Apparatus and techniques are presented such as can be used for electro-optic modulation and detection or other applications. For example, an optical metal grating is disposed on a thin metal film to couple light from broadside to the metal film as surface plasmon-polariton waves; below the metal film is located a thin insulating layer and a doped semiconductor region forming a metal-insulator-semiconductor structure. The device can be configured to operate as a reflection or transmission modulator, or as a photodetector, for example. Modulating the voltage applied to the metal-insulator-semiconductor structure modulates the carrier concentration in the semiconductor near the insulating layer, which modulates the refractive index of the semiconductor in this region, thus modulating the coupling efficiency to the surface plasmon-polaritons, thus modulating the reflectance and transmittance of the device. Modulated incident light produces a modulated photocurrent under bias which may be detected using electronics.