Optimal Control Of High-order Harmonic Generation In Crystals

Dynamics of the microscopic world has always attracted people's attention.For more than half a century,the development of laser technology has greatly expanded people's understanding of the ultrafast dynamics of microscopic particles.Laser mode locking,optical parametric amplification,chirped pulse amplification and other tech-nologies effectively enhance the peak intensity of the laser while compressing the pulse width.These ultrastrong and ultrafast pulses induce a coherent light source with a wider spectrum range and a narrower pulse width on the target materials.These ultrafast pulses on the femtosecond or even attosecond scale provide an effective means for exploring ultrafast dynamics.Many ultrafast phenomena induced by strong lasers,such as above-threshold ionization and high-order harmonics,have been widely studied in recent years.At the same time,the application of ultrafast coherent light sources realizes the coher-ent control of electronic dynamics.Although ultrashort pulses are compressed to the scale of tens of attoseconds,the intensity has not yet reached the level that can be widely used.Compared with the gas,the crystal has a higher density and strict periodicity,and it is expected that it can be used as a compact and integrated target material to produce high-order harmonics more efficiently.However,there are few studies on laser-induced ultrafast elctron dynamics in crystals,and the multielectron coherent dynamics in crys-tals is very complicated,which is a difficult point in the field.In this paper,the related scientific problems are systematically studied and the innovative results obtained are as follows.1.We study the multielectron excitations and reveal the multielectron interference effect in crystal intraband high-order harmonics,that can be used to increase intraband harmonics and reconstruct energy bands.Early researchs on intraband harmonics are mainly based on a single k point,which are not enough to fully reveal the nonlinear dy-namics of intraband harmonics.We propose a k-resolved semiclassical intraband har-monic model,which reveals the multielectron interference effect of crystal high-order harmonics,and the energy bands can be reconstructed by using the intraband harmon-ics.This interference effect dominates the nonperturbation characteristics of high-order harmonics,helps to improve the conversion efficiency of high-order harmonics,and provides more means to control the electron-hole ultrafast dynamics.2.We propose the mode locking mechanism of crystal high-order harmonics,which can effectively improve the conversion efficiency of interband high-order har-monics.By studying the spatially dependent phase of the high-order harmonics in crys-tals,we discover the mode-locking mechanism of high-order harmonics,which provides a theoretical solution for further enhancing the output power of crystal high-order har-monics and detecting ultrafast dynamics.This mode-locking mechanism can control the interference of quantum trajectories and realize the attosecond resolution of electron dynamics and selective enhance the crystal high-order harmonics of specific frequency band.3.We establish the connection between tunneling and multiphoton processes,and reveal the important role of electron decoherence in excitation processes.Quantum tunneling and multiphoton processes are regarded as two distinct excitation mechanisms in strong field induced dynamics.We study the effect of wavepacket interference in the process of optical excitation,linke these two excitation mechanisms,and point out that the transition of excitation mechanism from multiphoton to tunneling is essentially a process of electron decoherence.Quantum coherence control based on multiphoton absorption can inject controllable currents into semiconductors,as well as control the formation and breaking of chemical bonds.The combination of the coherent control and ultrafast optics is expected to achieve attosecond-resolved coherent control.For this reason,we propose a quantum coherent control theory with attosecond resolution.Based on this theory,we reveal the characteristics of subcycle dynamics in the excitation processes,and discover a new coherent control mechanism for current injection.Based on the theoretical study of electron ultrafast dynamics in crystals,this paper reveals the relationship between multiphoton and tunneling process and reveals the key role of multielectron interference in crystal high-order harmonics at the microscopic level,and proposes a mode locking mechanism of high-order harmonics at the macro level.This is of great value to the realization of crystal high-order harmonic light sources and their applications,and the research of ultrafast optical switches and terahertz radi-ations.