Benefiting from its compatibility with the complementary-metaloxide-semiconductor (CMOS) technology and low propagation loss, silicon nitride (Si3N4) is becoming a promising material for integrated photonics applications. Due to their wide transparency range, silicon nitride waveguides have been used for large-scale phased arrays at both infrared and visible wavelengths. In addition, both multilayer silicon-nitride on silicon, and silicon on silicon-nitride platforms are being investigated for low loss waveguiding. Other applications based on the Si3N4 platform include opto-mechanics, nonlinear optics enabled by its low nonlinear loss and on-chip spectroscopic sensing, to name a few. Grating couplers have been widely used for coupling between single mode optical fibers and high index contrast waveguides, because they offer high coupling efficiency and can be placed at specific surface locations enabling wafer-scale testing. Surface grating couplers with high coupling efficiencies (about 1 dB) have been demonstrated in the silicon-on-insulator (SOI) platform by using specific strategies including amplitude apodization, refractive index engineering by subwavelength structures, high-directionality blazing concept by interleaved trenches , etc. However, it is challenging to realize high efficiency silicon nitride grating couplers, because the comparatively low index contrast results in low grating strength and directionality (defined as the fraction of the total diffracted power that is radiated upwards). The coupling efficiency is typically limited to about 40-50% (3-4 dB loss).
In this work we demosntrate experimental results of a silicon nitride apodized-imaging grating coupler with a record coupling efficiency of -1.5dB and a 3dB bandwidth of 60nm in the C-band.
The SIN platform development was initiated within the EPSRC first grant EP/K02423X/1 HERMES: High dEnsity Silicon GeRManium intEgrated photonicS. The platform is currently developped further through collaboration under the EPSRC : A Platform Grant EP/N013247/1: Electronic-Photonic Convergence, EP/L021129/1 CORNERSTONE: Capability for OptoelectRoNics, mEtamateRialS, nanoTechnOlogy aNd sEnsing and European project H2020 COSMICC,