High intensity lights. Preferably direct sunlight outside. If inside keep them right under lights. Almost touching the lights otherwise they will stretch.
Discussion
It’s amazing that even with high intensity high pressure sodium or metal halide 1000W lamps, the intensity on the surface of your leaves roughly halves with every 12” of distance. Compare that to moving your hand 12” further from the sun, and it gives you some idea of the power of the sun.