Investigation and Improvement of 90 $^{\circ}$ Direct Bends of Metal–Insulator–Silicon–Insulator–Metal Waveguides

We investigate 90° direct bends of metal-insulator-silicon-insulator-metal (MISIM) waveguides, which are hybrid plasmonic waveguides with replaceable insulators. First, we fabricate them using fully standard CMOS technology and characterize them. The experimental excess loss of the two consecutive 90° direct bends is 11, 7.4, and 4.5 dB when the width of the Si line of the MISIM waveguide is about 160, 190, and 220 nm, respectively. Second, we analyze the experimental results using the 3-D finite-difference time-domain method. Through the analysis, we investigate possible loss mechanisms of the 90° direct bend, which have not been studied to our knowledge. It has been found that the Si lines should be narrow to reduce the excess losses of the 90° direct bends. However, the wide Si lines are better for ease of fabrication and for small propagation losses. Finally, we demonstrate a modified low-loss 90° direct bend of the MISIM waveguide with a wide Si line.