An optical device (20) includes an electro-optical layer, including a liquid crystal material (24) with a heliconical structure having a pitch that is less than 250 nm and is modifiable by an electric field. An array of excitation electrodes (28) extends over the electro-optical layer. Control circuitry (23) is coupled to apply control voltage waveforms to the excitation electrodes and is configured to modify the control voltage waveforms so as to locally modify a molecule director angle of the heliconical structure and thus to generate a specified phase modulation profile in the electro-optical layer.