Tomographic measurement of dielectric tensors?
Dielectric tensor tomography allows the direct measurement of the 3D dielectric tensors of optically anisotropic structures

Date:
March 22, 2022
Source:
The Korea Advanced Institute of Science and Technology (KAIST)
Summary:
A research team reported the direct measurement of dielectric
tensors of anisotropic structures including the spatial variations
of principal refractive indices and directors. The group also
demonstrated quantitative tomographic measurements of various
nematic liquid-crystal structures and their fast 3D nonequilibrium
dynamics using a 3D label- free tomographic method.



FULL STORY ==========================================================================
A research team reported the direct measurement of dielectric tensors
of anisotropic structures including the spatial variations of principal refractive indices and directors. The group also demonstrated quantitative tomographic measurements of various nematic liquid-crystal structures and
their fast 3D nonequilibrium dynamics using a 3D label-free tomographic
method. The method was described in Nature Materials.


========================================================================== Light-matter interactions are described by the dielectric tensor. Despite
their importance in basic science and applications, it has not been
possible to measure 3D dielectric tensors directly. The main challenge
was due to the vectorial nature of light scattering from a 3D anisotropic structure. Previous approaches only addressed 3D anisotropic information indirectly and were limited to two-dimensional, qualitative, strict
sample conditions or assumptions.

The research team developed a method enabling the tomographic
reconstruction of 3D dielectric tensors without any preparation or
assumptions. A sample is illuminated with a laser beam with various angles
and circularly polarization states. Then, the light fields scattered
from a sample are holographically measured and converted into vectorial diffraction components. Finally, by inversely solving a vectorial wave equation, the 3D dielectric tensor is reconstructed.

Professor YongKeun Park said, "There were a greater number of unknowns
in direct measuring than with the conventional approach. We applied our approach to measure additional holographic images by slightly tilting the incident angle." He said that the slightly tilted illumination provides
an additional orthogonal polarization, which makes the underdetermined
problem become the determined problem. "Although scattered fields are
dependent on the illumination angle, the Fourier differentiation theorem enables the extraction of the same dielectric tensor for the slightly
tilted illumination," Professor Park added.

His team's method was validated by reconstructing well-known liquid
crystal (LC) structures, including the twisted nematic, hybrid aligned
nematic, radial, and bipolar configurations. Furthermore, the research
team demonstrated the experimental measurements of the non-equilibrium
dynamics of annihilating, nucleating, and merging LC droplets, and the
LC polymer network with repeating 3D topological defects.

"This is the first experimental measurement of non-equilibrium dynamics
and 3D topological defects in LC structures in a label-free manner. Our
method enables the exploration of inaccessible nematic structures and interactions in non- equilibrium dynamics," first author Dr. Seungwoo
Shin explained.


========================================================================== Story Source: Materials provided by The_Korea_Advanced_Institute_of_Science_and_Technology_ (KAIST). Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Seungwoo Shin, Jonghee Eun, Sang Seok Lee, Changjae Lee, Herve
Hugonnet,
Dong Ki Yoon, Shin-Hyun Kim, Joonwoo Jeong, YongKeun
Park. Tomographic measurement of dielectric tensors at
optical frequency. Nature Materials, 2022; 21 (3): 317 DOI:
10.1038/s41563-022-01202-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220322111315.htm

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