Sub-wavelength Metallic Ring Apertures for Communications, Sensing and Nonlinear Optics
Doctor of Philosophy
In this thesis, we demonstrate sub-wavelength ring aperture arrays in a metal film which can be used for communications, material sensing, and nonlinear optics. We show in simulation and experiment extraordinary optical transmission through ring apertures on a metal film both in terahertz (THz) and mid-infrared (MIR) regions. For THz metallic ring aperture arrays, transmission of 60% is obtained with an aperture-to-area ratio of only 1.4%. We show that the high transmission can be suppressed by over 18 dB with a thin layer of free carriers in the silicon substrate underneath the metal film. We also experimentally demonstrate graphene-based active electro-optic modulation of THz waves. The metallic nanostructure provides ~4 times absorption enhancement and ~50% modulation depth is obtained with monolayer graphene. These results suggest that CMOS-compatible terahertz modulators can be built by controlling the carrier density near the aperture. We also demonstrate extraordinary optical transmission in MIR metallic ring aperture arrays. We observe enhanced field-matter interaction with the MIR ring apertures due to enhanced near field, and we present its applications in sensing and nonlinear optical effects. We demonstrate using the devices for enhancing the absorption of PMMA by ~8 times. We also show in simulation the enhanced sensing of monolayer graphene with different Fermi level. We obtain a 60 cm-1 shift of resonance frequency per 0.1 eV change of Fermi level in graphene, and a 6% change in transmission peak intensity. Next we experimentally demonstrate enhanced 2D IR spectrum by using the MIR concentric apertures. At last, we experimentally show polarization-independent Fano resonance in concentric metallic ring apertures both in THz and MIR regions. A high-Q and intensive dark mode is indirectly excited by coupling with a low-Q bright mode. The coupling is enabled by the intrinsic asymmetry between the two concentric rings. A coupled optical resonator model is used to analyze the coupling process between the bright and dark modes. We find the Q of the dark mode is 3~6 times higher than that of the bright mode. We show that the dark mode can be selectively disabled without affecting the bright mode due to its unique current flow pattern. We also observe enhanced field-matter interaction and the absorption-induced transparency effect caused by the intense E-field in the aperture when a material absorption line is aligned with the dark mode.
Nanophotonics; EOT; THz modulator; Fano resonance