| Summary: | 碩士 === 國立中央大學 === 光電科學與工程學系 === 105 === In this research, novel metallic absorbers with multi-sized nanohexagons arranged peri-
odically in metal-insulator-metal (MIM) configuration for broadband optical absorption
and plasmon-enhanced photoelectric conversion at visible frequencies are numerically and
experimentally explored. The resonance wavelength (λres) of a single gold (Au) hexagon
supported by silica film on Au or silver (Ag) bottom layer shifts approximately linearly
towards longer wavelengths in the reflectance spectrum with an increasing side length.
Broadband absorption for about λ0 < 500 nm in nanostructured Au-SiO2-Au absorbers
is mainly due to material absorption of the Au bottom layer. The design is shown to be
polarization insensitive at normal incidence and the absorptance spectrum (300 nm-1100
nm) is nearly independent of the incident angle up to about 40◦. 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 λ0 =
[400, 700] nm. In addition, the absorptance within top Au hexagons is enhanced up to
2.5 times for λ0 = [400, 550] nm with the Ag bottom layer mainly because of reflected
wave-enhanced localized surface plasmons and gap plasmon resonances.
In the multilayered aluminum (Al)-titanium oxide (TiO2)-Ag absorbers, plasmon-
enhanced photoelectric conversion is found to achieved mainly at edges and side walls of
the hexagons and also in the mid Al layer between hexagons. The simulated bandwidth
for the absorptance greater than 60% in the fabricated sub-optimum structure is about
293 nm for the TM wave at visible wavelengths. The optimum Al-TiO2-Ag absorbers with
nanohexagons is fabricated using electron beam lithography and the external quantum
efficiency (EQE) is measured. The calculated EQE and responsivity from the measured
photocurrent of the fabricated device is 0.0568% and 0.2899 mA/W at λ0 = 633 nm under
the bias of 0.5 V.
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