In recent years, intensive theoretical and experimental investigations using high-order harmonics have been performed. The optimization of attosecond pulse duration is important to achieve high temporal resolution needed for such application as inner-shell transition processes.
In this thesis, the spectrogram method was utilized to analyze the time-frequency analysis of high-order harmonics. It turned out that high-order harmonics have an intrinsic chirp during one-half cycle of a laser pulse. This chirp plays a crucial role in the shaping of attosecond pulses. In order to examine, more precisely, each part of the graph obtained from the spectrogram a procedure for defining instantaneous frequency is proposed. A criterion to select the optimum range of high-order harmonics for generating shortest attosecond pulse has been suggested.
