Abstract
This work introduces a novel reinterpretation of structured illumination (SI) microscopy for coherent imaging that allows three-dimensional imaging of complex refractive index (RI). To do so, we show that coherent SI is mathematically equivalent to a superposition of angled illuminations. It follows that raw acquisitions for standard SI-enhanced quantitative-phase images can be processed into complex electric-field maps describing sample diffraction under angled illuminations. Standard diffraction tomography (DT) computation can then be used to reconstruct the sample 3D RI distribution at sub-diffraction resolutions. We demonstrate this concept by using a SI-quantitative-phase imaging system to computationally reconstruct 3D RI distributions of human breast (MCF-7) and colorectal (HT-29) adenocarcinoma cells. Our experimental setup uses a spatial light modulator to generate structured patterns at the sample and collects angle-dependent sample diffraction using a common-path, off-axis interference configuration with no moving components. Furthermore, this technique holds promise for easy pairing with SI fluorescence microscopy, and important future extensions may include multimodal, sub-diffraction resolution, 3D RI and fluorescent visualizations.
Highlights
Refractive index (RI) is an important optical property in biological objects that is often exploited to visualize endogenous biological contrast with microscopies that specialize in label-free imaging
We demonstrate here that structured illumination (SI) microscopy, an imaging technique commonly associated with fluorescent super-resolution and a widefield technique that can enable optical-sectioning for 3D fluorescent imaging [14,15,16], can allow 3D reconstruction of RI distributions if operated in the coherent imaging realm
We emphasize that the electric-field measured at the image plane after SI of the object is a superposition of the electric-fields that would have been measured after object illumination with the tilted plane-waves individually composing the SI pattern
Summary
Refractive index (RI) is an important optical property in biological objects that is often exploited to visualize endogenous biological contrast with microscopies that specialize in label-free imaging Standard examples of such microscopies that are heavily used in the biological sciences include phase-contrast and differential interference microscopies. More advanced label-free imaging includes the set of quantitative-phase (QP) imaging techniques that generates complex-value electric-field maps that quantitatively encode optical-path-length (OPL), the integral of sample RI through the illumination path, into optical amplitude and phase components. Such QP techniques do not directly yield biological RI values. Though we focus on using SI for 3D RI visualization in this work, we emphasize that SI is compatible with both 3D RI and fluorescent imaging, and is a promising candidate technique for future 3D multimodal applications
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