Isolated Soft-X-Ray Attosecond Pulse Generation Using Synthesized Strong-Field Mid-IR Pulses

High-order harmonic generation (HHG) in the strong-field regime is widely used for producing coherent extreme ultraviolet and soft X-ray pulses. It is a gateway to experimental attosecond science because of the attosecond duration and timing precision of emitted photons.

Under this grant, we will demonstrate a mJ-level sub-cycle infrared optical field synthesizer and its applications to the generation of isolated attosecond pulses in the soft X-ray range (>200 eV) via cutoff extension and electron wavepacket (EWP) control of HHG. The temporal property of soft X-ray attosecond pulses will be characterized using a frequency-resolved optical gating apparatus with attosecond streaking, called FROG-CRAB. The high-energy field synthesizer is based on 0.87 um near-infrared (NIR) and 2.1-um mid-infrared (MIR) optical parametric chirped-pulse amplifiers (OPCPAs), generating a sub-cycle-duration synthesized pulses. The previously demonstrated energy of 15 uJ will be boosted up to 1 mJ using a high-energy, kHz-repetition-rate, picosecond cryogenic Yb:YAG pump laser, and stabilization units, ensureing long-term stability of energy and phase, will be implemented.

Using this novel pulse synthesizer, we will further investigate the sub-cycle coherent control of EWPs in the HHG process in atoms and molecules, probed by high-order harmonic spectroscopy (HHS). First, a dramatic increase of HHG efficiency by more than an order of magnitude will be demonstrated, as predicted by simulations. Second, using the MIR and synthesized waveforms, we will study multi-electron dynamics at Xe Cooper minimum of ~180 eV and Ar L-edge of ~250 eV. Ultimately, we attempt to find a way of coherently controlling the multi-orbital channel dynamics of HHG in molecules like CO2. Third, the precise numerical study of attosec-ond pulse generation and HHS will be performed using the simulation tools based on quantitative rescattering (QRS) theory and 3-dimensional propagation codes developed by KSU and MIT, opening a method of precision HHS.