Theoretical Investigation Of Nanolithography Based On Asymmetric Metal-cladding Dielectric Waveguide

In this letter,sub-wavelength grating and hierarchical sub-wavelength photonic structures are inscribed by waveguide modes interference,where the waveguide modes are existing in an asymmetric metal-cladding dielectric waveguide excited by the prism coupling method.The various dependences of the periods of sub-wavelength grating and hierarchical sub-wavelength photonic structures on relative parameter were investigated in detail.This nanolithography method will provide important theoretical support for extensive application of two nanostrucrure,which is simple operation,low cost,and high output.The fabrication method of hierarchical sub-wavelength photonic structures will provide possible way for inscribing it in the experiment.The details of this thesis are listed following:(1)A method of fabricating sub-wavelength gratings with different periods by employing an asymmetric metal-cladding dielectric waveguide is proposed and its theory is discussed.The multilayer waveguide structure consists of Al film,photoresist film,and air,and either the TE0 or TM0 waveguide mode is excited by a 325 nm laser and is used to inscribe the sub-wavelength grating.Theoretical analyses of the dispersion curves of the waveguide modes and the results of simulations performed using the finite element method to determine the distributions of the interference fields of the waveguide modes are discussed.The sub-wavelength gratings of different period and various depth-to-width ratio are inscribed by changing thickness and refractive index of photoresist,as well as which of the waveguide modes is excited and employed.The minimum period of sub-wavelength gratings is 90 nm.(2)The sub-wavelength grating is inscribed by two TE0 waveguide modes interference,where the TE0 waveguide mode is existing in an asymmetric metal-cladding dielectric waveguide excited by the prism coupling method.The dispersion curve of TE0 waveguide mode,the relationship between the period of the sub-wavelength grating and the exciting light source,the refractive index of the prism and the photoresist are theoretically analyzed in detail.The distribution of the interference optical field of TE0 waveguide modes in the multilayer structure including metal film,photoresist and air layer is numerically simulated using the finite element method.Comparing with surface plasmons interference lithography,TE0 waveguide modes interference can realize adjustable period sub-wavelength grating writing for thicker photoresist condition by changing exciting light source wavelength,the refractive index of prism,the refractive index of photoresist and especially the thickness of photoresist.(3)The theoretical investigation of hierarchical sub-wavelength photonic structures with various periods and numbers of layers is presented,which were fabricated using a high-order waveguide-mode interference field.A 442 nm laser was used to excite high-order waveguide modes in an asymmetric metal-cladding dielectric waveguide.The dispersion curve of the waveguide modes was theoretically analyzed,and the distribution of the interference field of high-order waveguide modes was numerically simulated using the finite-element method.The various dependences of the characteristics of hierarchical sub-wavelength photonic structures on the thickness and refractive index of the photoresist and the waveguide mode were investigated in detail.These hierarchical sub-wavelength photonic structures have various periods and numbers of layers and can be fabricated by a simple and low-cost method...