Quantum Control And Broadband Supercontinuum Generation By A Bichromatic Field

The appearance and development of the attosecond source based on the high harmonic generation brought up the revolutionary promotion of the ultrafast detecting technique. The extremely high time resolution of the attosecond pulses makes possible the study of the ultrafast processes inside the material. The attosecond pulses have been sucessfully applied in the dections of the ultrafast electrionic process inside atom, the ultrafast plasmonic dynamics on the surface of the nano-material, and the ultrafast process inside the condensed matter. The time resolution and the detecting efficiency of the attosecond pumb-probe technique are determined by the bandwidth and the intensity of the attosecond source. However, the bandwidth of the single attosecond pulse from the conventional method is approximately 20 eV, which is hard to breakthrough the 100-as barrier, and the intensity is also limited. Recently, it has been theoretically and experimentally studied that the quantum control in a bichromatic field is an efficient way to obtain an attosecond pulse with the pulse duration under 100 attosecond. Due to the abundant physics characteristics, it is needed to further investigate how to obtain the single attosecond pulses with much broader bandwidth, shorter duration and higher intensity via optimizing this control process. Aim at the above questions, the main contents and the innovations of our works are:(1) We investigate the optimization of the broadband supercontinuum generation by the conherent quantum control in a bichromatic field. First, we propose that the using a uv pulse to enhance the efficiency of the generated broadband supercontinuum for a multicycle bichromatic pusle and also select the short quamtum path, which can support the efficient generation of the 100-as pulse. Secondly, we propose a method to abtain a broadband supercontinuum and also achieve the quantum path selection using a non-linearly polarized bichromatic field. In this control scheme, the intensity variation of the control pulse can hardly change the bandwidth and the location of the generatd supercontinuum, and then a broadband single attosecond pulse with high signal-to-noise ratio can be stably produced.(2)We propose that using a static field or quasi-static field to non-conherently control the HHG process. It is found that a strong static field can significantly extend the spectrum cutoff. Moreover, most of the harmonics are attributed to the short path, i.e., these harmonics are locked in phase. The phase-locking harmonics are emitted once per cycle, which enables the generation of few-cycle attosecond pulses with stable carrier-envelope phases from pulse to pulse. We further investigate the broadband supercontinuum generation in such control scheme and find that few-cycle driving pulse synchromized by a low-frequency field can produce a supercontinuum with the bandwidth of 155 eV, and a sub-50-as pulse is directly obtained.(3) We extend the conherent quantum control scheme to the mid-infrared spectral range covered with the "water window", which is an effective way to produce broadband supereontinuum. We use a weak 800-nm near infrared pulse to control the HHG process to obtain a efficient broadband "water window " supercontinuum, which enables the efficient generation of single "water window" X-ray attosecond pulse with tunale central wavelengths. We further apply this control scheme to "polarization gating" technique, resulting in the effective generation of ultrabroadband supercontinuum covered with the spectral range from ultrviolet to "water window" X-ray. This supercontiuum enables the generation of sub-100-as pulses with tunable central wavelengths, and also support the generation of attosecond pulses with the duration far below 1 atomic unit (24 as).