Fractional high-harmonic combs by attosecond-precision split-spectrum pulse control

Philipp Raith, Christian Ott, Christopher P. Anderson, Andreas Kaldun, Kristina Meyer, Martin Laux, Yizhu Zhang, Thomas Pfeifer, M. Chergui, A. Taylor, S. Cundiff, R. de Vivie-Riedle (+1 others)
2013 EPJ Web of Conferences  
Few-cycle laser fields enable pulse-shaping control of high-order harmonic generation by time delaying variable broadband spectral sections. We report the experimental generation of fractional (noninteger) high-harmonic combs by the controlled interference of two attosecond pulse trains. Additionally the energy of the high harmonics is strongly tuned with the relative time delay. We quantify the tuning to directly result from the controlled variation of the instantaneous laser frequency at the
more » ... r frequency at the shaped driver pulse intensity maximum. High-order harmonic generation (HHG) has thus far been controlled by varying the carrier-envelope phase of the driving laser field [1], by the addition of a second driver field at a different frequency, e.g. the second harmonic [2], or by adaptive pulse control [3, 4] . Recently, sculpted optical waveforms shorter than a full laser cycle have been produced in a three-channel super-octave optical field synthesizer [5] . Here, we apply a similar continuous-spectrum pulse shaping method to perform comprehensive, coherent control of HHG [6]. We observe energy-tuning capability of the high-order harmonics and the generation of combs of fractional (noninteger) harmonics caused by the controlled spectral interference of two temporally separated attosecond pulse trains. To employ the splitspectrum time-delay control method, suggested for independent control of several attosecond pulse properties [7], we use a single broadband continuous laser spectrum which is divided into two adjustable spectral sections by a split mirror on a translation stage. The piezoelectric-driven split mirror is situated in the Fourier-plane of a Brewster-prism-based pulse shaper in 4-f configuration ( Figure 1a ) and enables the introduction of time delays between the spectral subfields with a precision below 400 as. High-order harmonics are produced with such shaped pulses in argon at a laser peak intensity of ~0.44PW/cm 2 and a backing pressure of ~60 mbar. Figure 1c shows the measured harmonic spectrum dependent on the time delay between the spectral components for the energy-asymmetric split spectrum depicted in Figure 1b . The high-order harmonics beat with time delay due to an alternating constructive and destructive interference of the driving subfields at the temporal center of the pulse. The energy of the high harmonics varies with time delay both on attosecond (subcycle) and on few tens of femtoseconds (subfields overlap) time scales. We extracted the physical mechanism and origin behind this pronounced energy tunability. It results from an interference-based modulation of the instantaneous frequency ) ( ) ( t t     of the shaped driver pulse at the temporal intensity maximum. The harmonic energy tuning further depends EPJ Web of Conferences
doi:10.1051/epjconf/20134101007 fatcat:ao2opkgf5nhktmtarf4mcmvk4u