Attenuation and Photodetection of Sub-Bandgap Slow Light in Silicon-on-Insulator Photonic Crystal Waveguides

Abstract

<p>A glass-clad, slow-light photonic-crystal waveguide is proposed as a solution to sub-bandgap light detection in silicon photonic circuits. Such detection in silicon is perceived as a challenge owing to silicon's indirect band gap and transparency to 1550nm wavelengths, yet is essential for achieving low-cost, high-yield integration with today's microelectronics industry. Photonic crystals can be engineered in such a way as to enhance light-matter interaction over a specific bandwidth via the reduction of the group velocity of the propagating wave (i.e. the slowing of light). The interaction enhanced for light detection in the present work is electron-hole pair generation at defect sites. The intrinsic electric field of a p-i-n junction enables light detection by separating the electron-hole pairs as a form of measurable current. The photonic-crystal waveguides are designed to have bandwidths in the proximity of a wavelength of 1550nm. Refractive indices of over 80 near the photonic-crystal waveguide's Brillouin zone boundary are measured using Fourier transform spectral interferometry and are found to correspond to numerical simulations. Defect-induced propagation loss was seen to scale with group index, from 400dB/cm at a group index of 8 to 1200dB/cm at a group index of 88. Scaling was sublinear, which is believed to be due to the spreading of modal volume at large group index values. Photodetectors were measured to have responsivities as high as 34mA/W near the photonic-crystal waveguide's Brillouin zone boundary for a reverse bias of 20V and a remarkably short detector length of 80um. The fabrication of each device is fully CMOS-compatible for the sake of cost-effective integration with silicon microelectronics.</p>