Description
Conventional imaging systems measure the intensity or amplitude of the optical field. Transparent samples, such as biological cells, are weakly absorbing and exhibit poor contrast in such systems. However, spatial changes in the refractive index and thickness within these samples lead to changes in the phase. Converting these phase variations into amplitude modulations provides a means to visualise the morphology of these samples, which can be performed with interferometers, phase contrast microscopes, or computationally. These approaches, however, require bulky and complex optical setups, are computationally intensive and require insertion/removal of optical components within the setup to change imaging modality.
More recently, it has been demonstrated that ultra-thin non-local metasurfaces can be used to perform mathematical operations on the optical field directly in the object plane. There is a relationship between spatial frequency and the angle of a plane wave to the optical axis, so by designing a metasurface with a specific angular response, we can filter the spatial frequency distribution of the field to process images in an ultra-compact form-factor.
Here, we use topology optimisation to design a metasurface that switches from a conventional brightfield imaging modality to a phase imaging modality simply by changing the polarisation. For x-polarised light, the metasurface performs an identity operation (conventional imaging), whereas for y-polarised light, it performs a first order derivative, permitting visualisation of phase gradients within the field. To perform such an imaging operation requires a transmission that differs for positive and negative illumination angles, which we generate through asymmetric diffraction into higher orders. We perform label-free imaging and extract quantitative phase gradient information on artificial phase samples, HeLa cells, and ovarian cancer tissue samples. This work enables ultra-compact, cost-effective phase microscopy systems for biological imaging, medical diagnostics, and material characterisation.
| I am the presenting author | Yes |
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