Controlling the intrinsic atomic phase in high-harmonic generation

Summary form only given. Recently, the process of high-order harmonic generation has received considerable attention. The dependence of the harmonic spectra on laser intensity, wavelength, polarization, and pulse width have all been studied experimentally and numerically. However, more detailed comparisons of models with experiment has proven to be difficult, primarily because of the lack of detailed and reliable experimental data. In this work, we provide a theoretical explanation for our experimental observation that the chirp of the excitation laser used to generate the harmonics dramatically changes the observed spectra of the high harmonic output. For positively chirped pump pulses, the individual harmonic peaks are discrete and well defined, while for negatively chirped pump pulses, the harmonic output spectra merge into a continuum. By considering the intrinsic phase shift of the harmonic emission with respect to the driving laser pulse, we show that these effects can be successfully explained using both analytic semiclassical theory and quantum models.