Inside a barium titanate crystal, there's a built-in electric field, like a permanent magnet, but for charges. When a flash of light hits it, electrons are knocked loose and start darting around, colliding with atoms. But the crystal isn't the same in all directions: it has an axis along which the field is strongest.

Scientists aimed a laser pulse at the crystal and simultaneously tracked two things: how the atomic lattice vibrates (ultrafast electron diffraction) and where the charges move (field electron microscopy). They found that when the light oscillated along that special axis, electrons transferred energy to the lattice vibrations twice as fast as with perpendicular polarization.

Then, the electrons and holes (the empty spots where electrons used to be) separated by the light run apart under the built-in field, like a couple being pulled in opposite directions on a dance floor. This creates a voltage that can be harvested. This stepwise path, from photon to lattice vibrations to charge separation, explains why barium titanate is so good at converting light into electricity.

So in this crystal, light works like a courier who first pushes through the crowd (the atoms), then guides couples (charges) to different exits. And if the courier takes the main street (along the field), the crowd parts faster.