Ultrafast inscription of laser induced graphene piezoresistive strain sensors in polyimide

Ultrafast laser processing has emerged as a promising technique for the direct fabrication of conductive carbon structures in polymers without masks, chemical reagents, or furnace-based carbonisation. Polyimide is a particularly suitable substrate due to its high thermal stability, mechanical robustness, and widespread use in flexible electronic platforms. Although laser-induced graphene has been extensively reported using continuous-wave and long-pulse laser systems, systematic studies employing ultrafast femtosecond sources remain limited. In particular, no structured parametric investigation has previously been performed on the Carbide femtosecond laser system used in this project. This thesis presents an experimental feasibility study of ultrafast laser-induced surface carbonisation of polyimide. The primary objective was to identify and characterise processing conditions that reproducibly generate electrically conductive carbon tracks. A defined parameter space was explored through controlled variation of pulse energy, scan speed, repetition rate, and hatch spacing. The work does not claim full optimisation; rather, it establishes baseline processing windows and carbonisation thresholds specific to the available laser platform. Surface modification was evaluated using optical microscopy and Raman spectroscopy. Raman spectra exhibit characteristic D and G bands associated with disordered graphitic carbon, confirming successful laser-induced carbonisation under selected conditions. Variations in the ID/IG intensity ratio correlate with changes in laser fluence and scanning parameters, indicating a measurable process–structure relationship. Electrical characterisation demonstrates consistent reductions in resistance for specific parameter sets, confirming the formation of conductive surface pathways. However, within the scope and timeframe of this project, conductive structures were confined to the surface. Systematic fabrication and verification of embedded three-dimensional conductive architectures were not achieved. Preliminary electromechanical testing was conducted to assess the piezoresistive response of selected conductive tracks under tensile loading. Resistance changes were observed during deformation, demonstrating proof-of-concept strain sensitivity. However, several tests extended beyond the elastic limit of the polyimide substrate, and strain control was not consistently restricted to the elastic regime. Consequently, the mechanical data presented should be considered exploratory and do not constitute a fully validated strain sensor characterisation study. The principal contribution of this thesis is the generation of reproducible experimental data defining the initial processing window for ultrafast surface carbonisation of polyimide using the Carbide femtosecond system. The results establish preliminary process–property correlations linking laser parameters, Raman signatures, and electrical conductivity. These findings provide a foundation for subsequent systematic optimisation. Based on the feasibility results obtained, an improved experimental methodology is proposed for future investigation. This includes: (1) statistically designed experiments with controlled fluence increments and repeatability analysis; (2) quantitative mapping of carbonisation thresholds; (3) controlled mechanical testing strictly within the elastic strain regime; and (4) structured evaluation of subsurface focusing conditions to determine the feasibility of true three-dimensional inscription. Implementation of this methodology would enable rigorous process optimisation and a comprehensive structure–property performance framework. In summary, this thesis presents a focused experimental study of ultrafast laser-induced surface carbonisation of polyimide, identifies reproducible conductive processing conditions, and defines a clear pathway for systematic development toward advanced flexible electronic applications.