Abstract: A through silicon optical modulator alters a phase of a light beam which enters the back of a silicon die. The modulator can be formed as a PMOS transistor fabricated in an n-well, or can be an NMOS transistor having a negative gate to substrate voltage. By modulating the well voltage (or gate potential) the phase of a portion of the reflected light is altered. Two accumulation layers are selectively formed in the light path which is reflected from the transistor gate electrode. The phase change is detected to provide a signal from the integrated circuit having the through silicon optical modulator structure.