Femtosecond laser micro-structuring of amorphous Polyether(ether)ketone (PEEK) at 775 nm and 387 nm

Laser micro-structuring of amorphous PEEK has been demonstrated with 180 fs/1 kHz NIR (775 nm) and NUV (387 nm) laser pulses. Significant differences in laser-material interaction is observed between the NIR and NUV wavelengths with single pulse ablation threshold found to be 2.01 ± 0.05 J cm-2 and 0.23 ± 0.02 J cm-2 at 775 nm and 387 nm respectively. This is connected to the requirement for multi-photon absorption at 775 nm, where PEEK is transparent, while at 387 nm, significant linear absorption within the material bandgap occurs, favouring sequential 2-photon absorption via reverse saturated absorption (RSA). Open aperture Z scan data yields a 2-photon absorption coefficient β387(I) which is intensity dependent, consistent with RSA. Multi-pulse exposure yields incubation coefficients of S775 = 0.72 ± 0.01 and S387 = 0.85 ± 0.02. This significant reduction in NUV incubation is consistent with a much higher level of electron excitation to the conduction band. Consequently, ablation of PEEK with NUV fs pulses is superior, exhibiting much reduced melting and re-deposition. Precision NUV polymer surface micro-structuring is accomplished while laser induced periodic surface structures (LIPSS) with pitch Λ ~ 0.4 μm are observed at the base of ablated regions, aligned parallel to the incident polarisation. The ease of NUV LIPSS generation allowed surface patterning of large scanned areas which exhibit white light diffraction due to this sub-micron periodic surface modulation.