With the rise of autonomous vehicles[1–3] and unmanned drones[4], light detection and ranging (lidar) has become an indispensable device for them. So far, the mechanical lidar[5] is still one of the most mature solutions, but its high cost and difficult assembly have plagued researchers. Besides, the short detection distance limits the wide application of flash lidars[6]. The integrated on-chip silicon (Si) optical phased array (OPA)[7–11], as an advanced solid-state beam steering device, can overcome the above defects and has gained significant interest for its energy saving and miniaturization. In application, the grating-emitter-based OPA is considered a viable candidate to achieve two-dimensional (2D) optical steering, i.e., phase steering in one direction and wavelength steering in the other direction. For phase steering, the OPA with a pitch close to a half-wavelength along the lateral direction is constructed to realize a wide beam steering range with low crosstalk, which is usually built upon metamaterial waveguides[12,13], corrugated waveguides[14], nano-structured Si waveguide arrays[15], etc. Notably, an OPA with a phase mismatched unequal width waveguide distribution has been applied, implementing a steering range of 110° and a maximum peak power of 720 mW[16]. Wavelength steering is often enabled by the grating dispersion when the wavelength of the input laser light is scanned. A macroscopic emitting aperture with its size of is typically required for ranging distances of interest for autonomous vehicles[17] because a larger aperture would generally enable a narrower width of the main beam lobe and thus a high angular imaging resolution. The Si waveguide grating with shallow etching is one possible approach to realizing long-length emitting for its weak emission rate[18]. However, in practice, they are challenging to fabricate. A more promising approach of integrating a Si nitride () overlayer on a Si waveguide has gained significant interest because of its low nonlinearity, broad transparency range, low propagation loss, and low index contrast characteristics[4]. On this basis, several surface gratings with novel structures, such as strip-line grating[19] and fishbone grating[20], have been realized in the modulation of the emission profile and the improvement of radiation beam quality. In addition, the downward radiant power of the grating always introduces destructive interference, which reduces the performance of the grating emitter. An effective approach of dual-layer grating misalignment has been proposed, consequently achieving more than 95% unidirectional radiation[21].