Various embodiments of the present invention are directed to external, electronically controllable modulators. In one embodiment, a modulating device (100,400) includes a first electrode (104,404), a second electrode (106,406), and an active region (102,402). The active region is configured so that at least a portion of the active region is disposed between the first electrode and the second electrode. Applying a voltage of an appropriate magnitude and polarity to the electrodes changes the conductivity of the active region which in turn shifts the phase and/or amplitude of electromagnetic radiation transmitted through the active region.