  Two-photon photopolymerization (TPP) has recently become a popular method for
the fabrication of three-dimensional (3D) micro- and nanostructures. The
reproduction fidelity of the designed micro- and nanostructures is influenced
by experimental writing conditions, including laser power, exposure time, etc.
To determine the appropriate writing parameters, characterization of
morphological features and surface roughness during the experiment is needed.
Traditional characterization methods for TPP, e.g., scanning electron
microscopy and atomic force microscopy, have limited speed and cannot study
internal structures without invasive approaches. Optical diffraction tomography
(ODT) is an emerging label-free 3D imaging technique based on reconstructing
the object's 3D refractive index (RI) distribution with diffraction-limited
resolution. Here, we propose a non-invasive solution to fully characterize the
TPP-fabricated structures using a high-speed ODT technique, which can eliminate
the need for complex sample preparation, such as fluorescence labelling or
metal-coating, and achieve a full 3D measurement time of 6 ms. By visualizing
and studying different TPP-fabricated structures, including embedded spirals
and cubes, via the ODT system, the fabrication quality, including 3D
morphological features, exposure levels, and surface roughness, can be examined
quantitatively. The results suggest our method can effectively improve the
fabrication quality and reproducibility of TPP, generating impacts on the
nanofabrication community.
