Time-frequency characteristics of high-order harmonics generated by intense femtosecond laser pulses

High-order harmonic generation in atomic gases under the influence of intense laser fields has received considerable attention owing to the expectation that it may be utilized to develop ultrashort extreme ultraviolet sources. Much effort is currently being devoted to controlling and optimizing the temporal, spectral and coherence properties of high harmonics, and enhancing the high harmonic generation efficiency. The Wigner distribution function that has been proven powerful for investigating quantum states in phase space is employed in our time-frequency domain analysis. The dynamically induced chirp of high harmonics is calculated both quantum mechanically and semiclassically, and a good agreement between them is reached.