Study on Functional Devices Using Ultrathin Metal Nanostrips

Department of Electrical EngineeringInfrared (IR) wireless communications has been widely studied due to advantages such as a large bandwidth and immunity to electromagnetic interference. Directly modulated light emitting diodes (LEDs) are usually used as a light source. In this case, however, the nature of LEDs limits modulation speed. In this thesis, an external modulator based on an ultrathin silver (Ag) film and an Ag strip array is theoretically investigated to solve the modulation speed limit problem. The investigated modulator consists of an ultrathin Ag film, a hafnium oxide (HfO2) layer, and a periodic array of a stack of an indium-tin-oxide (ITO) strip and an ultrathin Ag strip. 850 nm free-space beam incident on the array generates localized surface plasmon resonance (LSPR) around the Ag strip. The LSPR results in a rejection band in the transmission spectrum of the modulator. The Ag film, the HfO2 layer, and the ITO strip comprise a sort of metal-oxide-semiconductor (MOS) structure. By using the MOS structure, the optical properties of the ITO strip are electrically controlled, and the LSPR is tuned in consequence. In this way, the transmission of the modulator is modulated at a wavelength of 850 nm. The optical characteristics of the modulator are analyzed with respect to its geometrical parameters by using the finite difference time domain (FDTD) method. For the analysis, the optical properties of the ITO strip are obtained by analyzing electrically the MOS structure and using the Drude model. When a bias voltage of 46 V is applied between the Ag film and the Ag strip, the transmission at a wavelength of 850 nm is increased by 200 %. A preliminary experiment for realization of the modulator is carried out. E-beam lithography and lift-off processes are established. By using the processes, subtractive color filters based on an array of thin gold strips are fabricated and characterized. The experimental results show good agreement with the simulation results. The processes will be used to realize the modulator.ope