Split-Aperture Xolography - Linear Volumetric Photoactivation with Short Axial Dimension and Low out of Focus Excitation.

Spatially confined photo-excitation with the lowest possible activation of the remaining volume is of central importance for high-resolution high-density optical data storage, fluorescence microscopy, 3D-lithography, and 3D-printing. Two-photon absorption (2PA) enables such applications yet leads to slow processing speed due to the underlying non-linear absorption process. Here, Split-Aperture Xolography (SAX), is introduced which uses stepwise excitation of dual-color responsive molecules to initiate a linear volumetric photo-reaction process that is up to several orders of magnitude more efficient than 2PA. The capabilities of SAX are investigated in a scenario study for focusing systems with high numerical aperture (NA) using a Python implementation of vectorial diffraction theory. The intersecting half-cones generated by the split illuminated entrance aperture of the objective reduce the axial focal spot size of the activation distribution by up to a factor of two compared to 2PA targeting the same electronic transition. A steep average decline of the activation probability with the fifth power away from focus is found for a wide range of directions. This is significantly better in comparison to 2PA and prevents that undesired out-of-focus excitation events sum-up with subsequent irradiations. This approach is expected to be advantageous for volumetric methods at the nanoscale.