Detection of periodic structures through opaque metal layers by optical measurements of ultrafast electron dynamics

We report on femtosecond optical pump-probe measurements of ultrafast electron dynamics to detect the presence of gratings buried underneath optically opaque gold layers. Electron energy diffusion and cooling are found to be strongly affected by the presence and type of metal buried below the gold layer. As a result, the spatially periodic buried grating is encoded on the electron temperature near the top surface, leading to a spatially periodic modulation of the optical properties near the gold surface from which a delayed probe pulse can be diffracted. Our measurements show that these effects may be useful for optical detection and alignment applications in semiconductor device manufacturing.