Composite Antireflective Nanostructures

It has been an important research topic that how to improve the performance of photoelectric devices by using antireflective structures.In recent years,with the development of nanophotonics,nanostructures are widely used as antireflective strcutres.In these structures,SiO₂ nanocolumn arrays fabricated by using oblique angle deposition play a significant role in this area for its stable and novel optical properties.They can broaden the range of refractive index and provide more microscopic morphologies.Besides,Ag nanopartilces are also important for their localized surface plasmonic resonances,which are often adopted to reduce the surface reflection.However,it's a challenge to control the localized surface plasmonic resonances of Ag nanoparticles and make full use of their enhanced scattering and near-filed to reduce the reflection.In this dissertation,a broadband antireflective nanostructure with Ag nanoparticles on SiO₂nanocolumns has been systematically investigated.SiO₂ nanocolumn arrays were grown on substrates by oblique angle deposition through electron beam evaporation.Ag nanoparticles were made by depositing Ag thin films on SiO₂ nanocolumns through thermal evaporation.Due to SiO₂ nanocolumns,Ag nanoparticles have a certain size distribution which can support a broadband plasmonic resonances,resulting in a broadband antireflective performance.Oblique angle deposition technology is an effective way to prepare the SiO₂ nanocolumns.Through controlling the incident angle of flux,the diameters and tilt angles of SiO₂nanocolumns can be tuned.The mechanism under these phenomenon has been revealed via a combination of surface diffusion and flux density,which both are varying with the incident angle of flux.After that,a factor of parallel momentum was introduced to fix the classic rules,which makes the tilt angle of the nanocolumns be predicted more precisely.In addition,a formulation was derived based on density of flux,which makes a qualitative description about the density of nanocolumn arrays.All these make the SiO₂ nanocolumns prepared more accurately.Antireflective nanostructure with Ag nanoparticles on SiO₂ nanocolumns has been fabricated on the Si susbstrate.Experimental results show that the average reflection can reach 3.84%in the range 400-700 nm and 5.75%in the range 400-1100 nm,much lower than that of other strucutures used for comparing.Finite Difference Time Domain simulation results show that the localized surface plasmon resonances of Ag nanoparticels with different size lead to the reflection minimum at different wavelengths.Because the diameter distribution of SiO₂nanocolumns determine the size distribution of Ag nanoparticles,this makes the resonances occur in a wide wavelength range and lead to a broadband low reflection.The structure with Ag nanoparticles on SiO₂ nanocolumns on flexible polyimide substrate has also been investigated.Experimental results show that the structure can lower effectively the reflection on both flat and curved surfaces in a broad wavelength range.It is found that the increasing incident angle leads to the blue-shift of position of the minimum reflection due to the reduced travel distance of the resonated electrons and the weakened coupling of resonance between neighbouring Ag nanoparticles.The surfaces with smaller curvature radii have higher reflection than those with larger curvature radii.The reflection on a curved surface can be obtained by weighted summing of the angle-dependent reflection on a flat surface,which agrees well with experimental results.