A Liquid Crystals Modulated Optical Tunable Filter Based On Fano Resonance Of Au Nanorod Trimer

With the progress of the processing technology of nano devices,the surface plasmons of metal nanostructures has become a hot spot of research.The surface plasmons can be excited by incident light irradiating metal nanostructures,and the properties of surface plasmons are affected by factors such as the shape,material and size of metal nanostructures and so on.Due to the performance of traditional electronic devices is limited.In order to solve the bottleneck of current integrated circuits,the development of integrated optical circuit is imminent,and its importance is self-evident.The integrated optical device based on metal nanostructure breaks the diffraction limitation of traditional electronic devices,and has the advantages of high sensitivity,small volume and high transmission bandwidth and so on.Fano resonance,as a special form of surface plasmon,can be incited by rod,ring,and other nanoscale structures.Fano resonance has asymmetric and sharp alignment,and very sensitive to perceive changes in the surrounding environment.So Fano resonance is often applied in filters,biosensors and other fields.In this paper,a tunable filter is proposed by using the finite difference time domain method(FDTD).The main research is as follows:1)A "C"-shaped metal nanostructure is proposed,which consists of three metal nanorods with the same size.Because the structure is asymmetrical,the Fano resonance can be generated only in the direction of the determined polarization,and the Fano resonance will not be produced in the other directions.The incident light band that produces the Fano resonance is just in the same band as the absorption peak of the "C" structure,which provides us with a controllable transparent window.Therefore,when the incident light passes through the "C" structure,the transmittance of a certain band is indirectly controlled by controlling the polarization direction of the incident light.2)A tunable filter model is established.The incident light first passes the liquid crystal before irradiating to the "C" structure by using the polarized light characteristic of the twisted nematic liquid crystal.The state of the liquid crystal can be controlled by voltage.The homogeneous nematic liquid crystal does not change the polarization direction of the incident light.The twisted nematic liquid crystal can turn the polarization direction of the incident light to ninety degrees.The switch of the filter is controlled indirectly by controlling the state of the liquid crystal.3)The modulation depth of tunable filter is analyzed,and the different parameters of tunable filter are calculated.The factors that affect the modulation depth of filter are discussed,which lays the foundation for optimizing the modulation depth of filter.The technological defects which may be occured in the actual production and the error analysis brought by the process defects are also analyzed.