Silicon photonics
Silicon photonics sometimes combines silicon nitride (SiN) waveguides with silicon waveguides. Which correctly describes SiN waveguides?
- AWith a refractive index of about 2.0, their contrast with the surroundings (SiO2) is smaller than silicon's, making it easier to reduce propagation loss and temperature-induced changes in characteristics
- BTheir refractive index is higher than silicon's, so they can be bent sharply at smaller radii, making circuits even more compact
- CThey conduct electricity well, so fast modulators using the carrier dispersion effect can easily be built
Show answer and explanation
Answer: A. With a refractive index of about 2.0, their contrast with the surroundings (SiO2) is smaller than silicon's, making it easier to reduce propagation loss and temperature-induced changes in characteristics
SiN has a refractive index of about 2.0, and its contrast with SiO2 is smaller than that of silicon (about 3.5), so light confinement is weaker and bend radii are larger. In exchange, scattering from sidewall roughness is small, making it easy to build low-loss waveguides. Its thermo-optic coefficient is also small—a fraction of silicon's or less—so it suits filters that are robust to temperature changes. Its large band gap also means two-photon absorption is unlikely even at high optical power. On the other hand, SiN is an insulator, so modulators using the carrier dispersion effect cannot be built from it.