Enhancing high-order-harmonic generation by time delays between two-color, few-cycle pulses

Abstract

Use of time delays in high-order-harmonic generation (HHG) driven by intense two-color, few-cycle pulses is investigated in order to determine means of optimizing HHG intensities and plateau cutoff energies. Based upon numerical solutions of the time-dependent Schrödinger equation for the H atom as well as analytical analyses, we show that introducing a time delay between the two-color, few-cycle pulses can result in an enhancement of the intensity of the HHG spectrum by an order of magnitude (or more) at the cost of a reduction in the HHG plateau cutoff energy. Results for both positive and negative time delays as well as various pulse carrier-envelope phases are investigated and discussed.

Enhancing high-order harmonic generation by sculpting waveforms with chirp

“…The calculation of each A j (E) amplitude begins by finding a corresponding classical trajectory that starts at t r , see Refs. [5,22].…”

“…[5]. One is to solve the three-dimensional TDSE for an H atom interacting with a laser electric field F (t) that is linearly polarized along the z axis.…”

“…In this description, the harmonic spectrum is obtained by coherently adding the amplitudes corresponding to a handful of electron trajectories ionized from different half cycles in the vicinity of the maximum of the short-pulse envelope [5]. For each j th electron trajectory, one can calculate the ionization and recombination times t r , the instantaneous Keldysh parameterγ j at the time of ionization, the ionization factor I j (which largely determines the spectral intensity), and the cutoff energy E (j ) cut .…”

“…Specifically, HHG spectra can be very sensitive to the carrier-envelope phase (CEP) [2] and the chirp [3,4] of a single few-cycle pulse, or to the time delay between two-color synthesized few-cycle laser pulses [5]. An optimized few-cycle or many-cycle waveform of the laser field can, e.g., selectively enhance a single harmonic while suppressing neighboring harmonics [6][7][8], or, alternatively, greatly enhance HHG yields across a large range of harmonic photon energies [5,[9][10][11][12][13][14][15][16].…”

“…The values of the parameters are often obtained by optimization techniques using iterative algorithms with feedback loops [6,8,9,[11][12][13][14][15][16], but can also be determined on the basis of physical arguments, e.g., for the purpose of increasing ionization rates and/or recollision energies of the active electron in HHG processes [5,9,10]. Chirp is an additional parameter for controlling the driving laser waveform and, hence, HHG spectra.…”