3D Printing using Two Photon Polymerization (2PP)

Nanoscribe's technology for the fabrication of three-dimensional micro- and nanostructures in photo-sensitive materials is based on direct laser writing (DLW): a non-linear two-photon absorption process (2PA; Figure 1). A necessary condition for two photons of near-infrared light being absorbed simultaneously is a sufficiently high light intensity that is provided by a femtosecond pulsed laser beam.[ref][ref] Typically, the laser is focused into the resin and two-photon polymerization (2PP) is triggered only in the focal spot volume (Figure 2-1), where the light intensity exceeds a polymerization threshold.[ref] The resin is otherwise transparent to the wavelength of the photons (Figure 2-2). In contrast, one photon absorption (1PA) with an equivalent energy takes place along the whole light cone (Figure 2-2).
The smallest printable 3D volume is termed a voxel, which is analogous to a 2D pixel. Moving the laser focus along a trajectory in all three dimensions enables printing of structures built from multiple voxels and printed lines (Figure 3). This technology enables printing of structures with small, medium and large feature sizes in 3D as well as 2D patterns (  |  ).

One and two photon absorption, where an excited state S₁ is reached that triggers polymerization of a suitable resin.

Representation of the intensity distribution for one and two photon absorption.

Imaging fluorescence from one and two photon absorption processes.

Comparison between one and two photon absorption (courtesy of S. Ruzin and H. Aaron, UC Berkeley).

Printing process of a 3D free-form structure using 2PP.[ref] The laser focus is deflected to precise locations in the resin as per the structural design. Development of the structure removes unpolymerized resin and the desired structure remains attached to the substrate.

Further reading

Absorption probability

The nonlinear light absorption of the 2PP resin exhibits an absorption probability $P_{material}\propto L{P}_{avg}^n\cdot t$ with $n$ reflecting the multi-photon process (mainly two photon activation), $t$ the time of exposure and $L{P}_{avg}$ the average laser power.[ref] The absorption competes with a quenching process, leading to a threshold at which polymerization of the 2PP resin starts (Figure 4-1).[ref] The iso-intensity surface (Figure 4-2) in the focal spot where the intensity is equal to the threshold defines the size and shape of the printed ellipsoidal voxel.[ref] Figure 4-2 provides a plot of the squared intensities $I^2$, as this is proportional to the absorption probability for two photon absorption, $P_{material}\propto I^2$. In contrast, the one photon absorption probability is proportional to the intensity $I$.[ref][ref]

3D squared-intensity spacial distribution and threshold for 2PP.[ref][ref]

2D squared-intensity spacial distribution for 2PP.

3D and 2D spacial distribution of intensity and polymerization threshold.

Structural dimensions

It is possible to print line widths smaller than the diffraction-limited resolution of the objective lens.[ref] However, the period of the lines still is diffraction-limited by the objective lens.[ref] In addition, the proximity effect of the polymerization limits the line printing period.[ref] The proximity effect describes the (in this case) unintended, sub-threshold polymerization of resin between neighboring printed lines. Adjusting the dose in the focal spot leads to variable voxel sizes, with a lower limit of polymerization theshold and an upper limit of over-exposure. The dose and the distance between neighboring printed lines control the degree of polymerization.