An optical device includes first and second planar substrates having respective first and second faces and including first and second diffractive structure disposed respectively on the first and second faces. First and second planar electrodes are disposed respectively on the first and second faces. A mount holds the second planar substrate parallel to the first planar substrate, with the second face adjacent to the first face and with the first and second planar electrodes in mutual proximity, while permitting the second planar substrate to move transversely relative to the first planar substrate. A control circuit is coupled to apply an electrical potential between the first and second planar electrodes with a voltage sufficient to shift the second diffractive structure transversely relative to the first diffractive structure.