On-chip Ge-based refractive index sensor based on multislot Ge waveguide

On-chip optical sensors have been attracting significant attention of the researchers due to their portability, low cost, and in-situ detection compatibility. The realization of these compact and mass-producible sensors will be possible if all required optical and electronic components are fully integrated on the same chip. However, the main obstacle here is the lack of the room-temperature on-chip light source; therefore, an external source has been utilized for optical sensing applications. Our work introduces a germanium-based optical sensing structure, specifically designed to operate at low refractive indices, with all required optical components integrated on silicon in a CMOS-compatible manner. The structure features germanium laser cavity as light source, a germanium waveguide to laterally direct lasing output, and a multiple slot waveguide to function as the sensing region. 3-dimensional (3D) finite difference time domain (FDTD) simulations are carried out to estimate waveguide sensitivities, effective indices of the optical modes and the transmission spectra of the lasing output through the sensing region as a function of the refractive index of the sample varying in the range of 1 – 1.032. Our results demonstrate 27% difference in the intensity of the lasing output transmitted to the exit of the three parallel slots when the refractive index of the sensing region is increased from 1 to 1.032. Furthermore, we analyze the potential feasibility of our proposed design for gas sensing applications since the studied refractive index range covers refractive indices of several different gases. A figure of merit (FOM) of 11 RIU-1 has been achieved through this design within the refractive index range of 1 to 1.0015. Moreover, future directions are outlined to enhance the FOM, paving the way for the potential development of fully integrated gas sensors with embedded light source.