Manipulation Of Envanescent Waves Based On Surface Plasmon Polaritons

With the increasing demand of nano-scale fabrication and highly-integrated optoelectronic technology, plasmonics has attracted more and more interests due to the short wavelength and the ability of coupling/amplifying evanescent waves with high wavevectors. The surface plasmon polaritons (SPPs) is capable of manipulating these localized surface evanescent waves to break the conventional diffraction limit, making it great important for potential applications, such as subwavelength imaging, high-density storage and solar energy. In this dissertation, we mainly study the physical mechanism and theoretical model of manipulating surface evanescent waves using plamonics nanostructures, and experimental methods for verifying the transmission behavior of these surface waves with high wavevectors. Details are listed in the following.The subwavelength focusing of metallic nano-silts and nano-grating lens is analyzed. The performance of nano-slits metallic lens is mainly determined by the effective propagation constant in the slits waveguide. On the other hand, the distribution of electromagnetic field at the exit side and coupling between the surface waves and free space decide the focusing behavior of the nano-grating metallic lens. Besides, the metal-dielectric-metal (MDM) multilayer is proposed to induce the plasmon resonance inside the waveguide for a stronger focus. The focusing intensity of the proposed metallic lens is further optimized and enhanced to about 21 times comparing to the conventional grating metallic lens.The significant loss of subwavelength feather carried by evanescent waves mainly causes the conventional diffraction limit. These waves with high wavevectors are usually localized at surface of the object, and can’t reach to the imaging plane. The SPPs are able to couple and amplify the surface evanescent waves because of the short wavelength. Then, alternated stack metal-dielectric layers (ASMDLs) are proposed as an effective way to transmit and manipulate these evanescent waves. The effective material theory (EMT) is first employed to investigate these metamaterials to obtain the hyperbolic dispersion relation, in which the evanescent waves can propagate. Then, the optical transfer function (OTF) is calculated by the rigors coupled wave analysis (RCWA) to study the spatial filtering in ASMDLs. At last, we propose a moire fringes method to experimentally verify the evanescent waves filtering in ASMDLs. The multilayer is consisted of 8 pairs of Ag(20nm)-SiO2(30nm), sandwiched by the excited grating with 400nm period and detected grating with 200nm period. Only the diffraction orders of the excited grating with specific transverse wavevectors can pass through the multilayer. Then the moire fringes are generated by the interference between transmitted orders and detected gratings. The experimentally observed period of fringes is 59.76μm, which agrees with the theoretical prediction and simulation, proving the transmission and filtering inside ASMDLs. The filtering property, such as filter profile and transmission band, is further verified by the oblique incidence.The influence of surface roughness on the subwavelength imaging is further investigated. By comparing simulated random roughness with gauss distribution to the experimental measured one, it’s found that there exists correlation length of the measured roughness. Then, the measured roughness is introduced in the metal-photoresist-metal (GPG) subwavelength imaging structure. Through the roughness enables more transmission of high wavevectors, random interferential noise between neighborhood images is more severe with increasing roughness distortion. We show that additional loss is able to restrain the interferential noise caused by roughness and smooth the resonance peak in OTF curve while preserving the imaging integrity. Other situations, such as a single superlens with roughness at different interfaces, a multilayer superlens are further studied. Results show that the imaging surface (metal-photoresist interface) play a major role in superlens imaging, and the imaging is more vulnerable to roughness in a multilayer system. Furthermore, the additional loss is still able to improve the imaging in both situations.The previous study shows that roughness at different interface plays different roles in subwavelength imaging. The practical roughness distribution in the metal-photoresist-metal structure is experimentally obtained and analyzed. We find that the surface roughness can be flatted and planarized while fabricating on a photoresist layer. Besides, the influence of subsequent fabrication on the prior surface needs further consideration. The template stripping is employed to obtain the roughness distribution of the inner interface in multilayer systems. The evaporating silver film on the silicon substrate and spinning photoresist on silver film have less effect on the previous surface roughness, but evaporating silver film on the photoresist would distort the profile of original surface and improve the roughness.In conclusion, surface plasmon polaritons is an effective way to manipulate the localized evanescent waves for subwavelength focusing and imaging, which has great potential applications in biophotonics, high density optical storage and high efficient photoelectric conversion and so on. It also provides a convenient way to realize nano-optical devices and subwavelength imaging in practical application.