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Polarization-Resolved Extreme-Ultraviolet Second-Harmonic Generation from LiNbO3

Can B. Uzundal, Sasawat Jamnuch, Emma Berger, Clarisse Woodahl, Paul Manset, Yasuyuki Hirata, Toshihide Sumi, Angelique Amado, Hisazumi Akai, Yuya Kubota, Shigeki Owada, Kensuke Tono, Makina Yabashi, John W. Freeland, Craig P. Schwartz, Walter S. Drisdell, Iwao Matsuda, Tod A. Pascal, Alfred Zong, and Michael Zuerch
Phys. Rev. Lett. 127, 237402 – Published 30 November 2021
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    Abstract

    Second harmonic generation (SHG) spectroscopy ubiquitously enables the investigation of surface chemistry, interfacial chemistry, as well as symmetry properties in solids. Polarization-resolved SHG spectroscopy in the visible to infrared regime is regularly used to investigate electronic and magnetic order through their angular anisotropies within the crystal structure. However, the increasing complexity of novel materials and emerging phenomena hampers the interpretation of experiments solely based on the investigation of hybridized valence states. Here, polarization-resolved SHG in the extreme ultraviolet (XUV-SHG) is demonstrated for the first time, enabling element-resolved angular anisotropy investigations. In noncentrosymmetric LiNbO3, elemental contributions by lithium and niobium are clearly distinguished by energy dependent XUV-SHG measurements. This element-resolved and symmetry-sensitive experiment suggests that the displacement of Li ions in LiNbO3, which is known to lead to ferroelectricity, is accompanied by distortions to the Nb ion environment that breaks the inversion symmetry of the NbO6 octahedron as well. Our simulations show that the measured second harmonic spectrum is consistent with Li ion displacements from the centrosymmetric position while the Nb─O bonds are elongated and contracted by displacements of the O atoms. In addition, the polarization-resolved measurement of XUV-SHG shows excellent agreement with numerical predictions based on dipole-induced SHG commonly used in the optical wavelengths. Our result constitutes the first verification of the dipole-based SHG model in the XUV regime. The findings of this work pave the way for future angle and time-resolved XUV-SHG studies with elemental specificity in condensed matter systems.

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    • Received 2 April 2021
    • Revised 21 August 2021
    • Accepted 15 October 2021

    DOI:https://doi.org/10.1103/PhysRevLett.127.237402

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Dielectric propertiesFerroelectricitySecond order nonlinear optical processesStrong electromagnetic field effectsUltrafast phenomenaX-ray beams & optics
    1. Techniques
    High-harmonic generation
    Atomic, Molecular & OpticalCondensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Can B. Uzundal1,2, Sasawat Jamnuch3, Emma Berger1,2, Clarisse Woodahl1,4, Paul Manset5, Yasuyuki Hirata6, Toshihide Sumi7, Angelique Amado1,2, Hisazumi Akai7, Yuya Kubota8,9, Shigeki Owada8,9, Kensuke Tono8,9, Makina Yabashi8,9, John W. Freeland10, Craig P. Schwartz11, Walter S. Drisdell12,13, Iwao Matsuda14,7, Tod A. Pascal3,15,16,*, Alfred Zong1,2, and Michael Zuerch1,2,17,18,†

    • 1Department of Chemistry, University of California, Berkeley, California 94720, USA
    • 2Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
    • 3ATLAS Materials Science Laboratory, Department of NanoEngineering and Chemical Engineering, University of California, San Diego, La Jolla, California, 92023, USA
    • 4University of Florida, Gainesville, Florida 32611, USA
    • 5Ecole Normale Superieure de Paris, Paris, France
    • 6National Defense Academy of Japan, Yokosuka, Kanagawa 239-8686, Japan
    • 7Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
    • 8RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan
    • 9Japan Synchrotron Radiation Research Institute, (JASRI), 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan
    • 10Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
    • 11Nevada Extreme Conditions Laboratory, University of Nevada, Las Vegas, Las Vegas, Nevada 89154, USA
    • 12Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
    • 13Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
    • 14Trans-scale Quantum Science Institute, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
    • 15Materials Science and Engineering, University of California San Diego, La Jolla, California, 92023, USA
    • 16Sustainable Power and Energy Center, University of California San Diego, La Jolla, California, 92023, USA
    • 17Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany
    • 18Friedrich Schiller University Jena, 07743 Jena, Germany

    • *Corresponding author. tpascal@ucsd.edu
    • †Corresponding author. mwz@berkeley.edu

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    Vol. 127, Iss. 23 — 3 December 2021

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    • Figure 1
      Figure 1

      Schematic overviews of the two experiments. In both experiments the p-polarized fundamental is incident on an x-cut LiNbO3 sample at 45° with respect to the crystal surface. The 45° incident angle is the Brewster’s angle for XUV energies. SHG is also detected at 45° with respect to the crystal surface. (a) Schematic illustration of the spectral measurement. The residual fundamental and the emitted second harmonic of the incident FEL are analyzed by dispersing with a grating and imaging with an MCP detector. The inset shows the crystal structure of LiNbO3 in its ferroelectric state where black lines mark the unit cell. (b) Schematic illustration of the second harmonic polarization measurement. The residual fundamental and the emitted second harmonic are reflected off of a multilayer mirror tuned to 66 eV. The polarization of the residual fundamental and the second harmonic are simultaneously measured. Crystal drawing is produced by VESTA [36].

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    • Figure 2
      Figure 2

      Resonant transitions for LiNbO3 within the range of fundamental energies studied in the spectral experiment. (a) Schematic illustration of the possible transitions with respect to the electronic density of states. Two prominent transitions that involve the Li 1s and Nb 4p core states are labeled. For the Li 1s states (transitions 1 and 2), SHG occurs through a half-resonant scheme and it is facilitated by a virtual state, while the transition originating from Nb 4p (transition 3) is facilitated via the conduction band states of majority Nb 4d character. (b) Measured and theoretical values of the effective χeff(2)(ω) spectrum for LiNbO3 showing three prominent features corresponding to the labeled transitions from panel (a).

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    • Figure 3
      Figure 3

      Results of the polarization-resolved experiment on x-cut LiNbO3. Measured polarization of the incident FEL (blue squares) at 33 eV and the polarization of the SHG (red circles). p polarization with respect to the optical table is 90° while s polarization is 0°. The fitting error is shown for each point. The amplitude of the intensity is in arbitrary units and the scale is linear. The intensities of fundamental and second harmonic are individually normalized by their respective maximum.

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