Nonlinear Extreme Ultraviolet Spectroscopy: A Novel Probe for Surfaces and Symmetry Breaking with Elemental Resolution

Abstract:

Extreme ultraviolet second harmonic generation spectroscopy (XUV-SHG) is an emerging technique used to study inversion symmetry breaking with core-state specificity. This novel technique was only recently demonstrated for the first time measuring the surface spectrum of carbon films [1]. Pushing nonlinear spectroscopic techniques to the XUV and soft X-ray regime has several advantages. For example, light pulses in these regimes can penetrate materials providing access to buried interfaces and symmetry-broken states in bulk material with specificity to a single atomic species. Recent experiments demonstrated quantification of the interfacial bond geometry of an organic-inorganic interface [2] and measurement of a surface spectrum of titanium [3]. Measuring the angular distribution of SXR-SHG has enabled additional sensitivities such as to the nature of the symmetry state itself [4]. Recently, we utilized XUV-SHG spectroscopy to investigate the polar metal phase of LiOsO3 [5]. In polar metals the coexistence of polarity and metallicity is unexpected as the itinerant conducting electrons in metals are expected to screen long-range electrostatic forces that are typically required to stabilize a macroscopic polarization. The large difference of atomic number renders it challenging to study this material with electron and X-ray scattering techniques. We apply XUV-SHG to study the symmetry properties in this material with specificity to the lithium atoms in the lattice. In the experiment we focus an intense femtosecond X-ray laser beam obtained by a free-electron laser onto the material with photon energies in the range of 28 to 33 eV, which enables reaching a resonance condition for the Li 1s electrons around the K-edge at ~54 eV. A sensitivity to broken inversion symmetry appears above the Li K-edge. We compare the experimental spectra with numerical calculations based on time-dependent density functional theory that show how the spectrally-resolved SHG varies with Li-displacement. As the first demonstration of XUV-SHG spectroscopy around a phase transition, these results pave the way for using nonlinear XUV methods to investigate broken symmetry from an element-specific perspective. I will also briefly discuss recent results on surface-specific XUV-SHG spectroscopy on a solid-state electrolyte where we find reduced Li ion mobilities at the surface and its relation to phonon modes providing important input for the design of future all-solid-state batteries [6]. Advancements of enabling even shorter pulse durations in the attosecond regime at FELs provide interesting new opportunities as the inherently increased intensity ideally combines with exploiting nonlinear material responses as they are facilitated in XUV-SHG. In addition, inherent femtosecond to sub-femtosecond temporal resolution will enable studying phase transitions on the electronic timescale and provide unique opportunities for studying fundamental physical phenomena, chemical dynamics at interfaces, and materials sciences.

[1] R. K. Lam, et al., Phys. Rev. Lett. 120, 023901 (2018).

[2] C. P. Schwartz, et al., Phys. Rev. Lett. 127, 096801 (2021).

[3] T. Helk, et al., Sci. Adv. 7, eabe2265 (2021).

[4] C. Uzundal, et al., Phys. Rev. Lett. 127, 237402 (2021).

[5] E. Berger, et al., Nano Letters 21, 6095–6101 (2021).

[6] C. Woodahl, et al., “Structure of Lithium at a Perovskite Interface Probed by Second Harmonic Extreme Ultraviolet Spectroscopy” (2022), submitted.

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