High-harmonic generation (HHG) has been used to generate extreme ultra-violet (EUV) light sources to probe fast electron dynamics in the attosecond time scale. While traditionally observed in rare-gas atoms, HHG has also recently been reported in solids, with reduced threshold pump field and the additional advantage of producing stable EUV waveforms in a compact setup. Unfortunately, above-band-gap absorption restricts the HHG process to a very thin layer of the solid-state material (typically tens of nanometers in thickness), significantly limiting the generation efficiency. Here, we use a material operating in its epsilon-near-zero (ENZ) region, where the real part of its permittivity vanishes, to greatly boost the efficiency of the HHG process at the microscopic level. In experiments, we report high-harmonic emission up to the 9th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide, with an excitation intensity at the GW cm-2 level. Furthermore, the observed HHG signal exhibits a pronounced spectral red-shift as well as linewidth broadening, resulting from the photo-induced electron heating and the consequent time-dependent resonant frequency of the ENZ film. Our results provide a novel nanophotonic platform for strong field physics, reveal new degrees of freedom for spectral and temporal control of HHG, and open up possibilities of compact solid-state attosecond light sources
We report on our high speed camera designed for temporal characterization of attosecond pulses
(1as =10-18s) generated with the polarization gating technique. The uniform external magnetic field applied on the
time-of-flight spectrometer enlarges the acceptance angle (up to 65° for ~20-eV photoelectrons). By collecting two-dimensional
momentum images of the photoelectrons, which are ejected by the XUV pulses and streaked directly by the
co-propagating polarization gating electric field, we expect to derive the information about the XUV pulses. After the
characterization of XUV pulses, the same setup can be used to study complex dynamics of electrons in atoms and
molecules with time-resolved spectroscopy.
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