Axial resolution improvement in multiphoton polymerization by 4Pi excitation

A spherical diffraction limited voxel is highly desirable in multiphoton polymerization (MPP) technology, especially for photonic applications. However, currently an elliptic shape voxel having 1:3 or larger aspect ratio is typical in MPP. In order to improve this ratio the more advanced focusing techniques are required. Inheriting from the microscopy, we employed two opposing high numerical aperture microscope objectives in order to create 4Pi excitation in the photopolymer. Here, we describe this technique in detail and experimentally demonstrate the voxel axial length reduction by nearly three times down to 150 nm.