Abstract
We present a theoretical study of isolated attosecond pulse generation in pre-excited medium driven by an intense few-cycle laser pulse. We show that the generation of the macroscopic xuv supercontinuum is governed by the initial population of the excited state. Large initial population of the excited state leads to the high density of the free electrons in the media and strongly changes macroscopic properties of the driving pulse and the supercontinuum, and diminishes the isolated attosecond pulse generation. Instead, small initial population of 5% in the pre-excited media subjected to the few-cycle driving pulse can produce well phase-matched xuv supercontinuum, and a pure intense isolated attosecond pulse with the pulse duration of approximately 150 as and the pulse energy up to 0.5 nJ is directly obtained.
Highlights
Fascinating developments in attosecond science and technology lead to the generation of isolated xuv pulses as short as one hundred attoseconds [1]
Another alternative way is to prepare a coherent superposition state of the target, i.e., to populate part of the electron wave packet (EWP) at the excited state [23,24]. This part of EWP can be projected to the continuum, while the population of EWP at the ground state with much larger ionization potential is hardly ionized and prevents the saturation of high-order harmonic generation (HHG). Such pre-excited system with large ground-state potential can experience very intense driving field and emits harmonic photons with high energy and considerable efficiency, which serves as a good candidate to achieve intense isolated attosecond pulse generation
It is found that the temporal and spatial characteristics of the isolated attosecond pulse generation are governed by the initial population of the excited state, which is responsible of the ionization of the system and thereby the density of the free electrons in the medium
Summary
Fascinating developments in attosecond science and technology lead to the generation of isolated xuv pulses as short as one hundred attoseconds [1]. This xuv-assisted ionization enhancement can gate the ionization times to produce efficient isolated attosecond pulse [22] Another alternative way is to prepare a coherent superposition state of the target, i.e., to populate part of the electron wave packet (EWP) at the excited state [23,24]. This part of EWP can be projected to the continuum, while the population of EWP at the ground state with much larger ionization potential is hardly ionized and prevents the saturation of HHG Such pre-excited system with large ground-state potential can experience very intense driving field and emits harmonic photons with high energy and considerable efficiency, which serves as a good candidate to achieve intense isolated attosecond pulse generation. The calculation details can be found in [28]
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