Optical waveform synthesizer and its application to high-harmonic generation

Over the last decade, the control of atomic-scale electronic motion by optical fields strong enough to mitigate the atomic Coulomb potential has broken tremendous new ground with the advent of phase-controlled high-energy few-cycle pulse sources. Further investigation and control of these physical processes, including high-harmonic generation, ask for the capability of waveform shaping on sub-cycle time scales, which requires a fully phase-controlled multiple-octave-spanning spectrum. In this paper, we present a light source that enables sub-cycle waveform shaping with a two-octave-spanning spectrum and 15 mu J pulse energy based on coherent synthesis of pulses with different spectra, or wavelength multiplexing. The synthesized pulse has its shortest high-field transient lasting only 0.8 cycles (amplitude FWHM) of the centroid frequency. The benefit of the approach lies in its modular design and scalability in both bandwidth and pulse energy. Full phase control allows for the synthesis of any optical waveform supported by the amplified spectrum. A numerical study shows the uniqueness of the light source for direct isolated soft-x-ray pulse generation based on high-harmonic generation, greatly reducing and eventually even eliminating the need for gating techniques or spectral filtering. The demonstrated system is the prototype of a class of novel optical tools for attosecond control of strong-field physics experiments.