The lack of available table-top extreme ultraviolet (XUV) sources with high enough fluxes and coherence properties has limited the availability of nonlinear XUV and x-ray spectroscopies to free-electron lasers (FELs). Here, we demonstrate second harmonic generation (SHG) on a table-top XUV source by observing SHG near the Ti M2,3 edge with a high-harmonic seeded soft x-ray laser. Furthermore, this experiment represents the first SHG experiment in the XUV. First-principles electronic structure calculations suggest the surface specificity and separate the observed signal into its resonant and nonresonant contributions. The realization of XUV-SHG on a table-top source opens up more accessible opportunities for the study of element-specific dynamics in multicomponent systems where surface, interfacial, and bulk-phase asymmetries play a driving role.
The coexistence of ferroelectricity and metallicity seems paradoxical,
since the itinerant electrons in metals should screen the long-range
dipole interactions necessary for dipole ordering. The recent discovery
of the polar metal LiOsO
3
was therefore surprising [as
discussed earlier in Y. Shi et al.,
Nat. Mater
.
2013
,
12
, 1024]. It is thought that the coordination
preferences of the Li play a key role in stabilizing the LiOsO
3
polar metal phase, but an investigation from the combined
viewpoints of core-state specificity and symmetry has yet to be done.
Here, we apply the novel technique of extreme ultraviolet second harmonic
generation (XUV-SHG) and find a sensitivity to the broken inversion
symmetry in the polar metal phase of LiOsO
3
with an enhanced
feature above the Li K-edge that reflects the degree of Li atom displacement
as corroborated by density functional theory calculations. These results
pave the way for time-resolved probing of symmetry-breaking structural
phase transitions on femtosecond time scales with element specificity.
Charge transport processes at interfaces play a crucial role in many processes. Here, the first soft x-ray second harmonic generation (SXR SHG) interfacial spectrum of a buried interface (boron-Parylene N) is reported. SXR SHG shows distinct spectral features that are not observed in x-ray absorption spectra, demonstrating its extraordinary interfacial sensitivity. Comparison to electronic structure calculations indicates a boron-organic separation distance of 1.9 Å, with changes of less than 1 Å resulting in easily detectable SXR SHG spectral shifts (ca. hundreds of milli-electron volts).
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