Silicon-rich silicon nitride thin films for subwavelength grating metalens

In this work, high performance subwavelength grating metalens is demonstrated with high-refractive-index silicon-rich silicon nitride material compatible with CMOS fabrication processes. Conventional metalens materials namely TiO₂ and GaN require expensive and time-consuming deposition processes such as atomic layer deposition (ALD) and metal-organic chemical vapor deposition (MOCVD). In order to improve the cost efficiency of metalens and its performance, the trade-offs between refractive index, fabrication difficulty and metalens performance is studied. We propose a feasible approach that is silicon-rich nitride (SiNx) as metalens material, which balance the trade-offs between refractive index and fabrication difficulty to large extent. With the advantage of ultra-high refractive index SiNx (n = 2.74) at 685 nm incidence, we are able to shrink the pitch size to unprecedented 220 nm. A propagation-phase-based grating metalens is fabricated and characterized for proof of concept. In addition, the optical parameters (n & k) can easily be adjusted through the deposition process. Our work has also promised a new degree of freedom for future optimization of metalens.