| Surface plasmon polaritons(SPPs)are a special kind of electromagnetic field excited by the interactions of light and free-electrons on the metal surface.It is greatly confined on the metal surface and could propagate in subwavelength scale Thus SPPs have attracted much research interests in recent years.The application of SPPs enables the realization of integrated photonic devices beyond the diffraction limit.Much effort has been paid to develop SPP-based subwavelength devices in recent years because of the increasing demand for miniaturization and integration of modern optical device,in addtion to the development of micro/nanofabrication technologies and theoretical research.Among them,the metallic nanostructures have received special attention due to its unique ability in spatial field modulation and sub-wavelength focusing.Based on the basic theory of SPP and using numerical simulation methods,we have investigated the sub-wavelength focusing in far-field beyond the resolution limit of several kinds of metallic nanostructures lenses which are designed based on different mechanisms.The main issues addressed in this dissertation are as follows:1.A quadric cylinder plasmonic lens(PL)with gold film slits filled with dielectric is proposed.The proposed PL is composed of the gold film with only five dielectric-filled nanoslits.The exit surface of the gold film is processed into quadric shape that can be parabolic,elliptical or hyperbolic cylinders.The film is fabricated to form five uniformly spaced nanoslits with different widths and depths.All five slits are symmetrically arranged with respect to the center of lens and filled with a dielectric medium(i.e.,SiO2).Under the illumination of TM polarized beams,the surface plasmon polaritons are excited at the entrance surface of the PL,then pass through the SiO2-filled slits while acquiring specific phase retardations,and are finally coupled to the light waves in the free space.Each light wave originating from the slit can be regarded as an individual point source,and the constructive interference of light waves from slits gives rise to the beam focusing at the focal plane of the PL.We investigate the phase modulation mechanism of the PL and find that the focusing performance relies on the shape of exit surface,filling medium and geometric parameters of the slits.A suitable phase modulation can be achieved by adjusting the structure parameters of the PL with a specific exit surface shape.Three kinds of quadratic cylindrical PLs,i.e.,parabolic,elliptical and hyperbolic cylindrical ones with continuous or stepped exit surface are designed to realize the focusing of TM polarized subwavelength beams in visible spectrum.The finite difference time domain method(FDTD)is employed to compute the light field and to investigate the focusing characteristics of the proposed PL.The focusing performance is evalutaed by calclulating the focal length,depth of focus(DOF)and full-width half-maximum(FWHM).The simulation results confirm that the proposed PL with a 2-?m-diameter aperture can achieve the subwavelength focusing at a focal length of micron scale.The attainable smallest FWHM of the focal spot is 0.405?0(?0 denoting the wavelength of the incident light)which is well beyond the diffraction limit.It is also worth mentioning that the step-like cylindrical PL can yield a sharper focal spot than the continuous cylindrical PL.For example,the FWHM of focal spot produced by the stepped elliptical cylindrical PL is about 92%of that produced by the continuous elliptical cylindrical PL.The proposed PL has the advantages of simple and compact structure with much smaller lateral dimension and easy integration with other photonic devices.Our study helps design the easy-to-fabricate PLs and facilitates applications of plasmonic devices.2.A PL composed of gold film with an annular array of nanoholes and two concentric grooves is proposed.This PL is formed by an annular array of nanoholes surrounded on both sides by two concentric grooves and enables the circular polarization illumination,which is appreciated since the circular polarization is much more readily available than the radial polarization.The proposed PL can improve the focusing performance in the far-field region via the constructive interference of light.By numerical calculation,the dependence of the phase difference between the light waves radiating from the grooves and the nanoholes on the groove position is investigated.Through linear fitting,the positions of the grooves in the focusing structure needed to achieve a specified focal length can be easily determined.The electric field distribution of the PL is simulated by the FDTD method,which confirms the presence of a far-field focus.The simulation demonstrates that the proposed PL can tightly focus the light into a central axisymmetric subwavelength focus spot with a FWHM less than half of the illuminating wavelength and an elongated DOF.The light intensity of the focal spot significantly increases by three-fold at the focal spot,compared to that without two concentric grooves.The focusing interaction between two adjacent PLs in a PL array is also investigated,and the numerical simulation shows how the focusing interaction impacts the ultimate focusing performance when the distance between two adjacent PLs is increased.This provides a new route to improving the focusing performance of PL arrays.3.A PL composed of gold film with two arrays of elliptical nanoholes and multiple concentric circular grooves is proposed.This PL is a metal film structure consisting of a combination of multiple concentric circular grooves and two arrays of elliptical nanoholes distributed along Archimedes curves.The specially designed nanoholes can couple linearly polarized illumination into the SPPs propagating toward the circular grooves;the grooves effectively can scatter the SPPs into a propagation wave behind the exit surface,producing a subwavelength focus.The focusing properties of the plasmonic lens are theoretically and numerically investigated.The plasmonic lens can produce interesting focal fields with super-resolution and long depth of focus,such as optical needle or tube.It should be noted that this lens can create a centrosymmetric focus even under linearly polarized illumination.4.A broadband spectrum PL composed of gold film with an annular array of nanoholes and three concentric slits is proposed.All the holes and slits are drilled through the gold film.Furthrmore,a PL array is also designed and evaluated for different array size.The single PL and PL arrays are both illuminated by the circularly polarized light,which is preferred in practice.The distribution of electric field of PL is calculated by the FDTD method,which confirms the presence of a far-field focus.The central axisymmetric subwavelength focusing is realized.The nanostructured PLs in previous reports have some shortcomings,such as the divergence of transmitted light field and requirement for particular illumnation wavelength.In our work,however,the designed lens can achieve broadband spectrum focusing;the lens is designed for 660 nm wavelength,but it can realize the good far-field focusing in the range from 560 nm to 760 nm.Through the analysis of Rayleigh-sommerfeld integrals,we find why the greater the incident light wavelength is,the smaller the focal length becomes.We further investigate the focusing interaction between two adjacent units of the array.Through the numerical simulation,we find the relation between the distance of adjacent units and illumination wavelength and the array size on final focus features.Our study provides a novel way for designing metallic film PL array suitable for the practical needs. |
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