3D nanopolymerization and damage threshold dependence on laser wavelength and pulse duration

Abstract

The dependence of the polymerization and optical damage thresholds in multi-photon polymerization (MPP) lithography was studied using a broadly-tunable laser system with group delay dispersion (GDD) control. The order of non-linearity and the light-matter interaction mechanisms were investigated using the resolution bridges method for non-photosensitized SZ2080^TM and photosensitized SZ2080^TM + IRG369 prepolymers. Energy deposition, voxel dimension growth, and the size of the dynamic fabrication window (DFW) were measured in the 700-1300 nm wavelength range at three different pulse durations measured at the sample - 100, 200 and 300 fs. Polymerization was observed at all wavelengths and pulse durations without significant differences in the achieved minimal spatial dimension ( $<300$ nm). This was achieved despite the broad range of excitation wavelengths used which spanned two- and three-photon absorption bands, and the differences in the absorption spectra of the prepolymers. The lateral and longitudinal voxel growth dynamics revealed an abrupt change in the power dependence of polymerization and a significant variation of the DFW - from 1 at 1250 nm to 29 at 700 nm. This result can be interpreted as a consequence of a change in the instantaneous refractive index and a lowering of the polymerization but not the damage threshold. The optimization of energy delivery to the material by a wavelength-tunable laser source with pulse duration control was experimentally validated. These findings are uncovering the complexity of polymerization mechanisms and are useful in further development of MPP technology.

Keywords: 3D printing; multi-photon phenomena; non-linear absorption; optical damage threshold; photo-polymerization; ultrashort laser pulses.

Figure 1:

Layout of the experimental setup: wavelength-tunable laser system with pulse GDD control (CRONUS-3P), τ measurement (CARPE) and beam attenuation control (VNDF), custom-made microscope setup: piezo-stage stack for X and Y movement, Z-axis stage, focusing objective, CCD camera.

Figure 2:

OPA parameters at system output, (a) – average power and pulse energy, and (b) – minimum pulse duration. Fourier-transform-limited values are given in the 400–480 nm range.

Figure 3:

Group delay dispersion control: (a) – Principal diagram of the compressor. M – mirror, P – prism, R – retroreflector, W – glass plate. Double arrows represent compressor tuning by translating the components in the greyed-out areas. (b) – Compressor GDD tuning range compared to the dispersion of several high-NA immersive objectives.

Figure 4:

Experimental procedure: (a) – focusing of the pre-chirped pulse into the sample; (b) – multi-photon polymerization of RB structure; (c) – development of the sample; (d) – SEM image of an entire RB object with lines and support pillars. Scale bar is 20 µm.

Figure 5:

Absorbance spectra of the prepolymer and the photoinitiator. Blue lines represent measured 1P absorbance spectra (solid line) [34] and the corresponding expected 2P (dashed line) and 3P (dotted line) absorbance spectra of the SZ2080^TM prepolymer, marked as calc. in the legend. Red lines represent measured 1P absorbance spectra of the IRG369 photoinitiator (solid line), the same spectra shifted to the 2PA maximum measured using the Z-scan technique (dashed line) [35], and the corresponding expected 3PA spectra (dotted line). Vertical lines mark typical wavelengths used in MPP.

Figure 6:

An example of the RBs manufactured using 100 fs pulses at two different λ in SZ2080^TM + IRG369 prepolymer at different intensities: 700 nm – upper two rows, and 800 nm – bottom row. The first row shows optical images taken during the MPP process. The bottom two rows show SEM images after full development in solvent and CPD. The blue and red zones mark intensities at which the polymerization and optical damage thresholds were observed. The dashed red zone shows an example of bubble formation seen in optical images.

Figure 7:

From the top: (1) polymerization threshold (solid lines with solid dot markers) and optical damage threshold (solid lines with cross markers) dependence on λ, when τ = 100, 200 and 300 fs. SZ2080^TM + IRG369 prepolymer includes damage thresholds recorded optically during the MPP process (dashed lines with empty circle markers); (2) fluence level at the same thresholds.

Figure 8:

DFW dependence on λ and τ in SZ2080^TM + IRG369 prepolymer. Measured and expected (marked as calc. in the legend) multi-photon absorbance spectra of SZ2080^TM and IRG369 are presented on an inverted secondary Y axis on the right.

Figure 9:

Experimental results of voxel growth. (a) Minimum and maximum voxel size dependence on λ and τ; (b) and (c) lateral and longitudinal voxel size dependence on the applied intensity with different τ for 800 nm and 1100 nm wavelengths respectively in SZ2080^TM + IRG369 prepolymer. Solid lines represent voxel growth model curves. Dashed horizontal line marks the diffraction limit. Polarization of the writing beam was set to linear. Sample SEM images of lines polymerized at 1.6 TW/cm² and 6.1 TW/cm² intensity levels are provided for the 800 nm, 100 fs case.