The research of NIR absorption of plasmonic enhanced composite microstructure

Sub-wavelength microstructure optical devices based on silicon grating structure have unique spectral characteristics, which can produce the enhanced absorption performance of visible wide band. According to the extremely low absorption in NIR wide band
$0.78-2.5 \mu \mathrm{m}$
, this paper proposes two composite microstructures which contain silicon grating and Ag nano-sphere or nano-pit based on surface plasmon resonance to produce optical absorption enhancement. The simulation results based on Finite Difference Time Domain method show that When the Ag nano-sphere and the Ag nano-pit are added to the silicon column micro-structure, the combined microstructure is all above 9.3% in NIR band, and the highest absorption rate can achieve 95.6% theoretically. The layer Al
2
O
3
is increased in the grating surface according to the equivalent medium theory, and that result in the absorption efficiency of both composite microstructures is improved in NIR band. In conclusion, this paper provides a new method to enhance optical absorption in photodetectors, solar cells, optical communication, biomedicine and other fields.