The Synergistic Effect Of Built-in Electric Field During Strong-field Electron Emission From Solids

Electrons are the most fundamental particles that constitute matter,and their dynamical behavior influence the ultrafast processes within atoms and molecular dynamics processes,which consequently determine the electronic structure properties of solid materials.Electronic structures in solids with collective effects,such as the energy band structure of semiconductors,the free electron gas of metals,and their dynamic response to electromagnetic fields,are decisive factors for a number of physical properties and chemical reactions.The tracking and manipulation of the microscopic dynamics of electrons inside solids is also the key to revealing the macroscopic properties of solids and developing designed optoelectronic devices.Strong field ionization can excite electrons inside matter from undetectable bound states to detectable continuum states,thus becoming one of the fundamental tools to study electron dynamics processes in the microscopic world.In recent years,people’s understanding of the structure of atoms/molecules and their ultrafast dynamics under strong field ionization has gradually become clear.However,compared to atoms and molecules,the strong field ionization processes in solids are more complicated.The complex electronic structure and geometry,the mixed interaction mechanisms with laser fields,the elastic and inelastic scattering of electrons inside the solid,and the charge accumulation and damage threshold issues have posed great challenges to theoretical and experimental studies of strong field ionization in solids.In this thesis,the strong field ionization processes of three types of materials with different dimensional electronic structures,namely atoms(argon atomic gas),twodimensional materials(silicon-based monolayer graphene surface)and solid particles(copper nanosphere particles)under the radiation of femtosecond laser,have been investigated by the the photoelectron momentum distribution detected with velocity map imaging.The main contents are as follows:(1)The full three-dimensional momentum distribution of photoelectrons in the strong near-circularly polarized laser field of argon atoms has been measured by the technique of inverse projection chromatographic reconstruction technique,and the Coulomb focusing phenomenon of the transverse(i.e.,the direction not affected by the laser field)momentum during the strong field ionization has been investigated.The experimental results show that the transverse momentum distribution of the ionized electrons in the 410 nm near-circularly polarized field becomes narrower with increasing laser intensity,which is contrary to the law predicted by the conventional adiabatic theory.By analyzing the semiclassical electron trajectories obtained from non-adiabatic theory simulations,it is found that some electrons are temporarily trapped by the Coulomb potential during the evolution process,which leads to Coulomb focusing in the quasi-bound state,and consequently causes the narrowing of the transverse momentum.This work will help to understand and control the electron dynamics in coupled Coulomb systems,and also provide a reference for subsequent studies of more complex strong-field ionization in solids.(2)The photoelectron emission efficiency of single-crystal silicon wafers is significantly enhanced by covering them with a monolayer of graphene.This enhancement effect can be attributed to the synergistic effect on photoelectron emission by the built-in electric field formed between the monolayer graphene and the single-crystal silicon.On this basis,a micro-capacitor is formed by introducing a crevice structure on the surface of silicon-based graphene.Under the radiation of the incident laser field,a plasmonic enhancement electric field is formed inside the capacitor,which not only further increases the photoelectron yield,but also makes the polarization dependence modulated from the original highest electron yield when the laser polarization is perpendicular to the sample surface to the highest when it is parallel,thus providing a new development direction for solid surface defect detection technology.This work provides a new research direction for the design and all-optical modulation of high-quality electron sources.(3)The photoelectron emission process of copper nanoparticles over a large dynamic laser intensity range was investigated by combining the power of the aerodynamic lens for constantly refreshing samples in the particle beam and the power of the velocity map imaging for quickly obtaining detailed information about the electron kinetic energy and angular distributions.Electron emission with yields across six orders of magnitude and wellresolved angular distribution were recorded by operating the aerosolized VMI in either stacking or single-shot mode.Based on the distinct structural features in the twodimensional electron momentum distribution,the dominant photoelectron emission mechanisms at various light intensities were identified,including multiphoton ionization by surface electric field,and multiphoton and thermal ionization by built-in electric field,thus revealing the transition mechanism of the nanoplasmonic-nanoplasma state under the influence of the built-in electric field,and the corresponding physical model was constructed to confirm it.The correlated electron decay(CED)phenomenon in metal nanoparticles is also observed for the first time.Due to the extra high atomic density and full degeneracy compared to rare-gas clusters,the accompanied more substantial space charge effect will shift the characteristic peaks to higher energy.