Passive axial confinement in two-photon lithography with a 3D-printed pupil filter

Two-photon lithography (TPL) enables true three-dimensional nanoscale fabrication, but the writing voxel is inherently anisotropic, with the focal volume substantially more extended along the optical axis than in the transverse plane. This axial voxel elongation remains a central limitation in TPL, restricting layer separation, suspended-feature fidelity, and three-dimensional shape control. We report a passive, 3D-printed Toraldo-style pupil filter that retrofits a high-numerical-aperture (high-NA) bottom-up TPL system to confine the axial voxel, without the need for spatial light modulator (SLM) or digital micromirror device (DMD) beam shaping. A full-vector stratified-media model, matched to the objective-oil-coverslip-resin stack, predicts a reduction of the two-photon axial full width at half maximum (FWHM) from 0.517 to 0.418 µm. A lateral-offset sensitivity analysis assesses misalignment tolerance. Gold-nanoparticle axial scans measure a reduction from 0.755 to 0.470 µm, and scanning electron microscopy (SEM) measurements of suspended crosses show a reduced z/x aspect ratio from 2.71 to 1.64. These results demonstrate that a passive printed element can match or exceed the axial confinement performance of active SLM-based designs, establishing passive pupil engineering as a practical and low-cost route to axial confinement in high-NA TPL, with sidelobe-controlled exposure as an exploitable implementation constraint.