Nonequivalence of spatial shifts and wigner delays at interfaces

: We isolate spatial shifts and Wigner delays for reflection at the same interface and demonstrate that they can carry different information. The spatial shifts associated with Wood anomalies on gratings can be either positive or negative, while the corresponding delays are both positive. In the standard case of total reflection at a glass-air interface, a differential two-photon absorption correlation technique allows us to measure for the first time a delay of up to 35 fs in agreement with the associated 10.5 microm spatial shift. The method also allows us to isolate giant Wigner delays. The existence of similar delays in different areas of physics is discussed.