is still no mature theoretical model that can help us understand the interaction between electrons,spins,and phonons on the femtosecond-picosecond(fs-ps)time scale and the dynamic process of particles and quasiparticles.Furthermore,low-dimensional functional materials may exhibit unique dynamic behavior due to their reduced dimensions.Therefore,studying the ultrafast dynamics in low-dimensional functional materials is of great physical and practical significance.This article focuses on the ultrafast dynamics of coherent phonons and spins induced by femtosecond laser in several typical low-dimensional functional materials.On the one hand,the goal is to explore the ultrafast dynamics and derived effects of particles and quasi-particles in low-dimensional materials,aiming to achieve highfrequency optoelectronic modulation and high-speed information read/write in lowdimensional functional materials.On the other hand,a comparative study with threedimensional functional materials is conducted to investigate the correlation between the dynamic behavior of low-dimensional functional materials and the lattice dimensionality from a macroscopic perspective,providing experimental support for the construction of the dynamic model of low-dimensional functional materials.The main research contents of this article are as follows:(1)Construction of time-resolved pump detection system under magnetic field.An time-resolved pump-probe detection system under magnetic field was constructed based on the principles of time-resolved pump-probe,magneto-optic Kerr effect,and phase-sensitive detection.Thanks to the perfect combination of SpectraPhysics femtosecond lasers and Newport’s electric displacement platform,the system can perform time-resolved measurements of ultrafast dynamics processes of 3.3 nanoseconds(ns)with a maximum time resolution of 150 fs.In addition,relying on the Oxford superconducting optical magnet,the system can provide a test environment of 1.5-300 K and 0-7 T for the tested samples.The construction of this system provides an efficient and stable testing platform for the study of phonon and spin ultrafast dynamics in low-dimensional functional materials.(2)Ultrafast dynamics of coherent phonons and GHz birefringence in LaRhO3/SrTiO3 heterojunctions thin film.The ultrafast dynamics of coherent acoustic phonons and GHz birefringence induced by phonons were investigated in LaRhO3/SrTiO3(LRO/STO)heterostructure thin films using time-resolved pumpprobe detection techniques.The research results show that femtosecond pulse laser can induce low-damping,high-amplitude coherent acoustic phonons in the transducer/STO heterojunction thin film.In addition,by optimizing a series of transducer materials,it was found that LRO semiconductor thin films have relatively high photon-phonon energy conversion efficiency.Due to the high sensitivity of STO to lattice perturbations,coherent phonons can induce GHz-frequency optical birefringence in LRO/STO heterostructure thin films,and the oscillation amplitude of birefringence has a significant crystal orientation dependence.Finally,we also achieved coherent control of laser-induced acoustic phonons and phonon-induced GHz birefringence using dualpump laser.This research demonstrates a new path for birefringence modulation using femtosecond pulse laser,providing guidance for the development of low-dimensional high-frequency optoelectronic modulator devices.(3)Ultra-long spin relaxation in two-dimensional van der Waals ferromagnetic Cr2Ge2Te6.In order to investigate the correlation between laserinduced spin dynamics and lattice dimensionality in two-dimensional van der Waals(2D vdW)ferromagnetic materials,and to promote the development of lowdimensional optically-controlled high-speed magnetic storage devices,we conducted a comparative study using time-resolved magneto-optical Kerr effect(TR-MOKE)detection technique on the laser-induced spin ultrafast dynamics of three Te-based materials:3D ferromagnetic metal Cr3T4(CT),2D ferromagnetic metal Fe3GeTe2(FGT),and 2D ferromagnetic semiconductor Cr2Ge2Te6(CGT).Our study found that unlike the one-step ultrafast demagnetization exhibited by 3D CT,the ultrafast demagnetization process of 2D vdW FGT and CGT is mainly dominated by two-step demagnetization,indicating that the electronic-spin coupling in 2D vdW ferromagnetic materials is relatively weak.Particularly interestingly,an extremely long remagnetization process was observed in 2D vdW CGT.Even after undergoing a spin relaxation process of 3500 ps following laser-induced ultrafast demagnetization,no magnetic recovery was found,making it the longest observed remagnetization process to our knowledge,which may be one of the characteristics of laser-induced spin dynamics in 2D vdW ferromagnetic semiconductors.In addition,based on the modified three-temperature model,the crucial roles played by lattice dimensionality and thermal diffusion anisotropy in the laser-induced spin dynamics of 2D vdW ferromagnets were revealed.This work not only opens the door to understanding the magnetic origin of 2D ferromagnetic materials,but also provides guidance for the development of lowdimensional optically-controlled magnetic storage devices.(4)Spin wave study of two-dimensional van der Waals ferromagnetic Fe3GeTe2.The frequency of spin wave oscillations and the damping factor determine the fastest rate at which "0" and "1" conversion can be achieved in future lightcontrolled magnetic storage devices.In order to explore the ultrafast dynamics of laserinduced spin waves in 2D vdW magnetic materials and screen suitable 2D vdW magnetic functional materials for light-controlled low-dimensional high-frequency magnetic storage devices,we used time-resolved magneto-optical Kerr detection technology to study the laser-induced spin dynamics process of nanoscale FGT thin films and conducted comparative studies on laser-induced spin waves of five common vdW ferromagnetic samples.Firstly,we found that under a 1 T magnetic field intensity,femtosecond pulsed lasers can induce FGT to produce spin waves with frequencies as high as～140 GHz,which means that "0" and "1" information can be switched in as little as 7 ps,the highest frequency of spin wave oscillation observed in 2D vdW ferromagnetic materials under similar conditions so far.Secondly,the experimental results showed that temperature,pump laser energy density,and magnetic field intensity can effectively control the frequency and effective damping factor of FGT spin waves.A frequency tuning of up to～60 GHz can be achieved when the environmental temperature is raised from 10 K to 210 K.Due to its extremely strong magnetic anisotropy,FGT has no significant dimensional effect on its spin dynamics behavior.Finally,the spin dynamics processes of five 2D vdW ferromagnetic materials were comparatively studied,and numerical simulations were conducted based on the LLG equation to analyze the test results.It was found that the frequency of spin waves in 2D vdW ferromagnets is positively correlated with the strength of the material’s magnetic anisotropy field,while the intrinsic damping factor α0 is determined by the spin-orbit coupling strength ξ and the d-band width W of the magnetic atoms.This work provides guidance for the development of vdW-based light-controlled magnetic storage devices.