Two-photon lithography (2PL) is a powerful tool for microfabrication, capable of fabricating structures with sub-100 nm feature sizes. 2PL uses two photons to create a three-dimensional image of a target in a photosensitive material. The resulting image can be used to etch features into the target material, typically using focused ion beam milling or electron beam writing.
2PL was first demonstrated in the early 1990s, and has since been used to fabricate a variety of nanostructures including nanoparticles, nanowires, and quantum dots. 2PL is particularly well suited for fabricating photonic crystals and other periodic structures. In addition, 2PL can be used to create complex 3D shapes such as cavities and channels.
2PL is typically performed using an ultraviolet (UV) laser operating at 400-500 nm wavelength. The UV light is passed through a mask which contains the desired pattern. The light then interacts with a photosensitive material, typically containing molecules that fluoresce when exposed to UV light. The molecules absorb two photons from the UV light, which causes them to emit visible light. This process is called two-photon absorption (TPA).
The fluorescent molecules convert the energy of the absorbed photons into heat, which locally raises the temperature of thephotosensitive material above its glass transition temperature (Tg). This allows the polymer to flow and fill in any voids in the desired pattern. After cooling below Tg, the polymer solidifies and retains its new shape.
The main advantage of 2PL over traditional photolithography is that it allows for much smaller feature sizes to be fabricated. This is due to the fact that TPA only occurs when two photons are absorbed simultaneously by a molecule. As such, there must be a very high density of photon pairs present in order for TPA to occur frequently enough to produce an image with sufficient contrast. For this reason, 2PL generally requires shorter exposure times than traditional photolithography (on the order of seconds or less).
In addition, because 2PL relies on nonlinear processes for imaging, it can be used to create patterns that would be difficult or impossibleto generate using traditional photolithography alone