Extension of high-order harmonic generation cutoff via coherent control of intense few-cycle chirped laser pulses

We present an ab initio quantum investigation of the high-order harmonic generation HHG cutoff exten- sion using intense few-cycle chirped laser pulses. For a few-cycle chirped driving laser pulse, it is shown that significant cutoff extension can be achieved through the optimization of the chirping rate parameters. The HHG power spectrum is calculated by solving accurately and efficiently the time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. The time-frequency characteristics of the HHG power spectrum are analyzed in detail by means of the wavelet transform of the time-dependent induced dipole acceleration. In addition, we perform classical trajectory simulation of the strong-field electron dynam- ics and electron return map. It is found that the quantum and classical results provide complementary and consistent information regarding the underlying mechanisms responsible for the substantial extension of the cutoff region. Recent developments in attosecond metrology have al- lowed the production of laser pulses with reproducible tem- poral evolution of the electric field by using phase-stabilized optical frequency combs 1,2. Attosecond laser pulses can be produced by means of the high-order harmonic generation HHG process in rare gases 3-5. Coherent control tech- niques can be applied to the study of the interaction between matter and laser fields to obtain a desirable outcome by ma- nipulating the temporal shape of the interacting laser field with high precision 6. It has also been shown theoretically and experimentally that the enhancement of selective har- monics can be accomplished by adjusting the pulse shape of the excitation laser pulse via an evolutionary algorithm opti- mization scheme 7,8. Similar HHG enhancement can be achieved by coherently controlling the dynamics of the elec- tron wave packet via chirping the femtosecond laser pulse 9. Indeed, there has been an increasing interest recently in the use of frequency- and amplitude-chirped pulses to per- form quantum control of physical and chemical processes. For instance, chirped pulses have been applied to the control of population transfer in Rydberg atoms 10, and in the mea- surement of attosecond electronic dynamics 11. In this paper, we present a detailed ab initio quantum investigation of the HHG cutoff extension by means of the optimization of the chirping parameters of intense few-cycle driving laser pulses. The time-dependent Schrödinger equa- tion TDSE is solved accurately and efficiently by means of the time-dependent generalized pseudospectral TDGPS technique 12. The time-frequency characteristics of the emitted pulses are analyzed by means of the wavelet trans- form of the induced dipole acceleration 13. Our results show that a significant extension in the harmonic generation cutoff can be realized by the use of optimized chirped laser pulses. Since the extended cutoff position is now located at higher frequency, the time duration of the attosecond pulse produced is also reduced. The HHG and attosecond xuv pulse generation 14 can be studied by solving the following TDSE in atomic units: