Direct Laser Writing by 4Pi Multiphoton Polymerization

Multiphoton polymerization (MPP) is a versatile tool allowing fabrication of three-dimensional microstructures with a resolution of hundreds of nanometers for lateral resolution. However, axial resolution is at least 3 times larger than the transverse dimension. This anisotropy in horizontal and vertical resolutions limits the size and geometry of the structures. Focal spot having 1:1 aspect ratio could significantly enhance the final resolution of the fabricated structures for some applications (e.g. photonics) where the voxel axial resolution, and not the transversal feature size is the limiting factor. Currently, even by using high numerical aperture state-of-the-art objective the focused laser spot will be elongated in axial dimension forming ellipsoid voxel. In order to improve this aspect ratio, the more advanced focusing techniques are required. Inheriting from the microscopy, we employed 4Pi excitation, which requires two opposing high numerical aperture objectives to focus two counter-propagating laser beams such that the foci spatially and temporally coincide. In such a manner, the two opposing focused laser beams allow to produce an intensity peak whose axial dimension is several times smaller than the values obtained with a focusing from one side only. Using 4Pi excitation with low numerical aperture objectives creates a focal spot with many interference fringes in axial direction.