# Integrated Optical Phased Arrays for Beam Forming and Steering
## 5 Material Platforms
### Si Photonics Platforms
#### 5.1.3. OPAs Based on the Si/Si\({}_{3}\)N\({}_{4}\) Platform  
Structures based on the Si/Si\({}_{3}\)N\({}_{4}\) platform, which is also named Si\({}_{3}\)N\({}_{4}\) overlay grating [109], Si\({}_{3}\)N\({}_{4}\) assisted grating [90], and Si/Si\({}_{3}\)N\({}_{4}\) dual-layer structure [115], combine the advantages of Si and Si\({}_{3}\)N\({}_{4}\), and enable millimeter-scale WGAs with weak perturbations and uniform emission for the realization of OPAs with large apertures and narrow beamwidths. The WGA performance sensitivity to the shallow etched depth is mitigated on the Si/Si\({}_{3}\)N\({}_{4}\) platform compared to pure Si, thanks to the reduced refractive index of Si\({}_{3}\)N\({}_{4}\). It also allows the operation aperture of the OPAs to be extended [65; 109]. In addition, a Si/Si\({}_{3}\)N\({}_{4}\) WGA with a nano-scale Al\({}_{2}\)O\({}_{3}\) layer between the Si\({}_{3}\)N\({}_{4}\) and Si, which serves as an etch-stop layer, is robust to fabrication variations [109].  
The millimeter-long aperiodic Si/Si\({}_{3}\)N\({}_{4}\) WGA presented in Figure 8e simultaneously enabled a diffraction-limited beamwidth of 0.089\({}^{\circ}\) and a wide FOV [109]. Light is highly confined in the Si input waveguide and the mode is weakly perturbed by a deposited Si\({}_{3}\)N\({}_{4}\) layer. The millimeter-long WGA was robust to fabrication variations [109]. In another work, over 1-mm-long Si/Si\({}_{3}\)N\({}_{4}\) aperiodic WGA enabled uniform emission and a narrow divergence [66]. Aperiodic WGA of even 1-cm-length has been realized, with a metal mirror underneath to achieve unidirectional radiation, and a diffraction-limited beamwidth of 0.02\({}^{\circ}\) was demonstrated [127]. A fabricated 5 mm-long OPA with 60 nm partially etched Si\({}_{3}\)N\({}_{4}\) on the Si waveguide enabled crosstalk below \(-\)12 dB. An FOV of 66\({}^{\circ}\)\(\times\) 3.3\({}^{\circ}\) was achieved via phase modulations and wavelength tuning [90]. Broadband high efficiency of more than 86% from 1.5 to 1.6 \(\upmu\)m has been achieved in another Si/Si\({}_{3}\)N\({}_{4}\) dual-layer WGA [115].  
As mentioned above, a Si-based OPA chip cannot handle high power due to the strong nonlinear absorption effect [88], which results in relatively low emission power of the OPA. This issue can be partly solved by using the combined Si/Si\({}_{3}\)N\({}_{4}\) platform, where extremely low-loss Si\({}_{3}\)N\({}_{4}\) waveguides function as the power carrier and splitter and Si WGAs are used as emitters. Near lossless vertical transition between a Si\({}_{3}\)N\({}_{4}\) layer and a Si layer was achieved by a pair of adiabatic linear tapers [88; 89]. A 32-channel Si/Si\({}_{3}\)N\({}_{4}\) dual-layer OPA presented in Figure 11b enabled an FOV of 96\({}^{\circ}\)\(\times\) 14\({}^{\circ}\) and a beamwidth of 2.3\({}^{\circ}\)\(\times\) 2.8\({}^{\circ}\). A pair of 100-\(\upmu\)m-long adiabatic linear tapers functioned as a vertical coupler on the WGA-based OPA chip [88]. A similar 64-channel OPA enabled a beamwidth of 0.4\({}^{\circ}\)\(\times\) 0.4\({}^{\circ}\)\(\times\) 0.4\({}^{\circ}\)[89]. The Si\({}_{3}\)N\({}_{4}\)/Si double-layer structure can also be used to implement EF OPAs.  
Si\({}_{3}\)N\({}_{4}\)-on-Si WGAs are promising for highly desirable large-scale OPAs and for high-power applications. Compared with Si OPAs, devices based on the Si/Si\({}_{3}\)N\({}_{4}\) platform need additional deposition and etch processes. High-efficient vertical couplers between the double layers should be designed for the Si/Si\({}_{3}\)N\({}_{4}\) platform-based OPAs.