The　wide　bandgap　semiconductor　material　gallium　nitride　(GaN)　has　a　wide　range　of　applications　in　optoelectronic　devices.　As　a　consequence　of　the　evolution　of　the　device,　the　conventional　processing　techniques　are　no　longer　capable　of　fulfilling　the　necessary　requirements.　The　utilisation　of　a　top-hat　femtosecond　laser　can　lead　to　a　significant　enhancement　in　the　quality　and　efficiency　of　the　processing,　due　to　the　benefits　conferred　by　the　short　pulse,　high　peak　power　and　uniform　energy　distribution.　In　this　paper,　a　multi-physics　field　model　is　established　using　COMSOL　Multiphysics®　to　study　the　thermal　field　dynamics　and　the　spatial　and　temporal　variations　in　free　electron　density　on　the　surface　of　GaN　following　irradiation　by　a　top-hat　femtosecond　laser.　The　aim　is　to　gain　insight　into　the　influence　of　this　process　on　the　optical　properties　and　ablation　morphology.　The　model　is　then　validated　by　comparing　the　results　with　experimental　data,　which　demonstrate　the　grooving　process　of　GaN　with　a　flat　surface　edge,　a　smooth　bottom　of　the　groove,　and　a　perpendicular　sidewall.　The　findings　indicate　that　alterations　in　the　free　electron　density　impact　the　optical　characteristics　of　the　material　surface.　The　distribution　of　free　electrons　within　the　material　is　a　determining　factor　in　the　ablation　morphology.　The　experimental　results　support　the　hypothesis　that　the　top-hat　laser　has　significant　potential　for　use　in　the　preparation　of　GaN　devices.　©　2025　SPIE.