Silicon photonics
Why is the carrier dispersion effect (plasma dispersion effect) used to change the refractive index with an electrical signal in silicon optical modulators?
- ASilicon is a direct-gap material, so injecting carriers makes it emit strongly, and that light changes the refractive index
- BChanging the electron or hole concentration does not change the refractive index, only absorption, which makes it easy to handle
- CSilicon's crystal is centrosymmetric, so the Pockels effect, in which an electric field directly changes the refractive index, is almost absent
Show answer and explanation
Answer: C. Silicon's crystal is centrosymmetric, so the Pockels effect, in which an electric field directly changes the refractive index, is almost absent
In crystals such as lithium niobate, fast modulators can be built using the Pockels effect, in which the refractive index changes in proportion to the electric field, but silicon's centrosymmetric crystal structure gives it almost no such effect. Instead, the carrier dispersion effect is used, in which changing the concentration of electrons and holes (free carriers) changes the refractive index and absorption. Increasing the carrier concentration lowers the refractive index and increases absorption at the same time. Because the index change is tiny (about 10⁻³ or less), modulators are made a few mm long to accumulate enough phase change. Silicon is an indirect-gap material.