Study On The Fabrication And Performance Of Metal Nanoring Arrays And Related Structures

The nanoscale ring arrays and their related structures,split-ring or known as nanocrescent arrays,have drawn much attention due to their unique optical and electromagnetic properties which may open up various applications in the fields of photovoltaics,optical antennas,bio/chemical sensors,and metamaterials.Metallic nanocrescent structures are one of the most interesting metamaterial structures for their excellent tuning properties.Properties of nanoring arrays can be controlled by their geometrical parameters including inner radii,outer radii,height and period.In addition to nanorings,nanocrescents cause much more interests in metamaterial.It's reported that split-ring shaped resonators show strong electric and magnetic resonances.More importantly,unlike rings exhibiting no polarization effects,split-ring structures exhibit a polarization-dependent absorption spectrum due to their topographical asymmetry.One of main challenges faced by the nanotechnology is the fabrication of specially shaped nanostructures in a facile,low-cost and high-efficiency way.Electric beam lithography(EBL)and focused ion beam(FIB)lithography are ideal techniques for fabricating such nanoring and nanocrescent patterns,which make it possible to accurately control their geometric shapes.However,these point-by-point lithography processes are time-consuming,inefficient and expensive,and not suitable for large-area fabrication.An alternative approach for the fabrication of nanoring and nanocrescent structures over large area employs nanospheres as shadow masks,namely,shadow nanosphere lithography.But,on the other hand,the nanosphere lithography is prone to inevitable defects and lack of long-range ordering.Nanoimprint lithography(NIL)is a high-throughput and cost-effective technique to fabricate ordered nanopatterns with high resolution,thus it is popular to be applied in nanostructures fabrication.In this paper,we report an approach for the fabrication of ordered arrays of nanorings over large area from the templates of nanopillar arrays prepared by NIL This fabrication method combined UV-curing nanolithography technology,metal deposition and etching process,which made it possible to tune the geometric parameters of nanorings,such as height,diameter and linewidth.First,we deposited Au layer in the direction normal to the substrate surface by electron beam evaporation(EBE).Then,we adopted argon ion beam etching(IBE)process in normal direction to the substrate to remove the Au layer on the top and bottom of the nanopillars.Finally,the resist in the loops of these nanorings was removed by RIE process.By this approach,we fabricated a hexagonal packed Au nanoring array with an outer radius of 380nm,wall width of 40nm,height of 150nm and pitch of 600nm.We measured the transmission spectra of the fabricated nanoring structures using a commercial Fourier-transform infrared spectrometer and numerical simulated by COMSOL Multiphysics.A typical Au nanoring array presents a strong transmission dip at the wavelength of 2134 nm.This measured optical response is in good agreement with the numerically simulations,and the main spectral features could be qualitatively explained by the electrical field distribution of nanorings.Two modified versions of this fabrication method were also utilized to create nanocrescent arrays.The first method was used to etching metal by ion beam etching(IBE)at a desired angle ?,which was defined as the angle between the normal to the substrate plane and the incident metal vapor flux.In the second method,a layer of nickel was deposited at a controlled angle ?.Then,a gold layer was vertically deposited on sample surface.After that,double lift-off steps were used to form the final Au nanoscrescent structures.Both of these two methods could easily tune the width of nanocrescent through the angle ?.However,nanocresents fabricated by the first method were not precise enough.We chose the patterns fabricated by the second method to do the transmission spectra measurement.The large-area gold nanocrescent arrays showed strongly polarization-dependent transmission bands.When the incident electric field is set along the opening direction of the nanocrescent,a strong resonance centered at a wavelength of 1840 nm is observed.This resonance is associated with the dipole resonance along this direction,which can be verified by the electric field distribution simulated by COMSOL.While for an orthogonal polarization configuration(the exciting electric field is set perpendicular to the opening of the nanocrescent),the large transmission dip vanished to be replaced by almost a decreasing curve with the increasing of wavelength,which basically fits the simulated spectrum well.In addition,we fabricated silicon imprint mold with nanocrescent shaped holes by etching silicon through nickel mask.A hybrid imprint mold duplicated from silicon mold could successfully replicate nanocrescent structures by UV-curing nanoimprint lithography.