Jong-Ho Choe

Substrate effect on aperture resonances in a thin metal film

We present a simple theoretical model to study the effect of a substrate on the resonance of an aperture in a thin metal film. The transmitted energy through an aperture is shown to be governed by the coupling of aperture waveguide mode to the incoming and the outgoing electromagnetic waves into the substrate region. Aperture resonance in the energy transmission thus depends critically on the refractive index of a substrate. We explain the substrate effect on aperture resonance in terms of destructive interference among evanescent modes or impedance mismatch. Our model shows an excellent agreement with a rigorous FDTD calculation and is consistent with previous experimental observations.

Slot antenna as a bound charge oscillator

We study the scattering properties of an optical slot antenna formed from a narrow rectangular hole in a metal film. We show that slot antennas can be modeled as bound charge oscillators mediating resonant light scattering. A simple closed-form expression for the scattering spectrum of a slot antenna is obtained that reveals the nature of a bound charge oscillator and also the effect of a substrate. We find that the spectral width of scattering resonance is dominated by a radiative damping caused by the Abraham-Lorentz force acting on a bound charge. The bound charge oscillator model provides not only an intuitive physical picture for the scattering of an optical slot antenna but also reasonable numerical agreements with rigorous calculations using the finite-difference time-domain method.

Strong visible magnetic resonance of size-controlled silicon-nanoblock metasurfaces

To extend the operating window of all-dielectric metamaterials into the visible regime, obtaining controllable magnetic resonance is essential. We experimentally demonstrated strong magnetic resonance at 595 nm using an array of amorphous silicon (a-Si) nanoblocks. The results of both theoretical calculations and experiments show that magnetic resonance can be tuned continuously by appropriately varying the size and thickness of a-Si nanoblocks. We also experimentally achieved a magnetic resonance Q-factor of ~10, which is a higher value than that yielded by a metallic split-ring resonator in the visible regime.

Tunable directional radiation of a dipole inside a cuboid slot on a dielectric substrate

Far-field directional radiation of a single dipole in a cuboid slot is investigated in the presence of a dielectric substrate. Due to strong near field coupling between the dipole source and the surfaces of the slot and the dielectric, the far-field radiation shows strongly anisotropic pattern depending on the dipole radiation energy. By tuning local resonances within the air-slot interface or the substrate-slot interface, highly directional radiation either to free space or to the substrate space can be obtained. In the visible spectrum ranging from 1.2 eV to 3.5 eV, up to 18 fold directivity can be obtained. The up-to-down ratio can be tuned from −7.5 dB to 10 dB. We identify induced eigenmodes responsible for highly unidirectional radiations as a function of the emitter spectrum and slot thickness to assess controllability of radiation power and direction.