Visualizing 3D Anisotropic Molecular Orientation in Polarization Holographic Optical Elements via Dielectric Tensor Tomography

Abstract

The ability to unveil the spatial distribution of refractive index (RI) within volumetric holographic optical elements (HOEs) is critical for quantitating their diffractive behaviors. Angle-resolved far-field measurements of diffractive intensity have been prevalent toward this end. However, this century-old approach is unable to directly visualize the spatial distribution of RI at mesoscopic scale. More significantly, visualization of molecular orientation within photoaddressable polymers (PAPs), which serve as standard recording media for polarization HOEs (pHOEs), remains uncharted territory. The recent advent of dielectric tensor tomography (DTT) has paved the way for full characterization of 3D anisotropic dielectric tensors, encompassing principal RIs and their optic axes. This study embarks on direct visualization of the 3D spatial distribution of anisotropic molecular orientations within holographically recorded PAPs. Illuminating these PAPs with polarized light at varying angles, the diffracted vector fields essential for reconstructing the dielectric tensor tomogram are captured. After diagonalizing the dielectric tensors, periodic rotations of the anisotropic molecule orientations can be visualized in the PAPs, which have never been achieved so far. The homogeneity of grating patterns produced under diverse manufacturing conditions is also examined and juxtaposed. 3D anisotropic structures within polarization holographic optical elements are characterized using dielectric tensor tomography. Periodic rotations of anisotropic molecular orientations and the principal refractive index distributions of the polarization volume gratings are directly visualized, enabling the examination of the homogeneity of grating patterns under diverse manufacturing conditions. image