Fig. 2: Implementation of two-photon direct-laser writing for 3D subnanometer focusing.

a–c Schematics of 3D nanoprinted fiducials. a A 3 µm diameter and a 20% spherical cap (b) fiducial (same shape as in a) on a 3 µm high pedestal and c, fiducials assembled in a 3D architecture. The design in (c) covers a vertical range of 20 µm. d Subnanometer real-time 3D focus locking. Focus-locking was performed from 0 to 20 μm in 1 μm increments. Each z-step was acquired for 1 h (n = 3), with a combined duration of 63 h. The residual movement (Δx/Δy/Δz) was determined using a duplicate out-of-loop structure acquired simultaneously. Data are presented as mean values of the residual movement and error bars correspond to the standard deviation across the experimental repeats (n = 3). The sample stability across all depths in all directions was less than 0.8 nm. e, f are examples of prefabricated fiducial arrays. e SEM images of multiple fiducial arrays (n = 15 arrays). The arrays on the right have spherical fiducials enclosed within a protective box (right). f Zoom-in of highlighted areas in (e). Scale bars = 200 µm in (e) and scale bars = 50 µm in (f).