SOI photodiode with surface plasmon antenna: From sensitivity enhancement to refractive index measurement for biosensing

Abstract

In order to enhance the performance and functionality of the pn-junction photodiode in silicon on insulator (SOI), metallic line-and-space (L/S) surface plasmon (SP) antenna is introduced. In case of the 100-nm-thick silicon, external quantum efficiency (QE) of the photodiode can be increased by nearly an order of magnitude. The antenna shows wavelength and polarization selectivities, and incident angle dependence, and the peak wavelength and the polarization angle can be tailored only by changing the layout design. This is especially beneficial for integrating a variety of photodiodes with different characteristics in a single chip. Wavelengths of the QE peaks for various L/S pitches and light incident angles were examined in detail, and it was suggested that the coupling between the diffracted light from the L/S and the propagation modes in the SOI slab waveguide caused the QE enhancement. Related to this operation mechanism, it was found that the photodiode could detect the refractive index of the medium around the SP antenna as the shift of the peak wavelength when the incident light was tilted. This may lead to the fluorescence-label-free biosensing, featuring simple optics and high throughput attained by the parallelism with a large number of integrated photodiodes. Consequently, a wider application of this photodiode can be expected along with the evolution of the SOI-based large-scale integrated circuits (LSIs).