Spatiotemporal Control Of Quantum Path And Broadband Isolated Attosecond Pulse Generation

High-order harmonic generation (HHG) is well known as a high-nonlinear process when atoms or molecules encounter a strong femtosecond laser. The most direct consequence of this process is to generate coherent light in the extreme ultraviolet (UV) and soft X-ray region, or the very attracted source of isolated attosecond pulses, making it possible to capture structure and dynamics with the unprecedented resolution on both the attosecond and angstrom scale. However, the potential applications of isolated attosecond pulses would be limited by the intensity, bandwidth, duration, beam quality and the dependence of control process on driving laser parameter and stability. Nowadays, it is still very important to further explore how to control HHG process for much higher efficiency, broader bandwidth, shorter pulse duration, better spatial quality and how to loosen the dependence of control process on driving laser parameter and stability, which is the aim of our work. The main contents and results of our work include:(1) We propose a method to powerfully control the electron acceleration process with multi-cycle two-color fields, named "long-distance" acceleration mechanism. In this mechanism, the harmonic cutoff is dramatically extended to IP+26UP and the harmonics above IP+15UP are emitted almost in phase, leading to a smooth supercontinuum with the bandwidth of11UP. By adjusting the intensity ratio of two-color fields, we can obtain supercontinuum with tunable central wavelength from extreme ultraviolet to the whole soft X-ray "water-window" domain. To overcome the defect of low efficiency from the wave packet spreading of the "long-distance" acceleration mechanism, we further propose a "double control gating" method, which combines the "long-distance" acceleration mechanism and the ionization property of He+ions. This method can simultaneously control the ionization and acceleration process, and generate broadband isolated attosecond pulse with high efficiency and spatiotemporal quality.(2) We investigate HHG in a spatially inhomogeneous field with mid-infrared wavelength and few-cycle duration. It is shown that the spatiotemporal synthesis results in dramatic control of the quantum path, not only broadening the harmonic plateau region but also selecting the short path, then a more than300eV supercontinuum is significantly obtained. Also, this spatiotemporal control of electron dynamics potentially reduces attosecond chirp, and close-to-Fourier-limit27as isolated pulses are straightforwardly filtered out without any chirp compensation. Furthermore, such short isolated attosecond pulses can be produced for almost all the carrier-envelope phases (CEP) of the few-cycle laser pulse.(3) We propose a new approach, which combines the temporal ionization character of a pre-excited medium and the spatial inhomogeneity character of plasmon-enhanced fields, to control the HHG process both in time and in space. This spatiotemporal control of quantum path can lead to broadband supercontinuum with high efficiency, and its implementation on isolated attosecond emission is robust against the CEP shift of the plasmon-enhanced field.(4) We investigate HHG in UV-assisted plasmonic fields. It is found that the quantum paths contributing to the harmonics can be dramatically enhanced by adding a femtosecond UV pulse, which permits high efficient harmonic radiation under relatively low driving intensity condition and thus potentially avoids thermal damage of nanosystems. The results also show that the UV pulse, acting as a temporal trigger, in combination with the spatial inhomogeneity effect of the plasmonic field, is robust for the control of quantum path and broadband supercontinuum generation, and this process is general and not limited to a certain driving pulse duration, inhomogeneous distribution form, and CEP of the plasmonic electric field.