  Sub-diffraction resolution imaging has played a pivotal role in biological
research by visualizing key, but previously unresolvable, sub-cellular
structures. Unfortunately, applications of far-field sub-diffraction resolution
are currently divided between fluorescent and coherent-diffraction regimes, and
a multimodal sub-diffraction technique that bridges this gap has not yet been
demonstrated. Here we report that structured illumination (SI) allows
multimodal sub-diffraction imaging of both coherent quantitative-phase (QP) and
fluorescence. Due to the conventional fluorescent applications of SI, we first
demonstrated the principle of SI-enabled three-dimensional (3D) QP
sub-diffraction imaging with calibration microspheres. Image analysis confirmed
enhanced lateral and axial resolutions over diffraction-limited QP imaging, and
established striking parallels between coherent SI and conventional optical
diffraction tomography. We next introduce an optical system utilizing SI to
achieve 3D sub-diffraction, multimodal QP/fluorescent visualization of A549
biological cells fluorescently tagged for F-actin. Our results suggest that SI
has unique utility in studying biological phenomena with significant molecular,
biophysical, and biochemical components.
