HHG Driving with a CEP-Locked Three-Color Driver Pulse
Andrius Baltuska
Photonics Institute, Gußhausstraße 27-387, A-1040, Vienna, Austria
We realize a multicolor, multi-cycle combination of commonly CEP-locked three waves from a single
femtosecond OPA driven by a CEP-stable 7-mJ kHz Yb laser system and report on HHG driven with a
combination of three colors. The presented scheme is an efficient and technically straightforward approach to
adding Fourier components at an octave frequency spacing and is possibly the easiest experimental
implementation of the multicolor driving conditions outlined in the theoretical proposal for the “perfect-wave”
driver [1]. In the “perfect-wave” driving concept, the scaling of the kinetic energy of the re-scattering electron
wavepacket, the conditions for a returning trajectory, and minimization of harmful target pre-ionization are all
pre-programmed in the optical cycle shape. Our experimental apparatus is based on a single collinear optical
parametric amplifier (OPA). The OPA serves as a source of three colors that are mutually locked to a single
carrier-envelope phase (CEP). The CEP stabilization requirement in waveform synthesis arises automatically for
combinations of two and more different-color pulses whenever the mechanism of color multiplication is other
than direct optical harmonic generation of the fundamental frequency pulse. By downshifting the base frequency
into the mid-IR (idler wave), we provide room for more non-UV Fourier components and thus avoid problems
with atmospheric absorption of driver pulses and enhanced multiphoton UV ionization of the HHG target. In an
OPA, similar intensities at the frequencies of the three mixing waves, pump (3), signal (1), and idler (2) are
easily ensured and two additional Fourier components are easily attained by frequency-doubling 3 and 1.
The results of the HHG driven by three incommensurate frequency fields are summarized Fig 1. First, due
to a 3 increased beating period with respect to a single-color 1-μm driving wave and due to induced asymmetry
of half-cycles, the density of the harmonic peaks increases by a factor of six. Whereas the peak amplitude of
individual harmonic peaks drops by a factor of ~2, the total flux integrated over the spectrum increases
dramatically because of the higher peak density. Moreover, the spectrum generated using the two-color driver
extends beyond the transmission edge of Aluminum filter, demonstrating that the cutoff can be pushed above the
saturation limit of the 1-μm 180 fs pulse. When the third color pulse is added, the HHG spectrum becomes very
sensitive to the CEP settings, as evidenced by the phase-dependent cutoff modulation, signifying that sub-cycle
shaping is a tool that allows optimizing the dynamics of the electron in a strong laser field.
a)
c)
b)
Fig. 1. High order harmonic spectra generated with multiple colors. a) comparison of HHG spectrum generated in Argon using single
color at 1.03 µm and two color field composed of 1.03 µm (ω) and 1.545 µm (ω/1.5) waves. b) Cutoff region of the two-color spectrum that
extends beyond the Al filter transmission edge at 72.5 eV. c) a series of measured XUV high-order harmonic spectra, which depend upon the
shape of the optical cycle of the generating intense infrared laser pulses.
[1]
L. E. Chipperfield et al., Phys. Rev. Lett. 102, 063003 (2009).