Experimental Study of Indirect Phase Tuning-Based Plasmonic Structures for Finely Focusing

Three types of indirect phase tuning-based plasmonic structures with subwavelength circular grooves/slits and/or central apertures corrugated on Au film supported by glass substrate: depth modulation, width modulation, and hybrid depth-width modulation, were put forth in this paper. They were investigated experimentally by means of nanofabrication and near-filed scanning optical microscope characterization. The plasmonic structures were fabricated using the technique of focused ion beam direct milling. Our experimental results demonstrated that all of the phase tuning-based structures have focusing functions. Both the width and depth modulation-based structures can realize beam focusing and produce an elongated depth of focus. Moreover, after comparison among these three structures, we found that the width modulation-based structure has the best focusing performance.