# Integrated Optical Phased Arrays for Beam Forming and Steering
## 5 Material Platforms
### Si Photonics Platforms
#### 5.1.2 OPAs Based on the Si\({}_{3}\)N\({}_{4}\) Platform  
The demonstrated OPAs on SOI platform are mainly limited to applications in the NIR spectral range due to the material transparency window. The Si platform offers a high refractive index and active functionalities, such as TO phase shifters and photodiodes. However, owing to the high index contrast, Si devices suffer from significant scattering losses from surface roughness [121]. Additionally, it is challenging to achieve highly desirable large apertures with Si WGA-based OPAs. A variation of just 1 nm in the etch depth of a Si/SiO\({}_{2}\) grating increases more than 11 percent of the grating strength [109]. The lengths of Si WGAs to emission light are typically shorter than a few hundred micrometers [66]. Furthermore, Si devices suffer from strong nonlinear absorption effects in the telecommunication wavelength ranges, such as two-photon absorption, free carrier absorption, and large lowest-order nonlinear effects [119]. The nonlinearity-induced phase changes and material loss limit the scalability and output power of Si-based OPAs [65]. As a vital alternative to its Si counterpart, Si\({}_{3}\)N\({}_{4}\) is transparent from visible to MIR [124]. It is suitable for non-telecommunication applications such as augmented reality and virtual reality displays, quantum information processing, biological sensing, and biological simulations. Additionally, the lower index contrast between a Si\({}_{3}\)N\({}_{4}\) waveguide and its SiO\({}_{2}\) cladding reduces the waveguide losses due to the sidewall roughness scattering and dispersion, and lowered the sensitivity of device performance to variations in waveguide dimensions [119, 125]. Si\({}_{3}\)N\({}_{4}\) has a superior passive optical property. It is CMOS-compatible and more phase stable due to its five times smaller TO coefficient compared to Si [119]. Si\({}_{3}\)N\({}_{4}\) is spotlighted as a prominent substitute for Si without two-photon absorption and free-carrier absorption in the telecommunication wavelength range [119, 125]. The lowest-order nonlinear susceptibility of Si\({}_{3}\)N\({}_{4}\) is about 20 times smaller than that of Si [119], which allows for the low-loss propagation of high-power guided modes, making it an ideal material for large-scale PICs [65]. Together with the moderate refractive index, Si\({}_{3}\)N\({}_{4}\)-based OPA allows for further scaling up of the aperture size [65]. Owning to the aforementioned merits of Si\({}_{3}\)N\({}_{4}\), various integrated OPAs have been demonstrated on the Si\({}_{3}\)N\({}_{4}\) platform [41, 62, 65, 71, 93, 108]. A passive large-scale Si\({}_{3}\)N\({}_{4}\) OPA with a large aperture size of 4 \(\times\) 4 mm\({}^{2}\) was demonstrated at the wavelength of 1.55 \(\upmu\)m. A record small beamwidth of 0.021\({}^{\circ}\)\(\times\) 0.021\({}^{\circ}\) and a sidelobe suppression ratio of 10 dB were achieved by this OPA [65]. The first OPA in the visible range (635 nm) with a large aperture of 0.5 \(\times\) 0.5 mm\({}^{2}\) and a beamwidth of 0.064\({}^{\circ}\)\(\times\) 0.074\({}^{\circ}\) was demonstrated [65]. A sparse aperiodic Si\({}_{3}\)N\({}_{4}\) OPA was designed to mitigate the fabrication constraints at a short wavelength of 488 nm. An FOV of 50\({}^{\circ}\) and a beamwidth of 0.17\({}^{\circ}\) were achieved by the blue light OPA [41]. A 16-channel Si\({}_{3}\)N\({}_{4}\) OPA exhibited an FOV of \(\sim\)12\({}^{\circ}\) and a beamwidth of \(\sim\)1.1\({}^{\circ}\) at 1.064 \(\upmu\)m [93]. A dual-layer millimeter-scale Si\({}_{3}\)N\({}_{4}\) WGA-based OPA with unidirectional emission was achieved with extremely high upward radiation efficiency of 93% at 1.55 \(\upmu\)m [108]. The aperiodic antenna presented uniform emission on a millimeter scale for the first time, allow for a highly effective aperture with the elimination of blind spots [108].