We report a new method to fabricate self-enclosed optically transparent nanofluidic channel arrays with sub-10 nm channel width over large areas. Our method involves patterning nanoscale Si trenches using nanoimprint lithography (NIL), sealing the trenches into enclosed channels by ultrafast laser pulse melting and shrinking the channel sizes by self-limiting thermal oxidation. We demonstrate that 100 nm wide Si trenches can be sealed and shrunk to 9 nm wide and that lambda-phage DNA molecules can be effectively stretched by the channels.