Simulation Study On Polarization Properties Of Subwavelength Metallic Focusing Structure

The rapid development of optical technology leads to urgent demand for higher resolution of focusing and imaging system.Therefore,to develop the optical device that breaks the diffraction limit becomes even more important.The metallic subwavelength-focusing structures are significant functional devices used to achieve focusing beyond diffraction limit for its unique mechanism and performance.The structure is designed to excite surface plasmons,which could break the limitation of diffraction limit by controlling evanescent wave of high spatial frequency.The focusing structure has been widely used in plamonic lithography to improve the resolution of etching pattern.Although the ever-reduced spot size has achieved high-resolution performance,the challenge exists.The single-spot nanofocusing limits the speed of direct-writing methods and the polarization effect of structure may affect the etching precision.To solve the disadvantages mentioned above and improve the speed and precision of plasmonic lithography,this paper designs the new focusing structure to produce two spots simultaneously at focal plane and analyzes its polarization effect which affects the focusing performance.The main works are as follows:(1)Study on split-focusing theory.To design split-focusing structure,the relation between surface field of focusing structure and field at the focal plane should be discussed first.For near-field split focusing,the field distribution at focal plane can be represented by sinc function.Then the surface field distribution of split-focusing structure can be solved by angular spectrum method.For far-field split focusing,the ideal wave front for split focusing is designed by the principle of equivalent optical path.Therefore,we establish the relation of field at focal plane and phase distribution of transmitted light from focusing structure.The relation discussed above builds a foundation for split-focusing structure design.(2)Design of near-field metallic subwavelength-split-focusing structure and analysis of polarization properties.The circular hole/elliptical nanoring structure is designed on Si O₂substrate.The split focusing is achieved by exciting the surface plasmons of nanoring.Simulation result verifies that the source field distribution fits the theoretical one.Under the incidence of linearly polarized plane wave at 633nm wavelength,two circular split-focusing spots are achieved at the focal length of 36nm.The full width at half maximum of spot is approximately 50nm.The polarization vector method is used for analysis.Due to the polarization effect of structure,the relative position of dual spots alters following the change of azimuth angle.When the azimuth angle is near 90°,the plasmonic property of nanoring alters and split focusing disappears.According to simulation result,the applicable azimuth angle is in the range of 0°through 55°.The results provide guide in theory and methods for fabrication and application of related structure.(3)Design of far-field metallic subwavelength-split-focusing structure and polarization analysis.The concentric rings/rectangle nanohole structure is designed on Si O₂ substrate.Based on effective index of metallic slits,the focusing structure can construct wavefront for split focusing in a discrete form.Under the incidence of linearly polarized plane wave at633nm wavelength,the dual spots are achieved at focal length of 6?m and minimum full width at half maximum is around 320nm.The depth of focus is over 2?m.The polarization effect related to azimuth angle of structure is analyzed using Jones method.When azimuth of focusing structure is set between 0°and 90°,spot position alters and intensity of sidelobes raises.The sidelobe affects the contrast of the focusing spot.Therefore,to meet the practical demand,parameters concerned with spot contrast are simulated under different azimuth angles,which provides a good theoretical and technological basis for practical application.