The Design Of Surface Plasmon Focusing Lens With Dynamically Adjustable Focus Based On Changing Phase

Surface Plasmon Polaritons(SPPs) is the electromagnetic model caused by the interaction of photon and free electrons at the surface of metal, this kind of electromagnetic field is limited within a small region at the metal surface and enhanced, has the excellent property to break the diffraction limit, which is a big trouble in the traditional optics. It supplies a broad application prospect in the field of super resolution imaging, optoelectronic integrated circuits, biomedical sensing, and optical storage and so on. Under this premise, various optical schemes based on the mechanism of surface plasmon(SP) resonance have been concerned, among them the micro-nano planar lens optical structures are widely researched at both basic theories and experiments, not only because of the phenomenon of extraordinary optical transmission, but also due to the great focusing effect within a certain incident wavelength range in far field. Based on surface plasmon resonance. There have been several kinds of planar lens proposed, such as nano-slit or nano-hole arrays, zone plates and photonics crystals. Although these optical scheme have different structure, they almost share the same theory of focusing, which is to generate a constructive interference at a certain focal position through having the special phase distribution of the transmitted light.The performance of planar lens become better and better through continuing optimizing and further design, however, most of them lack the ability to change the focal length once being fabricated. The control of the focal position is important in active optical circuits and miniaturized integrated photonic components, because this affects the performance of the fine tuning of optical response and light-path switching. These also have some strategies try to enable the tunability of planar lens, for example, varying the angle of the incident light or using of anisotropic liquid crystals. All the above approaches get the tenability and the focal spot within nanometer scale, but they can only be realized by sophisticated incident light or complex controlling system.This article proposed the approach to change the focal position of the subwavelength surface plasmon focusing lens by using the ionic liquid to change the phase front of the emergent light. Thanks to the microfluidic technology has been more mature, to change the height of the ionic liquid to much easier than the other methods, so it’s easy to control the focal position for our subwavelength surface plasmon lens. Here, we designed two kinds of this structure respectively, one is with a single silver slit, and the other is with double gold slits. When the incident light irradiates the structure at the wavelength of 532 nm vertically, the light pass through the slit, it excites the SPPs at the metal surface with air, then the surface plasmon wave propagate along the metal surface, and form the focusing pattern in the far field after modulating by the ionic liquid in the grooves. This makes changing the focal position easier be possible by changing the filling characteristics of ionic liquid, instead of changing the parameters of the dielectric grating, and the adjusting performance is good. For the single silver silt structure, the adjusting range is about 0.42 micrometers, and it is suitable for slight adjustment since its changing amplitude is small. And for the double gold slits structure, the adjusting range is about 2.6 micrometers, which is several times about the incident wavelength, so it is good at wide range adjustment since its changing amplitude is bigger. Then the simulations were carried out for the structures by the simulation software based on the Finite Element Method(FEM), we slao did some calculations according to the basic theory, and the simulation results consistent with the results derived from the theory. All these suggest that the focal position adjustment schemes we proposed are really realizable, and have great potentials in practical application, such as optoelectronic integrated circuits, biomedical sensor, micro and nano fabrication and so on, so they have high research values.