Two-photon lithography (TPL) Figure 1. is a 3D printing technique for fabricating microscopic structures with high resolution features. Three dimensional structures are fabricated by moving a tightly focused laser beam inside a photoactive material (polymer, inorganic or hybrid materials). The laser beam initiates chemical reactions inside the photoactive sample to give a solidified microstructure. The structure to be fabricated is designed using a suitable 3D graphic software ( CAD design). In the next step the CAD model is mathematically sliced up into a group of 2D planes. The resulting 2D data is then used for layer by layer construction of the 3D structure as seen in Figure 2.
Components constituting Two-Photon Lithography Setup
The basic setup of two-photon lithography (TPL) system seen in Figure 1 can be broadly divided into three groups.
(i) Optics : Parts 1-4 in Figure 1. This group consists of a pulsed laser source. TPL system normally employs femto-second or nano-second pulsed lasers in TPL system. Pulsed lasers are capable of delivering very high powers for very short period of times. Typically the laser powers amount to few milli Watts to few tens of milli Watts (mW). Then there are optics containing lenses, beam-splitters, modulators etc. aimed at delivering light to the samples. The sample is oriented above a high numerical aperture lens capable of tightly focusing the laser on to the sample.
(ii) Mechanics : Part 5 in Figure 1. This group is crucial in manipulation of the samples along 3 dimensions. Usually mechanics is a combination of mechanical stages and piezo stages. Mechanical stages are lower resolution mechanical movements capable of moving the sample along the XY plane. Piezo stages are highly precise moving parts capable of delivering movements along X Y and Z axes at sub-nanometer resolutions.
(ii) Materials/Sample: Parts 5-6 in Figure 1. In classical TPL a thin transparent substrate a couple of hundred micrometers thick (~0.2 mm) is used. The sample is then dropped or spin coated on the substrate. The substrate can be a thin cover glass or plastic film. The laser is focused through the substrate into the sample. The microstructure is fabricated such that it is fixed on the surface of the substrate. The interface between the sample and the substrate should be identified accurately before the start of the fabrication. This step cannot be emphasized more because if the polymerization at the substrate-sample interface is not strong the whole fabricated structure can be washed off during development step.