Near-field visualization of focal depth modulation by step corrugated plasmonic slits

Nanometric plasmonic slits with stepped corrugations have been designed and fabricated to achieve plasmonic focusing and focal depth modulation. A scanning near-field optical microscope is employed to directly visualize the transmitted light from the slits. The near-field and far-field two-dimensional images taken at different planes parallel to the slit surface unambiguously demonstrated the focusing effect of the nanoslits. Furthermore, by forming stepped corrugations with either a concave or a convex profile on both sides of the slits, the phase of the transmitted beam can be effectively manipulated, thus allowing an accurate tuning of the focal depth.