An optical modulator that modulates light through the back side of a flip chip packaged integrated circuit die. In one embodiment, an optical modulator includes a p-n junction having a side wall that is substantially vertical or perpendicular relative to a surface of the integrated circuit die. A charged region is generated at the p-n junction and is modulated in response to an electrical signal of the integrated circuit die. An optical beam is directed through the back side, of the semiconductor substrate and through the charged region along the side wall p-n junction. The optical beam is deflected off a deflector back through the charged region along the side wall back out the back side. In one embodiment, the side wall p-n junction is provided with a metal oxide semiconductor (MOS) gate structure. In another embodiment, the side wall p-n junction is provided by an n- (or p-) well in a p- (or n-) epitaxy layer of the semiconductor substrate. In one embodiment, the well is a well ring structure. In another embodiment, there are a plurality of wells periodically located in the epitaxy layer of the semiconductor substrate. In one embodiment, the well or plurality of wells are surrounded with an optical beam confinement structure.