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(a) Molecular structure of croconic acid (H2C5O5). (b) and (c) show the crystal structure projected on the ab and ac planes, respectively. In (a) and (c), two structures with opposite ferroelectric polarizations are shown.
(a) Polarized reflectivity (R) spectra of H2C5O5 for E // a and E // c. (b) Refractive index n and distinction coefficient k spectra. (c) Imaginary part of the dielectric constant ε 2 deduced by the Kramers–Kronig transformation of the R spectrum for E // c. The broken lines represent the simulated spectra using the Lorentz model.
(a) Polarization-angle (ϕ) dependence of the SH intensities for the incident light of 1300 nm in H2C5O5. The light propagation direction is normal to the ac plane. (b) Schematic of the MF method. (c) Angle (θ)-dependence of the SH intensities in Z-cut quartz and H2C5O5 for the incident light of 1000 nm. Solid lines show simulated curves. (d) ε 2 and transmittance spectra (upper panel) and χ (2) spectrum (lower panel). The arrows show the positions of incident (ω) and SH (2ω) lights, and the broken line shows the simulated χ (2) curve.
(a) Molecular structures and MOs in an isolated molecule and a five-molecule cluster of H2C5O5. The lowest optical transitions are indicated by red arrows. (b) Molecular structure and MOs in an isolated molecule of C5O5. Details of the calculations and the definitions of MOs in [vi] and [vii] are reported in the supplemental material (see Ref. 10 ).
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