Engineered Second-Order Nonlinearity in Silicon Nitride.

Silicon nitride (Si₃N₄) is a low-loss, CMOS-compatible material platform widely used in integrated photonics. Si₃N₄films deposited in the foundry, usually through low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD), are amorphous (therefore, centrosymmetric). Such symmetry leads to a lack of second-order nonlinearity (χ⁽²⁾) in Si₃N₄which keeps it from key active functions such as Pockels electro-optic (EO) modulation and efficient second harmonic generation (SHG). We overcome this drawback and demonstrate a successful induction of χ⁽²⁾ in Si₃N₄ through electrical poling with an externally-applied field to align the Si-N bonds.

This alignment breaks the centrosymmetry of Si₃N₄, and enables the bulk χ⁽²⁾. The sample is heated to over 500°C to facilitate the poling. The comparison between the EO responses of poled and non-poled Si₃N₄, measured using a Si₃N₄ micro-ring modulator, shows an enhancement in the amplitude of the measured EO responses as well as a remarkable improvement in its speed from 3GHz to at least 15GHz (3dB bandwidth) after the poling, which confirms the χ⁽²⁾ nature of the EO response induced by poling. This achievement paves the way for realizing high-speed EO modulation and all types of χ⁽²⁾-related functions on a pure Si₃N₄ platform, allowing monolithic integration of photonics and CMOS electronics for photonic integrated circuits (PICs), LIDAR, data communications, and quantum optics.

PAPER

Zhang, Yi, et al. “Engineered second-order nonlinearity in silicon nitride.” Optical Materials Express 13.1 (2023): 237-246. https://opg.optica.org/ome/fulltext.cfm?uri=ome-13-1-237&id=524458