Resonance And Field Control Properties Of Optical Antennas In The Mid-infrared Region

The mid-infrared spectral region exists special phenomena of atmospheric transparency,thermal radiation and molecular fingerprint absorption,which make it attractive for both fundamental and applied studies.Optical antenna is an emerging concept in the area of micro/nano-photonics,similar to radiowave and microwave antennas,which enables energy coupling between free-space and localized area on subwavelength scale,and thus presents an effective way for light manipulation.Resonant modes formed in structured subwavelength metallic or dielectric antennas play significant roles in a wide range of applications including light emission devices,optoelectronic modulators and spectroscopic techniques.In this dissertation,resonance properties and physical mechanisms of mid-infrared antennas were studied.By using a metal-insulator-metal(MIM)geometry,a tapered patch antenna and an asymmetric cross-shaped antenna were designed and fabricated,which exhibited novel resonant modes such as broadband magnetic resonance mode,toroidal mode and magnetic quadruple mode,arising from superimposed or coupled magnetic dipoles.These resonant modes were shown to have unique properties of angle-insensitive broadband absorption,high quality factor and polarization-selective excitation.Structure-tuned broadband light absorption,cross-polarization conversion and high sensitive refractive index sensing were also demonstrated by using these resonances.These research results have enriched novel properties obtainable in optical antennas,and are helpful for understanding the fundamental properties of plasmonic antennas and for developing antenna-based devices.The main results obtained in this dissertation are as follows:1.On the basis of resonance superposition of multiple magnetic dipoles,a tapered patch antenna with graded width was designed and fabricated,which exhibited angle-insensitive near-perfect broadband light absorption.The magnetic dipole in MIM antenna structure has been widely used to realize perfect light absorption in various spectral regions.Based on the special angle-insensitive coupling of magnetic dipole to free-space,we proposed a tapered metal patch antenna with graded width.The structure consisted of 300 nm thick copper film,600 nm thick Al₂O₃ insulator and 100 nm thick gold strip array.The width of top gold strip varied gradually from 1.6?m to 4.2?m.Experimental measurements showed that absorption of the antenna sample was over80%within the band of 29.2?38 THz.This strong broadband absorption feature was maintained as the incident angle was varied from 0oto 75o,showing a novel property of angle-independent broadband light absorption.2.By using multiple coupled magnetic dipoles,an asymmetric cross-shaped antenna was designed and demonstrated,a toroidal resonant mode with high quality factor was realized,which showed special polarization properties and refractive indexsensing properties.As limited by ohmic loss of metals,electric and magnetic resonances in conventional metal microstructures are often very lossy and their quality factors are limited to less than 20.Here,based the coupling and hybridization of magnetic dipoles,we designed an asymmetric cross-shaped antenna,which was made of300 nm thick copper,500 nm thick Zn Se layer and metal bar array.The top layer four metal bars arranged in an asymmetric cross configuration.The width of the metal bars is150 nm,and their lengths are between 550-700 nm.Spectral measurements indicated that a toroidal dipole mode was formed at 82.6 THz with a ring of head-to-tail connected magnetic vortex.Its measured quality factor(Q)was 60.2.The toroidal dipole mode was only excited at special polarization angles of-45o,15oand 75o,and exhibited cross-polarization conversion with an efficiency of 17%at polarization angles of-90o,-60o,-30o,0o,30o,60oand 90o.In addition,the toroidal dipole mode was also applied for refractive index sensing with a figure-of-merit(FOM)of 4.The quality factor and refractive index sensing FOM are the highest values among similar metallic antennas in the infrared region.3.In the fabricated asymmetric cross-shaped antenna,a special high quality factor magnetic quadruple mode was also revealed.As a higher order mode of magnetic resonance,magnetic quadruple has favorable properties of low radiative loss and strongly enhanced near-field.However,due to its dark mode nature,it is usually difficult to be observed in optical antenna structures.In our fabricated asymmetric cross-shaped antenna,we measured a magnetic quadruple resonance at 80.4 THz in addition to the toroidal mode at 82.6 THz.The multiple expansion analysis suggested that the magnetic quadruple mode was formed with two pairs of magnetic dipoles with opposite directions.The measured resonant absorption was 86%and the quality factor was 57.4.This magnetic quadruple mode was only excited at special polarization angles of-75o,-15oand 45o,which were different from the toroidal dipole mode.In addition,the magnetic quadruple mode also exhibited cross-polarization conversion with an efficiency of 18.4%at polarization angles of-90o,-60o,-30o,0o,30o,60oand 90o,similar to the toroidal mode at 82.6 THz.Our measured magnetic quadruple response paves a pathway for studying high quality factor dark modes in optical antennas.