Improvements for Generating High-Order Harmonics and Attosecond Pulses with Ultrashort Laser Fields

Nonlinear processes of high-order harmonic generation (HHG) produced by ultrashort few-cycle laser pulses possess interesting features which HHG produced by long pulses of many cycles may not have. First, HHG spectra produced by ultrashort pulses are extremely sensitive to the driving pulse waveform, which can be controlled by laser parameters such as carrier-envelope phases (CEPs), time delays or frequency chirps. Second, HHG spectra produced by ultrashort pulses can exhibit broad uneven peaks which are different from usual odd-ordered harmonic peaks that long pulses produce. Based on the high sensitivity on pulse waveform of HHG spectra produced by ultrashort pulses, we investigate for a two-color few-cycle pulse how HHG can be enhanced by the use of time delays or frequency chirps. Both a numerical and an analytic method are employed to calculate HHG spectra from a single H atom. For the time delay case, our results show that a time delay between the two-color, few-cycle pulses can increase the intensity of an HHG spectrum by an order of magnitude (or more) compared to the no-time-delay case at the cost of a reduction in the HHG plateau cutoff energy. For the frequency chirp case, we show how changing signs of chirps in each of the two component few-cycle pulses leads to drastic changes in the HHG spectra. Both time-frequency analyses from the numerical method and a semiclassical interpretation from the analytic method provide clear physical explanations of how HHG spectra are changed by those time delays and chirp signs. Based on the broad uneven peaks in HHG spectra produced by ultrashort pulses, we investigate how the duration of an isolated attosecond pulse can be minimized by carefully selecting frequencies in an HHG spectrum and provide a solution for the attochirp problem in generating attosecond pulses. Specifically, three frequency-selection categories are studied: a single spectral range between cutoffs, a single spectral range across cutoff and a striped-frequency range. Our results show that among all three categories the striped-frequency range produces the shortest and strongest attosecond pulses for a broad HHG spectrum with transform-limited duration