Generation of 40-as few-cycle pulse through chirp manipulation

Abstract

We discuss theoretically how supercontinuum spectra produced from high-order harmonic generation processes in a synthesized same-color laser field is optimized by adjusting the chirp parameter of the controlling pulse. Furthermore, a 40-attosecond isolated pulse with an effective bandwidth of 121 orders is obtained from He+ ion when the chirp rate ratio of pulses has a small value. The numerical results show that the efficiency of single as pulse generation is enhanced, and the quantum paths are controlled successfully. Our simulation shows that the produced pulses with high signal-to-noise ratio are obtained straightforwardly without any phase compensation. These results are explained using the classical approach.OCIS codes: 190.0190, 140.0140. doi: 10.3788/COL201210.081901.High-order harmonic generation (HHG) has been of great interest due to its potential application as a coherent soft-X-ray source [1] and in the generation of attosecond (as) pulses [2,3] . Thus far, the HHG is considered the most promising approach in generating attosecond pulses. For practical application, the straightforward attosecond metrology prefers an isolated attosecond pulse, and as such, much effort has been exerted to obtain an isolated as pulse [4,5] . In the single-atom level, two of the most important methods of generating an isolated attosecond pulse involve the use of a few-cycle laser [6] and the polarization gating technique [7] . Control of quantum paths is another effective way of producing an isolated broadband ultrashort attosecond pulse [8] . To obtain a regular and intense short attosecond pulse, some promising ways have been proposed to control quantum paths, such as the two-color schemes [9,10] and preparing ions in a coherent superposition of the states [11] . Recently, Xu et al.[12] proved a combined same-color field scheme that can broaden the high-order harmonics plateau; they obtained an isolated 59-as pulse. In this letter, based on Ref.[12], we propose a method, in which the chirp of the laser pulse is controlled to generate a short and intense isolated broadband attosecond pulse. When the ratio of the chirp factors of the fundamental and controlling laser fields has a small value, the bandwidth of the harmonic plateau is broadened to 200 eV with some very slight modulations. Practically, to generate the 10 fs at 800-nm pulse, hollow fiber compression technique can easily be used [13] . For achieving more precise and accurate results and specially isolated attosecond with regular structure (i.e., without any phase compensation), we investigate the generation of high-order harmonics based on one-dimensional (1D) HHG calculations. We also used atomic units in all equations below.To verify our scheme, we investigated the HHG and the attosecond pulse generation by solving numerically the 1D time-dependent Schrodinger equation for He + ion. We chose the soft-core Coulomb potential V = −a/ √ b + x 2 for He + . We also set soft-core parameters to be a = 2 and b = 0.5, which can reproduce the groundstate binding ener...

Generation of isolated attosecond pulses of sub-atomic-time durations with multi-cycle chirped polarization gating pulses

“…From Fig. 2(a), we can clearly see that in this absence of the chirped effect, the harmonic emission occurs every half of an optical cycle of the laser pulse and each emission show up and down arms with nearly equivalent intensities, which is defined as the long trajectory and the short trajectory [12,16,19], respectively. Because of the interferences between different quantum trajectories, HHG spectrum is modulated significantly, as indicated in Fig.…”

“…As contrast to the attosecond pulse train, an isolated attosecond pulse, especially the one close to one atomic unit (i. e., ~24 as), is more benefit for measuring and controlling the ultrafast electronic dynamics inside the atoms, such as the inner-shell electronic relaxation and ionization by optical tunneling [4,15]. Hence, a series of methods have been proposed to generate the isolated attosecond pulses with multi-cycle laser pulses, such as two-color scheme [10,12,[16][17][18], the chirped control method [19][20][21][22], the polarization gating method [7,[23][24][25][26]. Recently, when a chirped control pulse combined with the polarization gating pulses interacts with the gas, an isolated ~70 attosecond pulse is filtered out from the supercontinuum spectrum [27,28].…”