HHG Drivingwith a CEP-Locked Three-Color Driver Pulse

AndriusBaltuska

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 apparatusis 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 3increasedbeating 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 becomesvery 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 ina strong laser field.

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).