Notes:

Imagine a series of discrete compartments, or cells, which are filled with a material that has a refractive index that can be controlled by applying a voltage across the cell. By setting the refractive index of the cells near the center to be higher than the cells near the edges, we can cause this array of cells to act like an ordinary lens and reshape the wavefront to focus a plane wave of light into a spot.

Of course the great advantage of this kind of lens is that it is completely general - just by re-programming the voltages that control each cell independently we can reshape the wavefront into anything that we like - at least in principle!