Highly efficient silicon nitride grating couplers with metal back-reflector enabled by cryogenic deep silicon etching

The development of efficient coupling interfaces is a crucial requirement to fully harness the on-chip performance of photonic integrated circuits. Optical access is commonly implemented by either edge or vertical coupling, the latter one often relying on diffractive grating structures. At present, edge couplers are preferred when broadband and efficient operation is required, while grating couplers are mostly employed for wafer-scale and fast prototyping of photonic devices, at the expenses of a decreased efficiency. Here we present apodized surface grating couplers, characterized by a negative diffraction angle resulting in a self-imaging effect of the Gaussian-like diffracted beam and a metal back-reflector, which can successfully redirect upward all the optical power otherwise leaking into the substrate. Simulations predict that the effective combination of these two properties can open to the possibility of realizing fiber-to-chip interconnects that feature a coupling efficiency larger than 95%, together with pliant positioning on chip. As a proof of principle of our coupling approach, in our implementation we choose Si₃N₄ as photonic platform in which the grating coupler is fully etched, while the fabrication of the metal mirror is enabled by cryogenic deep silicon backside etching. Preliminary experimental results show a coupling efficiency of approximately -3 dB at an operating wavelength of λ≃1550 nm, almost doubling the efficiency of grating couplers on the Si substrate (≃ -5 dB). We anticipate that, by a proper optimization of the fabrication process, couplers with insertion loss smaller than 0.5 dB can be demonstrated.