Surface Scattering And Resonance Properties Of Periodic Optical Antennas

As an emerging research hotspot in the field of micro-nano optics and infrared physics in recent years,optical antennas have great scientific value in basic theoretical research and new optical devices.Depending on different structural designs,optical antennas can show a variety of optical properties,and realize the regulation and energy exchange of free space light field in infrared and visible light frequency ranges,which is an effective method to realize artificial control of the light field,and has broad application prospects in fields of molecular sensing,photoelectric detection,polarization conversion and beam deflection.In view of complex physical mechanism and novel optical characteristics in optical antenna structures,basic properties and characteristic features of the surface scattering and mode resonances in periodic optical antennas were deeply studied in this dissertation.Based on the metal-dielectric-metal structure,optical leaky-wave antennas and periodic disc patch antennas were designed and fabricated.The spectral response properties of the surface scattering and resonance modes in these optical antenna samples were measured and analyzed.With the resonance frequency dynamically tuned by incident angles,the leaky-wave antenna mode and fingerprint detection of monolayer organic molecules were realized.The spectral features and physical mechanisms of surface scattering in periodic optical antennas were revealed,and the suppression effect of diffuse reflection in graphene optical antennas was discovered and studied.The results in this dissertation have a great heuristic value for understanding the basic photophysical properties of periodic optical antennas and novel infrared optical devices design and fabrication.The main innovative research work of this dissertation includes:1.The optical leaky-wave antenna with angle tuning properties was designed and fabricated,with tunable TM₀₁ waveguide mode and the fingerprint detection of monolayer molecules in a wide angle range realized,which revealed the basic principle and physical mechanisms of the interaction between optical antennas and monolayer molecules.Optical leaky-wave antenna samples fabricated in this dissertation consisted of a 300 nm thick copper film substrate,a 100 nm thick silica dielectric layer,and an array of 100 nm thick gold strip with 900 nm width.Our experimental results showed that as the incident angle varied from 15°to 75°,the resonance frequency of the TM₀₁waveguide mode in optical leaky-wave antenna samples moved from 2700 cm^-1 to 3190cm^-1,exhibiting an excellent angle tuning property.When optical leaky-wave antenna samples interacted with monolayer organic molecules,the intensity of molecule fingerprint signals was decided by frequency mismatches between the TM₀₁ waveguide mode and molecule fingerprints,and the maximum molecule fingerprint signals were obtained at frequency mismatches of±100 cm^-1.The variation trend of fingerprint signals was further shown to be related to the coupling intensity between the TM₀₁ waveguide mode and monolayer organic molecules using the coupled oscillator model.The results of this work are helpful to further understanding resonance properties of optical antennas and their interactions with organic molecules,which shed new light on the design and fabrication of novel molecular sensors.2.The periodic disc patch antenna was designed and fabricated,with properties and mechanisms of surface diffuse reflection revealed.The antenna structure was composed of a 300 nm thick copper film substrate,a 300 nm thick magnesium fluoride dielectric layer,and an array of 100 nm thick gold disks,with the period ranging from 6 to 10μm,and the disk diameter varying from 3 to 6μm.Experimental results and theoretical analysis both showed that the diffuse reflection of the periodic disc patch antenna originated from the diffraction effect in periodic structure,which occurred in the short-wave spectral ranges with wavelength shorter than the first-order Rayleigh anomaly.The intensity of diffuse reflection tended to gradually increase as the incident light wavelength decreased.In addition,when the range of diffuse scattering covered the antenna resonance,the intensity of diffuse reflection was enhanced at the resonance wavelength,which significantly affected the resonance absorption intensity and also had weak influence on the resonance wavelength of antennas.These results contribute to understanding the effect of surface diffuse reflection on resonance modes and spectral behavior in periodic optical antennas,and pave the way for the analysis,design and spectral characterization of periodic optical antennas.3.The optical antenna constructed by graphene strips was designed,and basic properties of resonance modes and surface diffuse reflection were explored.Based on the finite element numerical simulation method,we calculated the diffuse reflection spectrum and the cross-section of scattering and absorption in periodic graphene strip antennas with the variation of structural parameters and graphene material properties（Fermi leveland scattering time）.The numerical calculation results showed that the diffuse reflection of the graphene antenna was composed of several diffraction orders generated by the periodic structure,which occurred in the spectral range with wavelength shorter than the first-order Rayleigh anomaly,and its intensity was enhanced at the first and second order modes resonance wavelengths of graphene antennas,which were consistent with properties of the diffuse reflection in periodic disc patch antennas.Nevertheless,the intensity of diffuse reflection was strongly suppressed to be less than1%,which was originated from the large resonance wavelength to ribbon size ratio in antenna structures and ultra-thin thickness of graphene.These results are beneficial for understanding the physical mechanism of the interaction between graphene optical antennas and light fields,and have great research value for the design and characterization of graphene optical antennas.