If two photons are needed, the absorption response isn't linear. Instead, it scales with the square of the light intensity: if we double it, we can get four times the response. That helps confine the effect of a laser: If a laser beam is tightly focused, the exposure will be sharply confined to a small volume around the center or the focus. Then, to draw an arbitrary 3D shape, we need only move the focus around, by either moving the sample or the laser beam.