Half-cycle waveform control for producing a broad and intense harmonic spectral continuum and an isolated attosecond pulse

An effective method to control the half-cycle harmonic emission process through the combination of multiple laser beams is proposed. The control process can be separated into three steps. In the first step, by properly choosing the chirp and the chirp delay of the fundamental pulse the relevant negative and positive half-cycle waveform for extending the harmonic cutoff can be obtained. In the second step, using either an asymmetric inhomogeneous field or a half-cycle unipolar pulse, the harmonic cutoff from the negative or the positive half-cycle waveform can be further extended. In the third step, by properly adding an ultraviolet pulse, the harmonic yield can be enhanced due to ultraviolet resonance ionization. Finally, by superposing some harmonics on the harmonic spectra, a number of isolated attosecond pulses shorter than 70 as can be obtained.