Study On Dark Mode Properties In Asymmetric Optical Antennas

Dark mode is a kind of sub-radiative resonance in metasurface structure,which is different from regular electric resonance and magnetic resonance.Due to destructive interference effect,the far field radiation of dark mode is strongly suppressed.As originated from such “dark” property,dark mode doesn’t couple with free-space light and is difficult to be observed in spectral characterization.As compared with low order electric or magnetic resonance,dark mode has the unique properties of extremely narrow linewidth and strong localized field.Because of its “dark” property,dark mode excitation usually requires the introduction of structural perturbation or near-field coupling.The dark mode often appears as a hybridized asymmetric Fano lineshape in spectrum and the corresponding metasurface structure can be very complicated.In this thesis,we proposed and fabricated a “T” shape metasurface antenna,which exhibited a second order magnetic dark mode in the mid-infrared spectrum.The measured dark mode showed up as an obvious single peak in the far-field reflection spectrum.These results offer a simple way to realize dark mode resonance in metamaterial structures,and are of significance for the development of infrared metasurface devices.The main contents of this thesis are summarized as follows:(1)Based on the three-layered structure of metal-dielectric-metal(MIM),a “T”shape metasurface antenna was designed by introducing slight structural asymmetry in the array of rectangular metal patches.The “T” shape metasurface antenna consisted of a200 nm thick copper film,a 300 nm thick Zn Se layer and a top array of “T” shape gold patches.Our numerical simulation results showed that the metasurface antenna can excite a second order magnetic dark mode at 53.5 THz.The calculated linewidth and quality(Q)factor of dark mode is 42 nm and 135,respectively.This quality factor value is about 10 times that of the value for regular low order electric and magnetic resonances.By applying the dark mode in refractive index sensing of magnesium fluoride(Mg F)film,a sensitivity of 650 nm/RIU and a figure-of-merit(FOM)of 15 were obtained,which revealed the great potential of metesurface dark mode in refractive index sensing.In addition,our multipole expansion analysis suggested that the designed second order magnetic dark mode was equivalent to an electric dipole in nature.(2)By using the techniques of electron beam evaporation and electron beam lithography,we fabricated the “T” shape metasurface antenna and realized the second order magnetic dark mode in experiment.Our infrared spectral measurement shows that the first order magnetic resonance and the second order magnetic dark mode were excited in the metasurface sample at 27.6 THz and 51.53 THz,respectively.The absorptivity of the dark mode at 51.53 THz was 32.1%,and its Q-factor was 25.5,which is slightly smaller than the theoretical value.The discrepancies between experiment and theory can be originated from errors in structural fabrication and the material parameters.