張殷榮

n this research, modified absorbers in metal-insulator-metal (MIM) configurations for broadband radiation absorption and plasmon-enhanced photoelectric conversion at visible frequencies are explored. We first studied the resonance wavelength (res) of a single gold (Au) hexagon supported by the SiO2-Au and SiO2-silver (Ag) configurations as a function of side length of the Au hexagon. The optical resonance properties of the MIM structures are obtained from the 3-D finite-difference time-domain (3-D FDTD) simulations. We identified that the res of the single Au hexagon in the MIM configuration is shifting linearly towards the longer wavelengths side (> 700 nm) of the reflectance spectrum with increasing side length. We also observed the broadband absorption for wavelength < 500 nm in our Au-SiO2-Au absorber model is mainly due to the material absorption of Au bottom layer. The wavelength-averaged total absorptance of the Au-SiO2-Au and Au-SiO2-Ag absorbers are about 91.63% and 82.31%, respectively, for transverse magnetic (TM) wave at normal incidence for 0 = 400 nm-700 nm. Moreover, the insensitivity of our absorber models are evaluated by the simulated absorptance spectra as a function of polarization and incidence angles. Besides, we discovered that simply changing the Ag bottom in our broadband absorber model, the absorptance within the top Au hexagons is enhanced up to 2.5 times at wavelengths 0 = 400 nm-550 nm. Moreover, a multilayered aluminum (Al)-titanium oxide (TiO2)-Ag structure is systematically optimized using 3-D FDTD simulations for plasmon-enhanced photoelectric conversion at visible frequencies. The bandwidth of absorptance greater than 70% in the optimized structure is about 232 nm for TM wave at visible wavelengths. The optimum Al-TiO2-Ag device evaluated in terms of low reflectance for the photoelectric conversion is fabricated and the photon to electron conversion efficiency is measured. The wavelength and measurement averaged quantum efficiency (QE) of the fabricated two devices are 7.616% and 1.312%.