Focusing properties of aberration-free electromagnetic waves in thin dielectric slabs

Abstract

We study properties of electromagnetic fields that are focused into thin dielectric slabs using an approach based on physical-optics or Kirchhoff's approximation, making the results valid both for low and high Fresnel number focusing geometries. A variety of thin slab structures and focusing systems are studied with emphasis on the impact of the field that is due to the presence of the back interface of the slab which is critical in various fields such as optical recording, bio and material sciences. In particular, we investigate how the intensity of the focused field depends on variables such as aperture size, aperture-slab distance, slab refractive index, and slab thickness. Numerical results show that the contribution from the electric field due to the presence of the back interface of the slab is significant along the z axis, which passes through the center of the spherical aperture and the focal point. With fixed focal length, the axial field inside the slab is found to increase and the focal shift to decrease as the aperture-slab distance is reduced. With increasing relative refractive index of the slab, the axial field is found to decrease, but to be more concentrated around the focus. With increasing aperture size, the axial field is found to increase, while interference effects due to the presence of the back interface of the slab are found to decrease.