We have observed the second harmonic radiation (124 nm) of a subpicosecond KrF laser using an ultrashort scale length plasma produced by the same laser. It is demonstrated that an ultrashort scale plasma with a steep density gradient acts as a reflective nonlinear medium. A model predicts that the second harmonic conversion efficiency of 10−4would be obtainable at 124 nm. ©1995 American Institute of Physics.