A quantum well infrared photodetector and modulator are monolithically integrated to one another. The infrared scene impinges on an infrared receiving element such as a phase grating, which couples it to a modulator. The modulator includes asymmetric quantum wells which are shifted by an applied bias. By appropriately adjusting the bias, the modulator can pass a maximum amount of the input information or can pass less. The photodetector and modulator both use the same kinds of materials, preferably GaAs and Al.sub.x Ga.sub.1-x As, so they essentially become a single unit, obtaining better noise and operating characteristics from the combination.