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XFROG phase measurement of threshold harmonics in a Keldysh-scaled system


Attosecond pulses are created (by Fourier synthesis) from a comb of odd-order high harmonics resulting from the non-perturbative interaction of intense near-visible laser light with an atomic gas1,2,3. When produced by a mid-infrared laser, harmonics can have simultaneously high order, visible wavelength, and photon energy below the ionization threshold, Ip, of the generating atom. Methods requiring photon energies greater than Ip have been developed4 that measure the spectral amplitude and phase necessary for temporal reconstruction of the harmonic radiation. Here we report the temporal characterization of below-threshold harmonics using sum frequency generation cross-correlation frequency resolved optical gating5 (SFG XFROG), a technique sensitive to the relative delay between orders6, coupled with a novel approach that makes use of the Keldysh scaling7 in strong-field physics. The results surprisingly suggest non-perturbative generation of below threshold harmonics, providing a potential alternative to existing vacuum–ultraviolet frequency comb generation methods8,9.

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Figure 1: Spectra from the 3.6-µm/caesium scaled system.
Figure 2: Below-threshold harmonics XFROG spectrograms.
Figure 3: Reconstructed harmonics from the 3.6-µm/caesium scaled system.
Figure 4: dφ/dq for harmonic orders 5–13.

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This work was supported by the United States Department of Energy/Basic Energy Sciences contract no. DE-FG02-04ER15614. L.F.D. acknowledges support from the Hagenlocker Chair at OSU. We are grateful to L. Van Woerkom (Ohio State University) for the loan of the vacuum chamber and J. Tate (Louisiana State University) for useful discussions.

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E.P.P. designed and performed the XFROG experiment, analysed the results and wrote the manuscript. A.M.M. and E.S. assisted with the XFROG, collected photoelectron and VUV spectra and operated the laser. F.C. performed the TDSE modelling and analysis. E.S. and F.C. wrote sections of the Methods. K.K., P.A. and L.F.D. provided experimental and theoretical advice and edited the manuscript.

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Correspondence to Erik P. Power.

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Power, E., March, A., Catoire, F. et al. XFROG phase measurement of threshold harmonics in a Keldysh-scaled system. Nature Photon 4, 352–356 (2010).

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