Sub-diffraction limit imaging at visible-frequency using a hemispherical microsphere-thin slab composite spherical lens

Optical imaging techniques based on spherical lenses have demonstrated significant potential in nanomaterial surface characterization and biomedical diagnostics, primarily attributed to their capability for label-free, real-time, and high-resolution imaging. Here, we present a composite spherical lens (CSL) that can achieve sub-diffraction-limit imaging resolution. The CSL is composed of a photoresist thermal reflow-formed hemispherical microsphere composited with a photoresist thin slab. Physical optics simulations were performed to determine the focal lengths of the CSL, which were subsequently utilized to modify geometric optics theory. The modified theory accurately predicted the magnification of the CSL. Through combined computational and experimental methods, we investigated the influence of hemispherical microsphere orientation and CSL thickness on imaging performance. Results indicate that for a CSL incorporating 12.5-μm-radius hemispherical microspheres, optimal imaging performance is achieved with a face-down hemispherical microsphere configuration and a 7-μm-thick photoresist slab, enabling a lateral resolution of approximately 130 nm.