Femtosecond　(fs)　laser-thin　film　interaction　is　one　of　the　most　practical　methods　for　fabricating　functional　nanostructures.　However,　the　details　of　the　interaction　mechanism　remain　unclear.　In　this　study,　we　demonstrate　an　abnormal　ablation　effect　on　nanofilms　by　using　a　tightly　focused　single　fs　laser　pulse.　After　the　irradiation　of　a　single　Gaussian-shaped　femtosecond　laser　pulse,　a　molten　micro/nanopatch　at　the　irradiated　central　high-power　zone　is　isolated　from　the　surrounding　film.　The　confined　localized　threshold　effect　is　proposed　as　the　main　mechanism　for　the　phase　isolation.　With　this　effect,　the　high　refractive　index　dielectric　Ge2Sb2Te5　crystal　nanostructures　can　be　fabricated　by　directed　dewetting　of　the　isolated　molten　micro/nanopatch　on　Si　substrates.　After　the　laser　irradiation,　the　central　isolated　liquid　through　an　amorphous　GST　film　is　transformed　into　a　crystalline　state　after　resolidification.　The　isolated　central　micro/nanopatch　size　can　be　controlled　by　the　focused　spot　size　and　pulse　energy,　so　that　the　morphologies　(size,　geometrical　morphology,　and　distribution)　of　GST　nanostructures　can　be　flexibly　modulated.　Furthermore,　separated　solid　and　liquid　phase　states　detected　using　spatial-temporal-resolved　microscopy　validates　the　crucial　role　of　the　confined-localized　threshold　effect　in　the　dewetting　effect　based　on　the　separated　liquid　phase.　(C)　2020　Optical　Society　of　America　under　the　terms　of　the　OSA　Open　Access　Publishing　Agreement