Guilin Sun
Ansys Employee

FDTD solves Maxwell's Equations which do not have DC voltage incolved. In order to count for the index change at certain voltage, you will need to do a CHARGE simulation if the material is semiconductor. Once the charge distribution is obtained, it can be converted to index change and applied to the material. Please refer some online examples, such as

Charge distribution to change in refractive index theory

Traveling Wave Mach-Zehnder Modulator