The change in refractive index and optical absorption of a semiconductor when its free electron or hole concentration changes. Because silicon has almost no Pockels effect, this effect is used to shift phase in silicon modulators. Increasing the carrier concentration lowers the refractive index.
Japanese name: キャリア分散効果(プラズマ分散効果)
Plasma dispersion effect appears in 1 practice question across 1 topic, including Silicon photonics.
Field: Silicon photonics
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.
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