ACCORD: Amplified coherently combined optical radiation

Direct observation and understanding of electronic motion in solids requires the subatomic resolution in space and time. This unique spatio-temporal resolution is plausible by combining x-ray diffraction and attosecond spectroscopy, where, the instantaneous charge distribution can be probed by attosecond pulses producing a diffraction pattern and the movement of the electrons can be recorded by varying the arrival time of the XUV pulses with attosecond resolution. Therefore it is possible to record freeze-frame snapshots with picometer spatial resolution and attosecond exposure time, forming a 4-D movie of the evolution of electrons in solids. This method may elucidate electron dynamics in inner shells; therefore opening the door for time resolved intra-nuclear dynamics [1].

Undoubtedly this technique requires angstrom-wavelength isolated attosecond pulses. Therefore its realizability is the question of availability of a source which could push the frontiers of high-harmonic generation (HHG) and attosecond pulse generation into keV photon energies and hard x-ray regime. The capabilities of the current HHG sources, based on chirped pulse amplification (CPA) Ti:Sapphire technology, are limited to energies around a few hundred eV and to pulse durations of several tens of attoseconds [2]. Scaling attosecond pulses to high repetition rates and photon energies as high as several keV, demands few-cycle laser systems with high peak- and average-power, which is beyond the performance of the current laser technology.

Comprising optical parametric chirped pulse amplifiers (OPCPA) driven by 1-ps, ytterbium-based pump lasers; will provide the possibility to combine terawatt-scale peak powers with kilowatt-scale average powers in ultrashort optical pulse generation for the first time. Temporal superposition of the generated few-cycle pulses at different carrier wavelength employing the concept of controlled-transient waveform [3], where the electric field of sub-cycle light waves and consequently electron motion during the HHG process is controlled, will allow synthesis of sub-cycle waveforms at high peak and average powers holding promise to extend the HHG cutoff energy to keV x-ray pulses (Figure 1) [4,5].

The generated isolated keV attosecond pulses with this laser source will provide access to both more strongly bound electronic states and to atomic-scale electron trajectories by means of keV attosecond electron spectroscopy and diffraction, respectively. This capability, combined with the ability to induce and control strong-field electron phenomena in dielectrics and semiconductors with the unique sub-cycle-to-few-cycle visible-infrared waveforms, offers extremely exciting ample opportunities for joint experiments in the framework of the MPC.

Fig. 1: Schematic architecture of a three-channel OPCPA system seeded and pumped by near-1-ps ytterbium lasers. A part of its output is used for generating a phase-stable multi-octave supercontinuum signal, which is split into three channels, centered at 550 nm, 1 μm, and 2 μm, respectively. The different channels are pumped by different low-order harmonics of the multi-mJ level, kHz, Yb:YAG regenerative amplifier output. Each channel supports few-cycle pulses. By coherently combining the three few-cycle channels in VIS, NIR, and MIR, amplified, non-sinusoidal, multi-octave light transients spanning from 0.45-2.7 μm can be generated (1). Extension to FIR can be achieved by intrapulse DFG of the MIR channel. Coherent synthesis of the NIR, MIR and FIR channels will result in unique multi-octave infrared transients with a spectral coverage of 0.7-20 μm (2).

References:

[1] Baum, J. Phys. B, 2014.
[2] Krausz and Ivanov, Reviews of Modern Physics, 2009.
[3] Wirth et al., Science, 2011.
[4] Fattahi et al., Optica, 2015.
[5] H. Fattahi, Springer, 2015.

Project leaders:

Hanieh Fattahi(MPQ), hanieh.fattahi@mpq.mpg.de
Ferenc Krausz (MPQ), ferenc.krausz@mpq.mpg.de