Route to Attosecond Nonlinear Spectroscopy

We demonstrate generation of coherent microjoule-scale, low-order harmonic supercontinua in the deep and vacuum ultraviolet (4–9 eV), resulting from the nonlinear transformations of near-single-cycle laser pulses in a gas cell. We show theoretically that their formation is connected to a novel nonlinear regime, holding promise for the generation of powerful deep-UV and vacuum ultraviolet subfemtosecond pulses. Our work opens the route to pump-probe spectroscopy of subfemtosecond-scale valence-shell phenomena in atoms, molecules, and condensed matter.

Figure 1

(a–c) Dynamic evolution of the UV part of the supercontinuum generated by a near-single-cycle pulse, at different propagation distances along a quasistatic cell filled with pressurized neon. (d–f) Near-field spatiotemporal profiles of the propagated pulse along positions in the focus, according to (a–c).

Figure 2

Calculated far-field spatiotemporal structure of the driving pulse and the generated UV with (a),(b) and without (e),(f) ionization-induced defocusing. The FWHM temporal durations of the UV pulses are $\sim 950$ as in the case where ionization is on (c) and $\sim 3\mathrm{fs}$ if ionization is off (g). Spectra of the generated pulse after the filtering are depicted in (d) and (h), together with the filter function in (d). Ionization-induced optical nonlinearities yield a supercontinuum in the UV and VUV (d).

Figure 3

Generation and measurement of UV and VUV supercontinua.

Figure 4

UV and VUV supercontinuum generation in the few and near-single-cycle regime. Third and fifth harmonic spectra (a) generated with $\sim 6\mathrm{fs}$ pulses at 5 bar and supercontinua generated with sub-4 fs pulses at 5 bar (b), 7 bar (c), and 9 bar (d) of neon [solid (blue) line], are plotted together with the results of the numerical calculations [dashed (orange) line] and measured pulse energies after the filtering. The bandwidth-limited durations are about 2.6 fs (a), 1.2 fs (b), 1.1 fs (c), and 0.8 fs (d).