Focusing properties of the visible light wave through plasmonic lenses with subwavelength chirped slits

Focusing properties of a new kind of plasmonic lenses are investigated in the visible wavelength range through a subwavelength metallic chirped slit arrays which have the same depth but chirped widths. The chirped widths of slits are like a piece-wise-linear distribution which will be approximated by linearly increasing the width of a subwavelength feature and can build up a required phase front for focusing. We analyzed the focusing characteristics of different metallic lenses (silver and gold, respectively) with chirped widths that are obtained by generalizing the relevant phase delay for TE- and TM-polarized incident waves, for different f-numbers of lenses and for different material thickness, respectively. Meanwhile, the comparison of the metallic and dielectric lenses is also presented. The results of calculations show that, the metallic lenses are more sensitive to the polarization of incidence wave than that of dielectric lenses, and can get narrower full-width half-maximum (FWHM) beam width than that of dielectric lenses for TM-polarized incident waves, respectively. No matter which f-number we choose, the FWHM of dielectric lenses are higher than the plasmonic lenses, and the plasmonic lenses can get a higher focal resolution than dielectric lenses do. This kind of plasmonic lenses should have a good potential for applications in photonic and plasmonic integrated devices, sensing, and nano-optical manipulations, etc.