Abstract
Wide-field interferometric microscopy techniques have demonstrated their utility in sensing minute changes in the optical path length as well as visualization of sub-diffraction-limited nanoparticles. In this work, we demonstrate enhanced signal levels for nanoparticle detection by pupil function engineering in wide-field common-path interferometric microscopy. We quantify the improvements in nanoparticle signal achieved by novel optical filtering schemes, benchmark them against theory, and provide physical explanations for the signal enhancements. Our refined common-path interferometric microscopy technique provides an overall ten-fold enhancement in the visibility of low-index, non-resonant polystyrene nanospheres (r∼25 nm), resulting in nearly 8% signal-to-background ratio. Our method can be a highly sensitive, low-cost, label-free, high-throughput platform for accurate detection and characterization of weakly scattering low-index nanoparticles with sizes ranging from several hundred down to a few tens of nanometers, covering nearly the entire size spectrum of biological particles.
Highlights
Scientists have been fascinated with visualizing the microscopic world for hundreds of years [1,2]
We have demonstrated the integration of pupil function engineering into a wide-field interferometric imaging scheme
By implementing pupil modification in the illumination path, we have successfully shown the detection of low-index, non-resonant polystyrene nanospheres of 25 nm nominal radius with a more than 3% signal-to-background ratio in single-particle interferometric reflectance imaging sensor (SP-IRIS), representing a fivefold signal improvement over conventional full-NA Köhler illumination
Summary
The significant sensitivity advantage in interferometric nanoparticle detection stems from the fact that the particle signal consists of the cross (interference) term that scales with particle polarizability and particle volume [20]. Pupil function engineering has been employed extensively in optical techniques ranging from super-resolution microscopy to lithography [21,22,23,24,25] It allows for optimization of an optical system for a particular application, whether it be light confinement or extension of the resolving power of an imaging system in axial and lateral dimensions. We introduce pupil function engineering into the current system in two steps: first we use a mask in the illumination path to control the NA of the illumination, i.e., the angular content of excitation light, for efficient collection of the enhanced scattering of nanoparticles in the vicinity of the layered sensor surface. Our refined optical imaging method has the potential to be a highly sensitive, low-cost, label-free, high-throughput detection platform to study weakly scattering, low-index biological nanoparticles, such as viruses [26,27] and exosomes [28], with sizes ranging from several hundred down to a few tens of nanometers, covering nearly the entire range of biological nanoparticles
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