Investigations of the Properties of the Silver Dendritic Metasurfaces in Visible Light
Author:Fang Zhen Hua
Supervisor:luo chun rong
Metamaterials are composed of periodically artificial subwavelength composite structures.The microscopic structures of the unit cells are designed to meet the actual needs and possesses the special or unusual properties which are not found in natural materials.It enables many novel functions such as flat lenses,stealth cloaks,perfect lenses,super-resolution holograms and photoacoustic imaging.Compared with remarkable progress in the field of optical metamaterials,the three-dimension metamaterials are more difficult to fabricate.Thus,the researchers proposed the two-dimensional optical metasurfaces in view of the preparation challenge in visible light.In the meanwhile,the excellent properties of metamaterials can also be maintained.Convertional optical metamaterials are generally prepared by using laser,ion beam or electron beam etching,which face with the problems of high equipment and cost,long preparation period and extremely limited sample sizes.Therefore,the preparation of optical metasurface by chemical method is attracting more and more attentions.Based on the concept of bottom-up electrochemical deposition,a novel dendritic optical metasurface was proposed.Furthermore,the dendritic unit cluster was formed by these structural units,and the silver dendritic metasurfaces responding to the visible light were prepared by the electrochemical workstation.The anomalous Goos-H(?)nchen(GH)displacement and ’rainbow-trapped’ effects of the structures were verified by experiments.In addition,we also experimentally studied the polarization rotation properties,anomalous transmission and the anomalous optical spin Hall effect of the dendritic metasurface.The main results of this dissertation are listed as follows: 1.The preparation of silver dendritic metasurface in visible light Based on the concept of bottom up chemical preparation,a new type of metasurface in visible light composed of the silver dendritic units,was proposed.Dendritic units in the metasurface were randomly arranged to form a quasi-periodic structure,which has the advantages of simple preparation method and low cost.We used the AutoLab Electrochemical Workstation to accurately fabricate single and double-layer dendritic metasurface samples with adjustable resonance band in the visible light.The effects of external deposition conditions such as deposition voltage,temperature and electrolyte concentration on the morphology of silver dendrites were studied.The optimal process conditions were obtained for the preparation of silver dendritic metasurface.The resonant peak of silver dendritic metasurface could be controlled by adjusting the electrochemical deposition conditions.2.Experimental verification of the anomalous GH displacement of the dendritic metasurface The GH displacements of silver dendritic metasurface at different incident wavelengths were measured by the interference method.The experimental results show that the GH shift value of the sample is negative when the incident light wavelength is consistent with the resonant wavelength of the sample,and the positive GH shift was obtained when the incident light wavelength is in the non-resonant wavelength band.Interferometry is a simple and easy method to measure GH displacement.In addition,this method is also suitable for measuring GH displacement at other types of 2D metamaterial interfaces.The anomalous GH effect of the silver dendritic metasurface on the incident light in the resonance band was experimentally verified.The experimental measurement of anomalous GH shift is of great significance for the further study of light stop and ’rainbow trapped’ effects.3.Experimental observation of the ’rainbow-trapped’ effect of silver dendritic metasurface optical waveguide The wedge waveguide based on silver dendritic metasurface was designed and assembled to achieve the slowdown and stop of visible light at room temperature.The incident multicolor lights separate and stay at different positions of the wedge waveguides according to the wavelengths,forming a bright rainbow.By measuring the energy transmission in the waveguide,it is found that the light energy emitted by the waveguide is zero when a certain amount of light is incident on the resonant wavelength.When the wavelength of the incident light is different from the resonant wavelength,the energy of the emitted light is greater than zero.4.The polarization rotation properties of the silver dendritic metasurface Numerical simulation and experimental studies on the polarization rotation properties of silver dendritic metasurface were carried out.The results show that,when the incident light wavelength is consistent with the resonant wavelength of the sample and the linearly polarized light is perpendicularly incident to the surface of dendritic metasurface,cross-polarized transmitting light is obtained.The ultra-thin metasurface polarization converter was designed to study the polarization rotation in the reflection mode.It is demonstrated that the metasurface can achieve efficient cross-polarization conversion in the ultra-wide band of 8.4-20.7 GHz.Numerical simulations and experimental tests confirm that this metasurface can efficiently transform x(y)polarized waves to y(x)polarized waves and the conversion efficieny is over 92 % in the whole operating band.This simple design of the reflection mode metasurface can be easily extended to terahertz and the visible light region.5.Experimental study of anomalous optical spin Hall effect of silver dendritic metasurface An optical interferometric method was used to measure the optical spin Hall effect reflected from the surface of K9 crystals,silver dendritic metasurface and acanthosphere samples.This method is easy to set up and can be used to measure the shift of the entire transmission cross-section of the beam.It is found that the optical spin Hall effects of the silver dendritic metasurface and the acanthosphere sample are negative when the wavelength of the incident light is consistent with the resonant wavelength of the sample.On the other hand,when the wavelength of the incident light is different from the resonant wavelength of the sample,the optical spin Hall effect is positive.