Broadband spectral-phase control in high-order harmonic generation

I make evident a method for broadband spectral-phase control of light pulses produced in high-order harmonic generation (HHG). Using a feedback loop scheme, I demonstrate that a broad spectral region of the HHG output field can be coherently added to the intense infrared driving pulse with optimal attenuation and time delay parameters to control the phase of the HHG field generated in a second interaction region. I provide computational evidence of the control scheme by considering an optimal flattening of the spectral phase for the production of isolated attosecond pulses. This is proven by considering different spectral widths and for several central photon energies, such as 36 eV, 70 eV, and 120 eV. An iterative procedure of the method allows one to obtain single pulses with less than 1.2 cycles at a central photon energy of 36 eV. This control scheme is a fundamental tool that can be implemented for amplitude and phase shaping of any suitable spectral region in HHG. DOI: 10.1103/PhysRevA.87.013825