Super-narrow focusing and ultra-long working distance by different shapes of dielectric microlenses

The dielectric microlens, surmounts the diffraction limit, consider as a super-focusing and super-resolution imaging microlens. In this paper, the super-focusing and working distance between microlens and test sample are investigated through different shapes of dielectric microlenses. The result shows that the hollow sphere microlens, an innovative design of nanoparticle with a solid sphere including a hollow core region, is an optimal spherical microlens with super-resolution. After optimization, a hollow sphere microlens with the ratio of inner and outer radius r/R of 1:2 and the refractive index of 1.6 is designed and a super-narrow photonic nanojet with the full width at half maximum of 164 nm has been achieved under the incident wavelength of 500 nm. When the hollow sphere microlens is immersed in water or oil, it can focus the 500 nm incident light 3 μm away from the hollow sphere lower surface. Increasing the refractive index of hollow spheres, the resolution reaches 0.22 λ. With the super-focusing and ultra-long working distance, the hollow sphere microlens could be applied for detecting a test sample to acheive a super-resolution imaging and a test sample can be distinguished clearly when combined with a conventional optical microscope, which is particularly available for the test samples with the extreme matte surfaces.