Miniaturized microscopes with large field-of-view (FOV) have wide applications in biomedical imaging and automated machine vision. The glass based miniaturized microscopes inherit the system configuration from the conventional microscopes, as well as the excellent imaging qualities with the mature lens system design methods. They consist of a pair of miniaturized tube lens and objective lens, which significantly reduces the optical system size. This type of miniaturized microscope has been widely used in biomedicine imaging. Due to the bulky size and heavy weight of conventional glass lens system, however, it is very difficult to further reduce the overall size and weight.The metalens is a novel photonic optics that has the advantages of flat surface structure with a thin thickness in the order of operating wavelength, low weight that mainly comes from the substrate, high design flexibility that can integrate multiple functionalities together, low manufacturing cost when the mature industrial nanofabrication technology is applied. It is an assembly of well-designed nanostructures, also known as meta-unit, on a thin substrate that has been designed to modulate the phase of the light wavefront like a lens. With different designs of meta-unit, metalens can also modulate light polarization and amplitude. Past research has shown that metalens can focus light into a diffraction limited focal spot and have a high imaging quality within a small FOV. The metalens is believed to be a promising candidate for building the next generation compact imagers.
Despite those fascinating advantages, there are still many challenges before it can be applied to practical imaging applications, especially the miniaturized microscopy. This limitation is the tradeoff among numerical aperture (NA), FOV, and system compactness. Although spherical aberrations can be corrected in metalens, it suffers from comma aberrations which severely limits its FOV. There have been mainly two ways to overcome this challenge. The first one is using a single lens with a hard aperture. Although it has an extremely simple structure, it is limited to small NA applications. Another solution is a doublet metalens design that combines two layers of metalens together. This design can achieve high imaging qualities with a large NA, but it increases the complexity of fabrication. So far, both approaches have only been demonstrated for photography applications. The microscopy application is much more complicated due to the diverging beam incidence rather than the plane wave.
In this dissertation, we focus on addressing these challenges and building up a metalens based miniaturized microscope with extremely high compactness, large FOV, and high imaging quality. We first explored the design of a single layer of metalens array based miniaturized microscope. It has the advantages of a simple structure, arbitrary large FOV by tiling an arbitrary number of lenses together. However, the imaging quality is limited due to the proximity of metalens, object plane and the sensor plane. We validated this idea with a proof-of-concept demonstration. The second solution is using a double layer of metalens for miniaturized microscope. This method shares the same concept with the first method to expand the FOV by tiling lens units together, but each doublet lens unit has a much higher imaging quality than the singlet in the first method. We fabricated the metalens array and integrated it with a bare camera sensor for the proof-of-concept demonstration. On top of that, we further explored the design method of multiple layers of metalenses. We proposed a design principle for the multi-layer metalens based miniaturized microscope. We show two examples of designs with excellent imaging qualities.
