Enhancement of laser machining resolution using the two photon absorption effect

Abstract

One of the main strengths of ultrafast laser machining is its ability to produce very high resolution cuts. This is useful in a range of fields for manufacturing purposes, however being able to incorporate two photon absorption (TPA) to further enhance this would allow even smaller fabrication. In this study we look at the effect of doping amorphous polycarbonate with two different molecules (R1 and R2) which have been identified to have a high TPA cross-section. Using the diameter regression (D2) technique, we observe a notable decrease in the ablation thresholds as dopant concentration increases from 0 – 10%. In addition, these results provide a measurement of effective beam waist (ωeff). Beam waist values for R2 doped samples show a decrease to half the reference, which can be linked to an increase in the occurrence of multiphoton absorption. Linear ablation features produced at identical peak powers and pulse numbers show improved machining resolution as dopant concentration increases. This increase in resolution is more prolific for R2 than R1 despite their similar TPA cross sections. We believe that this is due to the relative bandgaps of the materials causing three photon absorption for R2 whereas multiphoton absorption is limited to the second order process for R1. This means by doping appropriate materials with these molecules it is possible to improve resolution whilst maintaining most properties of the main material.