The Research Of Ag-Al₂O₃-Ag Metamaterial's Optical Property Under Low Temperature

Metamaterial is a kind of engineering composite material,which is constituted with artificial micro-nano structure distributed periodically.It has many special physical properties which don't existing in nature,for example,negative refraction.However,due to lack of tenability and narrow bandwidth,the application of metamaterials is limited.Some of the recent research of tunable metamaterials provide many methods,such as changing the substrate material,the shape and size of the structure unit or the environment temperature.Metal has become a common choice to constitute the structure of metamaterial.Metals' dielectric function is a function of temperature,so this article aims to explore the feasibility of temperature control,by studying the properties of plasmonic metamaterial under normal and low temperature.In this paper,we chose the Ag-Al₂O₃-Ag sandwich structure as the unit of metamaterial.Coupling of visible wavelengths of electromagnetic wave and structure was investigated by means of finite element simulation in TE mode and TM mode and the result shows that the presence of electromagnetic resonance only exists in TM mode.By controlling variable method,this paper shows the influence of structure' different dimension parameters on the electromagnetic resonance and respectively analyzes the effect of metal' real and imaginary part of dielectric function on the spectrum.The decline of temperature will cause a small increase in the real part and a larger decrease in the imaginary part on the metal dielectric function.We discover that a red shift of electromagnetic resonance and a enhancement of the reflection spectrum will occur as temperature declining by simulation.The preparation of grating metamaterial has been completed by a series of micronano processing method and the parameters match what we want well.At normal temperature,spectrums which are suitable to simulation are got in measurement.This verifies the law of size factors on electromagnetic coupling.However,there exist some differences between measured spectrums and simulation of low temperature.Firstly,a large red shift exist in reflection spectrums.Secondly,two resonance peaks of the reflection spectrums trend to join together.Thirdly,we don't found obvious enhancement in spectrums.To explain the difference between experimental data and simulation of low temperature,we consider two reasons.One is the influence of the adhesive layer,the other is the increase of ?surf caused by low effective electron mean free path.The ?surf is a parameter which represents the effect of electron-surface scattering in dielectric fuction.When the structure or grain is small,the effective electron mean free path would be smaller than it in block.It is proved that the surface adhesion layer can make red shift occur in the reflection spectrum and make the two peaks merge with each other by simulation.We measure the reflection of the same thickness silver films with different grain size at room temperature and under low temperatures.The spectrums show that the relative variation of the reflectivity increases with the increase of grain size.This phenomenon confirmed the role of ?surf.