In 2PP, a high-intensity, femtosecond laser is focused onto a small volume of the photosensitive resin. The key aspect is that two photons must be absorbed simultaneously for the polymerization to occur. This absorption is non-linear, meaning it only happens at the focal point of the laser beam, allowing for high spatial resolution. The resin hardens at the focal point, and by moving the focal point in three dimensions, intricate 3D nanostructures can be created.
